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Treatment of the Subject.—In this paragraph the subject matterof the science of petrology is briefly surveyed; the object is to pointout the headings under which particular subjects are treated(there is a separate article on the terms printed in italics). Generalquestions as to the nature, origin and classification of rocks and themethods of examination are discussed in the present article,mineralogy comprises similar matter respecting the componentminerals;metamorphism,metasomatism,pneumatolysis and theformation ofconcretions are agencies which effect rocks and modifythem. Three classes of rocks are recognized the igneous, sedimentaryand metamorphic. The plutonic, or deep-seated rocks, whichcooled far below the surface, and occur asbatholites, bosses,laccolites,andveins, include the great classesgranite,syenite,diorite,gabbroandperidotite; related to the granites areaplite,greisen,pegmatite,schorl rock andmicropegmatite, to the syenites,borolanite,monzonite,nepheline-syenite andijolite, to the diorites,aphanite,napoleoniteandtonalite; to the gabbros,pyroxenite andtheralite, and toperidotites,picrite andserpentine. The hypabyssal intrusive rocks,occurring assills,veins,dikes,necks, &c., are represented byporphyryandporphyrite (includingbostonite,felsite andquartz-porphyry),diabase andlamprophyre; somepitchstones belong to thisgroup and containcrystallites andspherulites. The volcanic rocks,found typically as lava flows, includerhyolite andobsidian (withsometimesperlite),trachyte andphonolite (and leucitophyre whichis treated underleucite),andesite anddacite,basalt (with the relateddolerite,variolite andtachylyte),nephelinite andtephrite. Amongsedimentary rocks we recognize a volcanic group (includingtuff,agglomerate and some kinds ofpumice), an arenaceous series suchassand (some withglauconite),sandstone,quartzite,greywacke andgravel; an argillaceous group includingclay,firebrick,phyllite,lateriteshale andslate; a calcareous series withchalk,limestone (oftenformingstalactites andstalagmites),dolomite andmarls or argillaceouslimestones (flint occurs as nodules in chalk); the natural phosphatesmay be mentioned here. The metamorphic rocks are commonlygneisses andschists (includingmica-schist), other types areamphibolite,charnockite,eclogite,epidiorite,epidosite,granulite,itacolumite,hornfels,mylonite and thescapolite rocks.

A good rock-section should be about one-thousandth of an inchin thickness, and is by no means very difficult to make. A thinsplinter of the rock, about as large as a halfpenny maybe taken; it should be as fresh as possible and free fromobvious cracks. By grinding on a plate of planed steel or castiron with a little fine carborundum it is soon rendered flat on one sideSections.and is then transferred to a sheet of plate glass and smoothed withthe very finest emery till all minute pits and roughnesses are removedand the surface is a uniform plane. The rock-chip is then washed,and placed on a copper or iron plate which is heated by a spirit orgas lamp. A microscopic glass slip is also warmed on this platewith a drop of viscous natural Canada balsam on its surface. Themore volatile ingredients of the balsam are dispelled by the heat,and when that is accomplished the smooth, dry, warm rock is pressedfirmly into contact with the glass plate so that the film of balsamintervening may be as thin as possible and free from air-bubbles.The preparation is allowed to cool and then the rock chip is againground down as before, first with carborundum and, when it becomestransparent, with fine emery till the desired thickness is obtainedit is then cleaned, again heated with a little more balsam, andcovered with a cover glass. The labour of grinding the first surfacemay be avoided by cutting off a smooth slice with an iron disk armedwith crushed diamond powder. A second application of the slitterafter the first face is smoothed and cemented to the glass will inexpert hands leave a rock-section so thin as to be already transparent.In this way the preparation of a section may require only twentyminutes.

The microscope employed is usually one which is provided with arotating stage beneath which there is a polarizer, while above theobjective or the eyepiece an analyser is mounted; alternativelythe stage may be fixed and the polarizing andanalysing prisms may be capable of simultaneous rotation by meansof toothed wheels and a connecting-rod. if ordinary light and notpolarized light is desired, both prisms may be withdrawn from theMicroscope.axis of the instrument, if the polarizer only is inserted the lighttransmitted is plane polarized, with both prisms in position theslide is viewed between “crossed nicols” A microscopic rock-sectionin ordinary light if a suitable magnification (say 30) beemployed is seen to consist of grains or crystals varying in colour,size and shape. Some minerals are colourless and transparentCharacters of Minerals.(quartz, calcite, felspar, muscovite, &c.), othersare yellow or brown (rutile, tourmaline, biotite), green(diopside, hornblende, chlorite), blue (glaucophane), pink (garnet),&c. The same mineral may present a variety of colours, in thesame or different rocks, and these colours may be arranged inzones parallel to the surfaces of the crystals. Thus tourmalinemay be brown, yellow, pink, blue, green, violet, grey or colourless,but every mineral has one or more characteristic, because mostcommon tints The shapes of the crystals determine in ageneral way the outlines of the sections of them presented onthe slides If the mineral has one or more good cleavages theywill be indicated by systems of cracks (see Pl. III.) The refractiveindex is also clearly shown by the appearance of the sections,which are rough, with well-defined borders if they have a muchstronger refraction than the medium in which they are mounted.Some minerals decompose readily and become turbid and semi-transparent(e.g. felspar); others remain always perfectly fresh andclear (e.g. quartz), others yield characteristic secondary products(such as green chlorite after biotite). The inclusions in the crystalsare of great interest; one mineral may enclose another, or may containspaces occupied by glass, by fluids or by gases.

Lastly thestructure of the rock, that is to say, the relation of itscomponents to one another, is usually clearly indicated, whether itbe fragmental or massive; the presence of glassy matterin contradistinction to a completely crystalline or“holo-crystalline” condition; the nature and origin oforganic fragments; banding, foliation or lamination; the pumiceousMicro-structure.or porous structure of many lavas; these and many other characters,though often not visible in the hand specimens of a rock, are renderedobvious by the examination of a microscopic section. Many refinedmethods of observation may be introduced, such as the measurementof the size of the elements of the rock by the help of micrometers;their relative proportions by means of a glass plate ruled in smallsquares, the angles between cleavages or faces seen in section bythe use of the rotating graduated stage, and the estimation of therefractive index of the mineral by comparison with those of differentmounting media.

Further information is obtained by inserting the polarizer androtating the section. The light vibrates now only in one plane, andin passing through doubly refracting crystals in theslide is, speaking generally, broken up into two rays,which vibrate at right angles to one another. In manycoloured minerals such as biotite, hornblende, tourmaline, chlorite,Pleochroism.these two rays have different colours, and when a section containingany of these minerals is rotated the change of colour isoften very striking. This property, known as “pleochroism” (Gr.πλείων, more;χρώς, colour), is of great value in the determination ofrock-making minerals. It is often especially intense in small spotswhich surround minute enclosures of other minerals, such as zirconand epidote; these are known as “pleochroic halos.”

If the analyser be now inserted in such a position that it is crossedrelatively to the polarizer the field of view will be dark where thereare no minerals, or where the light passes through isotropicsubstances such as glass, liquids and cubic crystals.All other crystalline bodies, being doubly refracting,will appear bright in some position as the stage is rotated. TheDouble Refraction.

Extinction.only exception to this rule is provided by sections which areperpendicular to the optic axes of birefringent crystals; theseremain dark or nearly dark during a whole rotation, and as willbe seen later, their investigation is of special importance. Thedoubly refracting mineral sections, however, will in all casesappear black in certain positions as the stage isrotated. They are said to be “extinguished” whenthis takes place. If we note these positions we may measurethe angle between them and any cleavages, faces or otherstructures of the crystal by means of the rotating stage. Theseangles are characteristic of the system to which the mineral belongsand often of the mineral species itself (seeCrystallography.To facilitate measurement of extinction angles various kinds ofeyepieces have been devised, some having a stauroscopic calciteplate, others with two or four plates of quartz cemented together;these are often found to give more exact results than are obtainedby observing merely the position in which the mineral section is mostcompletely dark between crossed nicols.

The mineral sections when not extinguished are not only brightbut are coloured and the colours they show depend on several factors,the most important of which is the strength of the double refraction.If all the sections are of the same thickness as is nearly true of well-madeslides, the minerals with strongest double refraction yieldthe highest polarization colours. The order in which the coloursare arranged is that known as Newton’s scale, the lowest beingdark grey, then grey, white, yellow, orange, red, purple, blue andso on. The difference between the refractive indexes of the ordinaryand the extraordinary ray in quartz is ·009, and in a rock-sectionabout⁠1/500⁠ of an inch thick this mineral gives grey and whitepolarization tints; nepheline with weaker double refraction givesdark grey; augite on the other hand will give red and blue, whilecalcite with still stronger double refraction will appear pinkish orgreenish white. All sections of the same mineral, however, will nothave the same colour; it was stated above that sections perpendicularto an optic axis will be nearly black, and, in general, the more nearlyany section approaches this direction the lower its polarizationcolours will be. By taking the average, or the highest colourgiven by any mineral, the relative value of its double refraction canbe estimated; or if the thickness of the section be precisely knownthe difference between the two refractive indexes can be ascertained.If the slides be thick the colours will be on the whole higher than inthin slides.

It is often important to find out whether of the two axes of elasticity(or vibration traces) in the section is that of greater elasticity(or lesser refractive index). The quartz wedge or selenite plateenables us to do this. Suppose a doubly refracting mineral sectionso placed that it is “extinguished”; if now it is rotated through45° it will be brightly illuminated. If the quartz wedge be passedacross it so that the long axis of the wedge is parallel to the axisof elasticity in the section the polarization colours will rise or fall.If they rise the axes of greater elasticity in the two minerals areparallel; if they sink the axis of greater elasticity in the one is parallelto that of lesser elasticity in the other. In the latter case by pushingthe wedge sufficiently far complete darkness or compensation willresult. Selenite wedges, selenite plates, mica wedges and micaplates are also used for this purpose. A quartz wedge also may becalibrated by determining the amount of double refraction in allparts of its length. If now it be used to produce compensationor complete extinction in any doubly refracting mineral section, wecan ascertain what is the strength of the double refraction of thesection because it is obviously equal and opposite to that of a knownpart of the quartz wedge.

A further refinement of microscopic methods consists of the useof strongly convergent polarized light (konoscopic methods). Thisis obtained by a wide angled achromatic condenser above the polarizer,and a high power microscopic objective. Those sections aremost useful which are perpendicular to an optic axis, and consequentlyremain dark on rotation If they belong to uniaxial crystalsthey show a dark cross or convergent light between crossed nicols, ​the bars of which remain parallel to the wires in the field of the eye-piece.Sections perpendicular to an optic axis of a biaxial mineralunder the same conditions show a dark bar which on rotationbecomes curved to a hyperbolic shape. If the section is perpendicularto a “bisectrix” (seeCrystallography) a black cross is seenwhich on rotation opens out to form two hyperbolas, the apices ofwhich are turned towards one another. The optic axes emerge atthe apices of the hyperbolas and may be surrounded by colouredrings, though owing to the thinness of minerals in rock sectionsthese are only seen when the double refraction of the mineral isstrong. The distance between the axes as seen in the field of themicroscope depends partly on the axial angle of the crystal andpartly on the numerical aperture of the objective. If it is measuredby means of an eye-piece micrometer, the optic axial angle of themineral can be found by a simple calculation. The quartz wedge,quarter mica plate or selenite plate permit the determination of thepositive or negative character of the crystal by the changes in thecolour or shape of the figures observed in the field. These operationsare precisely similar to those employed by the mineralogist in theexamination of plates cut from crystals. It is sufficient to pointout that the petrological microscope in its modern development isan optical instrument of great precision, enabling us to determinephysical constants of crystallized substances as well as serving toproduce magnified images like the ordinary microscope. A greatvariety of accessory apparatus has been devised to fit it for thesespecial uses.

Fluids are used which do not attack the majority of the rock-makingminerals and at the same time have a high specific gravity.Solutions of potassium mercuric iodide (sp. gr. 3·196), cadmiumborotungstate (sp. gr 3·30), methlyene iodide (sp. gr. 3·32), bromoform(sp. gr. 2·86), or acetylene bromide (sp. gr. 3·00) are the principalmedia employed They may be diluted (with water, benzene,&c.) to any desired extent and again concentrated by evaporation.If the rock be a granite consisting of biotite (sp. gr. 3·1), Muscovite(sp. gr. 2·85), quartz (sp. gr. 2·65), oligoclase (sp. gr. 2·64) andorthoclase (sp. gr. 2·56) the crushed minerals will all float inmethylene iodide; on gradual dilution with benzene they will beprecipitated in the order given above. Although simple in theorythese methods are tedious in practice, especially as it is commonfor one rock-making mineral to enclose another. But experthandling of fresh and suitable rocks yields excellent results and muchpurer powders may be obtained by this means than by any other.

Complete chemical analyses of rocks are also widely made use ofand are of the first importance, especially when new species are underdescription. Rock analysis has of late years (largely under theinfluence of the chemical laboratory of the United States GeologicalSurvey) reached a high pitch of refinement and complexity. Asmany as twenty or twenty-five components may be determined, butfor practical purposes a knowledge of the relative proportions ofsilica, alumina, ferrous and ferric oxides, magnesia, lime, potash,soda and water will carry us a long way in determining the positionto which a rock is to be assigned in any of the conventional classifications.A chemical analysis is in itself usually sufficient to indicatewhether a rock is igneous or sedimentary and in either case to showwith considerable accuracy to what subdivision of these classes itbelongs. In the case of metamorphic rocks it often establisheswhether the original mass was a sediment or of volcanic origin.

The specific gravity of rocks is determined in the usual way bymeans of the balance and the pycnometer. It is greatest in thoserocks which contain most magnesia, iron and heavymetals; least in rocks rich in alkalis, silica and water.It diminishes with weathering,Specific Gravity.and generally those rockswhich are highly crystalline have higher specific gravities than thosewhich are wholly or partly vitreous when both have the samechemical composition. The specific gravity of the commoner rocksranges from about 2·5 to 3·2.

into marble by heating it in a closed gun-barrel, which prevented theescape of the carbonic acid at high temperatures. Adams andNicholson have carried this a stage farther by subjecting marbleto great pressure in hydraulic presses and have shown how thefoliated structures, frequent in natural marbles, may be producedartificially.

Theigneous rocks (crystalline and fragmental) form a well-definedgroup, differing in origin from all others. The crystalline or massivevarieties may occur in two different ways; the lavas havebeen poured out at the surface and have consolidatedafter ejection, under, conditions which are fairly wellunderstood, seeing that they may be examined at active volcanoesIgneous Rocks.in many parts of the world; the intrusive rocks, on the other hand,have been injected from below into cracks and fissures in the strataand have cooled there beneath masses which conceal them from viewtill exposed by denudation at a subsequent period. The membersof these two groups differ in many respects from one another, sothat it is often possible to assign a rock to one or other of them onmere superficial inspection. The lavas (or effusive rocks), havingcooled rapidly in contact with the air, are mostly finely crystallineor have at least fine-grained ground-mass representingthat part of the viscous semi-crystalline lava flow whichwas still liquid at the moment of eruption. At thistime they were exposed only to atmospheric pressure, andthe steam and other gases, which they contained in great quantity,were free to escape, many important modifications arise from this,Lavas or Effusive Types.the most striking being the frequent presence of numerous steamcavities (vesicular structure) often drawn out to elongated shapessubsequently filled up with minerals by infiltration (amygdaloidalstructure). As crystallization was going on while the mass wasstill creeping forward over the surface of the earth, the latestformed minerals (in the ground-mass) are commonly arranged insubparallel winding lines following the direction of movement(fluxion or fluidal structure) (see Pl. I. figs. 2 and 9, Pl. II. fig. 2), andthe larger early minerals which had previously crystallized may showthe same arrangement. Most lavas have fallen considerably belowtheir original temperatures before they are emitted In theirbehaviour they present a close analogy to hot solutions of saltsin water, which, when they approach the saturation temperature,first deposit a crop of large, well-formed crystals (labile stage) andsubsequently precipitate clouds of smaller less perfect crystallineparticles (metastable stage). In igneous rocks the first generationof crystals generally forms before the lava has emerged to the surface,that is to say, during the ascent from the subterranean depths to thecrater of the volcano. It has frequently been verified by observationthat freshly emitted lavas contain large crystals borne along in amolten, liquid mass. The large, well-formed, early crystals aresaid to be porphyritic (Pl. III. figs. 1, 2, 3); the smaller crystals of thesurrounding matrix or ground-mass belong to the post-effusion stage.More rarely lavas are completely fused at the moment of ejection,they may then cool to form a non-porphyritic, finely crystalline rock,or if more rapidly chilled may in large part be non-crystalline orglassy (vitreous rocks such as obsidian, tachylyte, pitchstone (Pl. I.figs. 1, 4, 5). A common feature of glassy rocks is the presence ofrounded bodies (spherulites. Gr.σφαῖρα, ball), consisting of fine divergentfibres radiating from a centre (Pl. I. figs. 7, 8); they consist ofimperfect crystals of felspar, mixed with quartz or tridymite; similarbodies are often produced artificially in glasses which are allowed tocool slowly. Rarely these spherulites are hollow or consist of concentricshells with spaces between (lithophysae Gr.λίθος, stone;φῦσα, bellows). Perlitic structure, also common in glasses, consistsin the presence of concentric rounded cracks owing to contractionon cooling (seePerlite).

The phenocrysts (Gr.φαίνειν, to show;κρύςταλλον, crystal) or porphyriticminerals are not only larger than those of the ground-mass.As the matrix was still liquid when they formed they werefree to take perfect crystalline shapes, not being interfered with bythe pressure of adjacent crystals They seem to have grown rapidly,as they are often filled with enclosures of glassy or finely crystallinematerial like that of the ground-mass (Pl. II. fig. 1). Microscopicexamination of the phenocrysts often reveals that they have had acomplex history. Very frequently they show successive layersof different composition, indicated by variations in colour or otheroptical properties; thus augite may be green at the centre and variousshades of brown outside this, or may be pale green centrally anddarker green with strong pleochroism (aegirine) at the periphery.In the felspars the centre is usually more basic and richer in limethan the surrounding faces, and successive zones may often be noted,each less basic than those which lie within it. Phenocrysts of quartz(and of other minerals), instead of sharp, perfect crystalline faces,may show rounded corroded surfaces (Pl. I. fig. 9), with the pointsblunted and irregular tongue-like projections of the matrix into thesubstance of the crystal. It is clear that after the mineral hadcrystallized it was partly again dissolved or corroded at some periodbefore the matrix solidified. Corroded phenocrysts of biotite andhornblende are very common in some lavas; they are surroundedby black rims of magnetite mixed with pale green augite. Thehornblende or biotite substance has proved unstable at a certainstage of consolidation and has been replaced by a paramorph ofaugite and magnetite which may be partially or completely substitutedfor the original crystal but still retains its characteristicoutlines.

Let us now consider the characteristics of a typical deep-seatedrock like granite or diorite (Pl. II. figs. 4, 5, 9). That these areigneous is proved by the manner in which they haveburst through the superincumbent strata, filling thecracks with ramifying veins; that they were at a veryhigh temperature is equally clear from the changes whichPlutonic or Abyssal Types.they have induced in the rocks in contact with them. But as theirheat could dissipate only very slowly, because of the masses whichcovered them, complete crystallization has taken place and novitreous rapidly chilled matter is present. As they have had timeto come to rest before crystallizing they are not fluidal. Theircontained gases have not been able to escape through the thick layerof strata beneath which they were injected, and may often be observedoccupying cavities in the minerals, or have occasioned manyimportant modifications in the crystallization of the rock. Becausetheir crystals are of approximately equal size these rocks are said tobe granular, there is typically no distinction between a first generationof large well-shaped crystals and a fine-grained ground-mass Theirminerals have formed, however, in a definite order, and each has hada period of crystallization which may be very distinct or may havecoincided with or overlapped the period of formation of some of theother ingredients. The earlier have originated at a time whenmost of the rock was still liquid and are more or less perfect, the laterare less regular in shape because they were compelled to occupythe interspaces left between the already formed crystals (Pl. II.figs. 5, 9). The former are said to be idiomorphic (or automorphic),the latter are anidiomorphic (allotriomorphic, xenomorphic).^[1]There are also many other characteristics which serve to distinguishthe members of these two groups. Orthoclase, for example, is thetypical felspar of granite, while its modification sanidine occurs inlavas of similar composition. The same distinction holds betweenelaeolite and nepheline. Leucite is common in lavas, very rare inplutonic rocks. Muscovite is confined to the intrusives. Thesedifferences show the influence of the physical conditions underwhich consolidation takes place.

There is a certain class of intrusive rocks which have risenupwards towards the surface, but have failed to reach it, and havesolidified in fissures as dikes and intrusive sills at nogreat depth. To this type the nameintrusive (orhypabyssal)is often given in distinction to theplutonic (orabyssal) which formed at greater depths As mightIntrusive or Hypabyssal Types.be expected, they show structures intermediate between those ofthe effusive and the plutonic rocks. They are very commonly porphyritic,not rarely vitreous, and sometimes even vesicular. In factmany of them are indistinguishable petrologically from lavas ofsimilar composition.

The attempt to form a special group of hypabyssal (intrusive anddike) rocks has met with much criticism and opposition. Such agroup certainly cannot rank as equally important and equally wellcharacterized with the plutonic and the effusive. But there aremany kinds of rock which are not found to occur normally in anyother manner. As examples we may cite the lamprophyres, theaplites and the porphyrites. These never occur as lava flows or asgreat plutonic bosses; if magmas of the same composition as theserocks occur in either of these ways they consolidate with differentassemblages of minerals and different structures.

In subdividing the plutonic, the hypabyssal and the effusiverocks, the principle is followed of grouping thosetogether which resemble one another in mineral constitutionand in chemical composition. In a broadsense these two properties are interdependent.Subdivisions of igneous Rock Class.

The commoner rock constituents are nearly all oxides; chlorine,sulphur and fluorine are the only important exceptions to this andtheir total amount in any rock is usually much less than1%. F. W. Clarke has calculated that a little morethan 47% of the earth’s crust consists of oxygen. Itoccurs principally in combination as oxides, of which the chiefChemical Characters.are silica, alumina, iron oxides, lime, magnesia. potash and soda.The silica functions principally as an acid, forming silicates, and allthe commonest minerals of igneous rocks are of this nature. Froma computation based on 1672 analyses of all kinds of rocks Clarkearrived at the following as the average percentage composition:SiO₂＝59·71, Al₂O₃＝15·41, Fe₂O₃＝2·63, FeO＝3·52, MgO＝4·36,CaO＝4·90, Na₂O＝3·55, K₂O＝2·80, H₂O＝1·52, TiO₂＝0·60, P₂O₅＝0·22, total 99·22%. All the other constituents occur only in verysmall quantities, usually much less than 1%.

These oxides do not combine in a haphazardway. The potash and soda, for example, with asufficient amount of alumina and silica, combine toproduce felspars. In some cases they may takeother forms, such as nepheline, leucite and muscovite,but in the great majority of instances theyare found as felspar. The phosphoric acidlime forms apatite. The titanium dioxide withferrous oxide gives rise to ilmenite. Part of thelime forms lime felspar. Magnesia and iron oxides,with silica crystallize as olivine or enstatite, or withalumina and lime form the complex ferro-magnesiansilicates of which the pyroxenes, amphiboles andbiotites are the chief. Any excess of silica abovewhat is required to neutralize the bases willseparate out as quartz; excess of alumina crystallizesas corundum. These must be regarded only asgeneral tendencies, which are modified by physicalconditions in a manner not as yet understood.It is possible by inspection of a rock analysis tosay approximately what minerals the rock will contain, but thereare numerous exceptions to any rule which can be laid down.

Hence we may say that except in acid or siliceous rocks containing66% of silica and over, quartz will not be abundant. In basicrocks (containing 60% silica or less) it is rare andaccidental. If magnesia and iron be above the averagewhile silica is low olivine may be expected; where silicais present in greater quantity other ferro-magnesianMineral Constitution.minerals, such as augite, hornblende, enstatite or biotite, occurrather than olivine. Unless potash is high and silica relativelylow leucite will not be present, for leucite does not occur withfree quartz. Nepheline, likewise, is usually found in rocks withmuch soda and comparatively little silica. With high alkalissoda-bearing pyroxenes and amphiboles may be present. Thelower the percentage of silica and the alkalis the greater is theprevalence of lime felspar as contracted with soda or potashfelspar. Clarke has calculated the relative abundance of theprincipal rock-forming minerals with the following results: Apatite＝0·6,titanium minerals＝1·5, quartz＝12·0, felspars＝59·5,biotite＝3·8, hornblende and pyroxene＝16·8, total＝94·2%.This, however, can only be a rough approximation. The otherdetermining factor, namely the physical conditions attending consolidation,plays on the whole a smaller part, yet is by no meansnegligible, as a few instances will prove There are certain mineralswhich are practically confined to deep-seated intrusive rocks,e.g.microcline, muscovite, diallage. Leucite is very rare in plutonicmasses; many minerals have special peculiarities in microscopiccharacter according to whether they crystallized in depth or nearthe surface,e.g. hypersthene, orthoclase, quartz. There are somecurious instances of rocks having the same chemical compositionbut consisting of entirely different minerals,e.g. the hornblendite ofGran, in Norway, containing only hornblende, has the same compositionas some of the camptonites of the same locality which containfelspar and hornblende of a different variety. In this connexionwe may repeat what has been said above about the corrosion ofporphyritic minerals in igneous rocks. In rhyolites and trachytesearly crystals of hornblende and biotite may be found in greatnumbers partially converted into augite and magnetite. The hornblendeand biotite were stable under the pressures and other conditionswhich obtained below the surface, but unstable at higherlevels. In the ground-mass of these rocks augite is almost universallypresent. But the plutonic representatives of the same magma,granite and syenite contain biotite and hornblende far more commonlythan augite.

Those rocks which contain most silica and on crystallizing yieldfree quartz are erected into a group generally designated the “acid”rocks. Those again which contain least silica and mostmagnesia and iron, so that quartz is absent while olivineis usually abundant, form the “basic” group. The“intermediate” rocks include those which are characterizedAcid, Intermediate and Basic Igneous Rocks.by the general absence of both quartz and olivineAn important subdivision of these contains a very highpercentage of alkalis, especially soda, and consequently has mineralssuch as nepheline and leucite not common in other rocks. It isoften separated from the others as the “alkali” or “soda” rocks,and there is a corresponding series of basic rocks. Lastly a smallsub-group rich in olivine and without felspar has been called the“ultra basic” rocks. They have very low percentages of silica butmuch iron and magnesia.

Except these last practically all rocks contain felspars or felspathoidminerals. In the acid rocks the common felspars are orthoclase,with perthite, microcline, oligoclase, all having much silicaand alkalis. In the basic rocks labradorite, anorthite and bytowniteprevail, being rich in lime and poor in silica, potash and soda.Augite is the commonest ferro-magnesian of the basic rocks, butbiotite and hornblende are on the whole more frequent in the acid.

The rocks which contain leucite or nepheline, either partly orwholly replacing felspar are not included in this table. They areessentially of intermediate or of basic character. We might in consequenceregard them as varieties of syenite diorite, gabbro, &c.in which felspathoid minerals occur, and indeed there are manytransitions between syenites of ordinary type and nepheline—orleucite—syenite, and between gabbro or dolerite and theralite oressexite. But as many minerals develop in these “alkali” rockswhich are uncommon elsewhere, it is convenient in a purely formalclassification like that which is outlined here to treat the wholeassemblage as a distinct series.

This classification is based essentially on the mineralogical constitutionof the igneous rocks. Any chemical distinctions betweenthe different groups, though implied, are relegated to a subordinateposition. It is admittedly artificial but it has grown up with thegrowth of the science and is still adopted as the basis on whichmore minute subdivisions are erected. The subdivisions are by nomeans of equal value. The syenites, for example, and the peridotites,are far less important than the granites, diorites and gabbros.Moreover, the effusive andesites do not always correspond to theplutonic diorites but partly also to the gabbros. As the differentkinds of rock, regarded as aggregates of minerals, pass graduallyinto one another, transitional types are very common and are oftenso important as to receive special names. The quartz-syenites andnordmarkites may be interposed between granite and syenite, thetonalites and adamellites between granite and diorite, the monzonitesbetween syenite and diorite, norites and hyperites betweendiorite and gabbro, and so on.

There is of course a large number of recognized rock species notincluded in the tables given. These are of two kinds, either belongingto groups which are subdivisions of those enumerated (bearingthe same relation to them that species do to genera) or rare andexceptional rocks that do not fall within any of the main subdivisionsproposed. The question may be asked—When is a rock entitledto be recognized as belonging to a distinct species or variety anddeserving a name for itself? It must, first of all, be proved tooccur in considerable quantity at some locality, or better still ata series of localities or to have been produced from different magmasat more than one period of the earth’s history In other words, itmust not be a mere anomaly. Moreover, it should have a distinctivemineral constitution, differing from other rocks, or somethingindividual in the characters of its minerals or of its structures.It is often surprising how peculiar types of rock, believed at first ​to be unique, turn up with identical features in widely scatteredregions,alnöite, for example, occurs in Norway, Scotland, Montreal,British Columbia, New York and Brazil,tinguaite in Scotland,Norway, Brazil, Montana, Portugal, &c. This indicates thatunderlying all the variations in mineralogical, structural andchemical properties there are definite relationships which tend torepeat themselves, producing the same types whenever the sameconditions are present.

Although in former years the view was widely current, especiallyin Germany, that igneous rocks belonging to different geologicalepochs should receive different names, it is now admitted on allsides that this cannot be upheld.

In 1902 a group of American petrographers brought forwarda proposal to discard all existing classifications of igneous rocksand to substitute for them a “quantitative” classification basedon chemical analysis. They showed how vague and often unscientificwas much of the existing terminology and argued that asthe chemical composition of an igneous rock was its most fundamentalcharacteristic it should be elevated to prime position.Geological occurrence, structure, mineralogical constitution, thehitherto accepted criteria for the discrimination of rock specieswere relegated to the background. The completed rock analysisis first to be interpreted in terms of the rock-forming mineralswhich might be expected to be formed when the magma crystallizes,e.g. quartz felspars of various kinds, olivine, akermannite, felspathoids,magnetite, corundum and so on, and the rocks are dividedinto groups strictly according to the relative proportion of theseminerals to one another. There is no need here to describe theminutia of the process adopted as the authors have stated themvery clearly in their treatise (Quantitative Classification of IgneousRocks, Chicago, 1902), and there is no indication that even in theUnited States it will ever displace the older classifications.

We can often observe in a series of eruptives belonging to oneperiod and a restricted area certain features which distinguishthem as a whole more or less completely from othersimilar assemblages. Such groups are often said tobe consanguineous, and to characterize a definite“petrological province.” Excellent examples of this are furnishedCons­anguinity.by the Devonian igneous rocks of southern Norway as described byBrögger, the Tertiary rocks of the Hebrides (Harker), the Italianlavas studied by H. S. Washington. On a larger scale the volcanoeswhich girdle the Pacific (Andes, Cordillera, Japan, &c.), and thosewhich occur on the volcanic islands of the Atlantic, show the samephenomena. Each of these groups has been formed presumablyfrom a single deep-seated magma or source of supply and duringa period which while necessarily prolonged was not of vast durationin a geological sense.

On the other hand, each of the great suites of eruptive rockswhich constitute such a petrological province embraces a greatrange of types. Prolonged eruptions have in a fewcases a somewhat monotonous character, owing to thepredominance of one kind of rock. Thus the lavas ofthe Hawaiian Islands are mostly basaltic, as are those of Oregon,Differentia­tion.Washington and the Deccan, all of which form geological massesof enormous magnitude. But it is more usual to find basalts,andesites, trachytes, dacites and many other rocks occurring ina single eruptive complex. The process by which a magma splitsup into a variety of partial products is known as “differentiation.”Its importance from the standpoint of theoretical petrology is verygreat, but as yet no adequate explanation of it has been offered.Differentiation may show itself in two ways. In the first typethe successive emissions from a volcanic focus may differ considerablyfrom one another. Thus in the Pentland Hills, near Edinburgh,the lavas which are of lower Devonian age, were first basaltic,then andesitic, trachytic and dacitic, and finally rhyolitic, and thissuccession was repeated a second time. Yet they all must havecome from the same focus, or at any rate from a group of focivery closely connected with one another. Occasionally it is foundthat the earlier lavas are of intermediate character and that basicalternate with acid during the later stages of the volcanic history.

Not less interesting are those cases in which a single body ofrock has in consolidation yielded a variety of petrographical typesoften widely divergent. This is best shown by great plutonicbosses which may be regarded as having once been vast subterraneanspaces filled with a nearly homogeneous liquid magma. Coolingtook place gradually from the outer surfaces where the igneousrock was in contact with the surrounding strata. The resultantlaccolite (Gr.λάκκος, pit, crater,λίθος, stone), stock or boss, maybe a few hundred yards or many miles in diameter and oftencontains a great diversity of crystalline rocks. Thus peridotite,gabbro, diorite, tonalite and granite, are often associated, usuallyin such a way that the more basic are the first-formed and lie nearestthe external surfaces of the mass. The reverse sequence occursoccasionally, the edges being highly acid while the central partsconsist of more basic rocks. Sometimes the later phases penetrateinto and vein the earlier; evidently there has been somemovement due to temporary increase of pressure when part of thelaccolite was solid and part still in a liquid state. This links thesephenomena with those above described where successive emissionsof different character have proceeded outwards from the focus.

According to modern views two explanations of these facts arepossible. Some geologists hold that the different rock faciesfound in association are often due to local absorption of surroundingrocks by the molten magma (“assimilation”). Effects of thiskind are to be expected, and have been clearly proved in manyplaces. There is, however, a general reluctance to admit that theyare of great importance. The nature and succession of the rockspecies do not as a rule show any relation to the sedimentary orother materials which may be supposed to have been dissolved;and where solution is known to have gone on the products areusually of abnormal character and easily distinguishable from thecommon rock types.

Hence it is generally supposed that differentiation is to beascribed to some physical or chemical processes which lead to thesplitting up of a magma into dissimilar portions, each of whichconsolidates as a distinct kind of rock. Two factors can be selectedas probably most potent. One important factor is cooling andanother is crystallization. According to physico-chemical laws theleast soluble substances will tend to diffuse towards the coolingsurfaces (Ludwig–Sorets’s principle). This is in accordance withthe majority of the observed facts and is probably avera causa ofdifferentiation, though what its potency may be is uncertain. As arock solidifies the minerals which crystallize follow one another ina more or less well-defined order, the most basic (according toRosenbusch’s law) being first to separate out. That in a generalway the peripheral portions of a laccolite consist mainly of thoseearly basic minerals suggests that the sequence of crystallizationhelps largely in determining the succession (and consequently thedistribution of rock species in a plutonic complex). Gravity alsomay play a part, for it is proved that in a solution at rest the heaviestcomponents will be concentrated towards the base. This must,however, be of secondary importance as in laccolites the top portionsoften consist of more basic and heavier varieties of rock than thecentres. It has also been argued that the earliest minerals beingheaviest and in any case denser than the fused magma aroundthem, will tend to sink by their own weight and to be congregatednear the bottom of the mass. Electric currents, magnetic attractionand convection currents have also been called in to account for thephenomena observed. Magmas have also been compared to liquidswhich, when they cool, split up into portions no longer completelysoluble in one another (liquation hypothesis). Each of these partialmagmas may dissolve a portion of the others and as the temperaturefalls and the conditions change a range of liquids differing incomposition may be supposed to arise.

All igneous magmas contain dissolved gases (steam, carbonicacid, sulphuretted hydrogen, chlorine, fluorine, boric acid, &c.).Of these water is the principal, and was formerly believed to havepercolated downwards from the earth’s surface to the heated rocksbelow, but is now generally admitted to be an integral part of themagma. Many peculiarities of the structure of the plutonic rocksas contrasted with the lavas may reasonably be accounted for bythe operation of these gases, which were unable to escape as thedeep-seated masses slowly cooled, while they were promptly givenup by the superficial effusions. The acid plutonic or intrusive rockshave never been reproduced by laboratory experiments, and theonly successful attempts to obtain their minerals artificially havebeen those in which special provision was made for the retentionof the “mineralizing” gases in the crucibles or sealed tubes employed.These gases often do not enter into the composition of the rock-formingminerals, for most of these are free from water, carbonicacid, &c. Hence as crystallization goes on the residual liquormust contain an ever-increasing proportion of volatile constituents.It is conceivable that in the final stages the still uncrystallizedpart of the magma has more resemblance to a solution of mineralmatter in superheated steam than to a dry igneous fusion. Quartz,for example, is the last mineral to form in a granite. It bearsmuch of the stamp of the quartz which we know has been depositedfrom aqueous solution in veins, &c. It is at the same time the mostinfusible of all the common minerals of rocks. Its late formationshows that in this case it arose at comparatively low temperaturesand points clearly to the special importance of the gases of themagma as determining the sequence of crystallization.

When solidification is nearly complete the gases can no longerbe retained in the rock and make their escape through fissurestowards the surface. They are powerful agents in attacking theminerals of the rocks which they traverse, and instances of theiroperation are found in the kaolinization of granites, tourmalinizationand formation of greisen, deposit of quartz veins, stanniferousand auriferous veins, apatite veins, and the group of changesknown as propylitization.^[2] These “pneumatolytic” (Gr.πνεῦμα,spirit, vapour,λύειν, to loose, dissolve) processes are of the firstimportance in the genesis of many ore deposits. They are a realpart of the history of the magma itself and constitute the terminalphases of the volcanic sequence.

The complicated succession from basic (or ultrabasic) to acid types exemplified in the history of many magmas is reflected with ​astonishing completeness in the history of individual products.In each class of rock crystallization follows a definite course. Thefirst minerals to separate belong to a group knownas the minor accessories; this includes zircon, apatite,Sequence of Crystalliza­tion.sphene, iron oxides; then follow in order olivine, augite,hornblende, biotite, plagioclase, felspar (beginning withthe varieties most rich in lime and ending with those which containmost soda), orthoclase, microcline and quartz (with micropegmatite).Many exceptions to this rule are known; the same mineral maycrystallize at two different periods; two or more minerals maycrystallize simultaneously or the stages in which they form mayoverlap. But the succession above given holds in the vast majorityof cases. Expressed in this way: the more basic minerals precedethe less basic; it is known as Rosenbusch’s law.

Types of Structure.—In some rocks there seems to be little tendencyfor the minerals to envelop one another. This is true of manygabbros, aplites and granites (Pl. III, fig. 7). The grains then lieside by side, with the faces of the latter moulded on or adapted tothe more perfect crystalline outlines of the earlier. More commonlysome closer relationship exists between them. When the smalleridiomorphic crystals of the first-formed are scattered irregularlythrough the larger and less perfect crystals of later origin, thestructure is said to be poikilitic (Gr.ποικίλος, many-coloured,mottled).Poikilitic. A variety of this, known as ophitic(Pl. III, fig. 6), is very characteristic of many dolerites and diabases,in which large plates of augite enclose many small laths of plagioclasefelspar. Biotite and hornblende frequently enclose felsparophitically; less commonly iron oxides and sphene do so. In peridotitesthe “lustre-mottled” structure arises from pyroxene orhornblende enveloping olivine in the same manner (Pl. III, fig. 8).In these cases no crystallographic relation exists between the twominerals (enclosing and enclosed).

But often the surrounding mineral has been laid down on thesurface of the other in such a way that they have certain crystallinefaces or axes parallel to one another. This is knownas parallel growth. It is best seen in zoned crystalsof plagioclase felspar, which may range in compositionfrom anorthite to oligoclase, the more acid layers being depositedParallel Growths.regularly on the surfaces of the more basic. Biotite and muscovite,hornblende and augite, enstatite and diallage, epidote and orthite,very frequently are associated in this way.

When two minerals crystallize simultaneously they may beintergrown in “graphic” fashion. The best example is quartzand orthoclase occurring together as micropegmatite(Pl. II, figs. 6 and 8). The quartz forms angulargrowths patches in the felspar, which though separated havethe same crystalline orientation and one position ofGraphic intergrowths.extinction, while the felspar on its part behaves in the same wayTwo porous crystals thus interpenetrate but the scattered parts ofeach mineral maintain their connexion with the others. Theremay be also a definite relation between the crystalline axes of thetwo crystals, though this is not known in all cases. Augite alsooccurs in graphic intergrowth with hornblende, olivine and felspar;and hornblende, cordierite, epidote and biotite in graphic intergrowthwith quartz.

Physical Chemistry of Igneous Rocks.—The great advances thathave been made in recent years in our knowledge of physicalchemistry have very important bearings on petrological investigations.Especially in the study of the genesis of igneous rocks weanticipate that by this means much light will be thrown on problemswhich are now very obscure and a complete revolution in our ideasof the conditions which affect crystallization may yet be the consequence.Already many important results have been gleaned.As yet little work of an exact and quantitative nature has beendone on actual rocks or on mixtures resembling them in composition,but at the Carnegie Institution in Washington, an elaborate seriesof experiments in the synthesis of minerals and the properties ofmixtures of these is being carried on, with all the refinementswhich modern science can suggest. The work of Doelter and ofVogt may also be mentioned in this connexion. At the same timethe mathematical theory of the physical processes involved hasreceived much attention, and serves both to direct and to elucidatethe experimental work.

A fused mixture of two minerals may be regarded as a solutionof one on the other. If such a solution be cooled down, crystallizationwill generally set in and if the two components beindependent (or do not form mixed crystals) one ofthem may be expected to start crystallizing. On furthercooling more of this mineral will separate out till at last a residueMagmas as Solutions.is left which contains the two components in definite proportions.This mixture, which is known as the eutectic mixture, has the lowestmelting-point of any which can be formed from these mineralsIf heat be still abstracted the eutectic will consolidate as a whole;its two mineral components will crystallize simultaneously Atany given pressure the composition of the eutectic mixture in sucha case is always the same.

Similarly, if there be three independent components (none ofwhich forms mixed crystals with the others), according to theirrelative amounts and to the composition of the eutectic mixtureone will begin to crystallize; then another will make its appearancein solid form, and when the excess of these has been removed, theternary eutectic (that mixture of the three which has the lowestmelting-point) will be produced and crystallization of all threecomponents will go on simultaneously

These processes have without doubt a very close analogy tothe formation of igneous rocks Thus in certain felsites or porphyrieswhich may be considered as being essentially mixtures ofquartz and felspar, a certain amount of quartz has crystallized outat an early period in the form of well-shaped porphyritic crystals,and thereafter the remainder of the rock has solidified as a veryfine-grained, cryptocrystalline or sometimes micrographic ground-masswhich consists of quartz and felspar in intimate intermixture.The latter closely resembles a eutectic, and chemical studies haveproved that within somewhat narrow limits the composition ofthese felsitic ground-masses is constant.

But the comparison must not be pushed too far, as there arealways other components than quartz and felspar (apatite, zircon,biotite and iron oxides being the most common), and in rocks ofthis type the gases dissolved in the magma play a very importantpart. As crystallization goes on, these gases are set free and theirpressure must increase to some extent. Moreover, the felspar isnot one mineral but two or perhaps three, there being always sodafelspar and potash felspar and usually also a small amount of limefelspar in these porphyries.

In a typical basic rock the conditions are even more complex.A dolerite, for example, usually contains, as its last products ofcrystallization, pyroxene and felspar. Of these the latter consistsof three distinct species, the former of an unknown number; and ineach case they can form mixed crystals, to a greater or less extentwith one another. From these considerations it will be clear that theproperties of solutions of two or three independent components, donot necessarily explain the process of crystallization in any igneousrock.

Very frequently in porphyries not only quartz but felspar alsois present in large well-formed early crystals. Similarly in basalts,augite and felspar may appear both as phenocrysts and as componentsof the ground-mass. As an explanation of this it has beensuggested that supersaturation has taken place. We may supposethat the augite which was in excess of the proportion necessary toform the felspar-augite, eutectic mixture, first separated out. Whenthe remaining solution reached the eutectic composition the felspardid not at once start crystallizing, perhaps because nuclei arenecessary to initiate crystal-growth and these were not at hand;augite went on crystallizing while felspar lagged behind. Thenfelspar began and as the mixture was now supersaturated with thatmineral a considerable amount of it was rapidly thrown out of thesolution. At the same time there would be a tendency for part ofthe augite, already crystallized, to be dissolved and its crystalswould be corroded, losing their sharp and perfect edges, as is oftenobserved in rocks of this group. When the necessary adjustmentshad been made the eutectic mixture would be established andthereafter the two minerals would consolidate simultaneously (ornearly so) till crystallization was complete.

There is a good deal of evidence to show that supersaturationis not unimportant in igneous magmas. The frequency with whichthey form glasses proves that under certain conditions the moltenrocks are highly viscous. Much will depend also on the presence,accidental or otherwise, of nuclei on which a mineral substancecan be deposited. It is known that minerals differ in their tendencyto crystallize, some doing so very readily while others are slow andbackward. The rate at which crystallization goes on depends onmany factors, and there are remarkable differences in this respectbetween minerals.

On the other hand, there is plenty of evidence to show thatsupersaturation, though probably one of the causes, is not the principalcause of the appearance of more than one mineral in twogenerations of crystals In some of the quartz-porphyries, forexample, there are phenocrysts not only of quartz and felspar butalso of micropegmatite. These prove that quartz and felspar werenot crystallizing successively or alternately but simultaneously.

The great majority of the minerals found in igneous rocks are notof simple composition, but are mixtures of various elementaryminerals in very different proportions This enormously complicatesthe theoretical problems of consolidation. It has been found,for example, that in the case of three minerals—one of which isindependent, while the two others can form mixed crystals—thereis a large number of possible sequences; and, what is very important,one mineral may separate out entirely at an early stage, or itscrystallization may be interrupted and not continuous. Theternary eutectic, which is produced by a mixture of three independentminerals, may not in such a case be the last substance to crystallize,and may not be present at all. This is very much in accordancewith the observed facts of petrology; for usually in a rock there isone mineral which indubitably was the last of all to finish crystallizingand contained no appreciable quantity of the others.

As yet we know little about such important questions as the composition of the eutectic mixtures of rock-minerals, their latent heat of fusion, specific heats, mutual solubilities, inversion temperatures, &c. Until we are in possession of a large body of accurate information on such points as these the theoretical treatment of ​the processes involved in the formation of igneous rocks cannot besuccessfully handled. But every day sees an increase in the amountof data available, and encourages us to believe that sooner or latersome of the simpler igneous rocks at any rate will be completelyexplicable on physico-chemical principles.

Rock masses of igneous origin have no sooner consolidated thanthey begin to change. The gases with which the magma is chargedare slowly dissipated, lava-flows often remain hot andsteaming for many years. These gases attack the componentsof the rock and deposit new minerals in cavitiesand fissures. The beautiful zeolites, so well known toPost-volcanic Changes.collectors of minerals, are largely of this origin. Even beforethese “post-volcanic” processes have ceased atmospheric decompositionbegins. Rain, frost, carbonic acid, oxygen and otheragents operate continuously, and do not cease till the whole masshas crumbled down and most of its ingredients have been resolvedinto new products. In the classification of rocks these secondarychanges are (generally considered unessential; rocks are classifiedand described as if they were ideally fresh, though this is rarelythe case in nature.

Epigenitic change (secondary processes) may be arranged undera number of headings, each of which is typical of a group of rocksor rock-forming minerals, though usually more thanone of these alterations will be found in progress in thesame rock. Silicification, the replacement of the mineralsby crystalline or crypto-crystalline silica, is most common in acidrocks, such as rhyolite, but is also found in serpentine, &c. KaolinizationSecondary Changes.is the decomposition of the felspars, which are the commonestminerals of igneous rocks, into kaolin (along with quartz, muscovite,&c.); it is best shown by granites and syenites. Serpentinizationis the alteration of olivine to serpentine (with magnetite); it istypical of peridotites, but occurs in most of the basic rocks. Inuralitization secondary hornblende replaces augite; this occursvery generally in diabases; chloritization is the alteration of augite(biotite or hornblende) to chlorite, and is seen in many diabases,diorites and greenstones. Epidotization occurs also in rocks ofthis group, and consists in the development of epidote from biotite,hornblende, augite or plagioclase felspar.

When the rocks exposed at the earth's surface give way beforethe attack of the agencies of denudation, they crumble down andare resolved into two parts. One of these consists of solid material(sand, clay and angular débris) insoluble in carbonated waters;the other part is dissolved and washed away. The undissolvedresidues, when they finally come to rest, formclastic sedimentaryrocks (sandstone, conglomerate, shale, &c.). The dissolved portionsare partly transferred to the sea, where they help to increaseits store of salts, and may again be precipitated ascrystallinesedimentary rocks; but they are also made use of by plants andby animals to form their skeletal and vital tissues From thislatter portion the rocks of organic origin are built up. Thesemay also contain certain ingredients derived from the atmosphere(nitrogen, carbon in coals, &c.)

We have thus three types of sediments of distinct origin, whichmay be named the clastic (or fragmental), the crystalline and theorganic.

The clastic materials may accumulatein situ, and then differchiefly in their disintegrated and weathered state from the parentrock masses on which they rest The best example ofthese are the soils, but in elevated regions angular brokenrock often covers large areas. More usually they are transportedby wind or water, and become sorted out according to their sizeClastic.and density. The coarsest débris comes first to rest and is leastworn and Weathered; it includes screes, gravels. coarse sands, &c.,and consolidates as conglomerates, breccias and pebbly grits. Thebedding of these rocks is rudimentary and imperfect, and as eachbed is traced along its outcrop it frequently changes its characterwith the strata on which it rests. The most finely divided sedimenttravels farthest, and is laid down in thin uniform sheets of wideextent. It is known as mud and clay; around the shores of ourcontinents, at distances of a hundred miles and more from land, greatsheets of mud are spread over the ocean floors. This mud containsminute particles of quartz and of felspar, but Consists essentiallyof finely divided scaly minerals, which by their small size and flatshape tend to remain suspended in water for a very long timeChlorite, white micas and kaolin are the best examples of this classof substances. Wind action is even more effective than water inseparating and removing these fine particles. They to a very largeextent escape mechanical attrition, because they are transported insuspension and are not swept along the ground or the bottom of thesea; hence they are mostly angular. Fragments of intermediatemagnitudes (from⁠1/100⁠ of an inch to⁠1/8⁠ of an inch are classed assands. They consist largely of quartz, because it does not weatherinto scaly minerals like felspar, and having but a poor cleavagedoes not split up into flakes like mica or chlorite. These quartzgrains have been rolled along and are usually rounded and worn(Pl. IV., fig. 1). More or less of garnet felspar, tourmaline, zircon,rutile, &c., are mixed with the quartz, because these are hardminerals not readily decomposed.

The mechanical sorting by the transporting agencies is usuallysomewhat incomplete, and mixed types of sediment result, such asgravels containing sand, or clays with coarser arenaceous particlesMoreover, successive layers of deposit may not always be entirelysimilar, and alternations of varying composition may follow oneanother in thin laminae:e.g. laminae of arenaceous material in bedsof clay and shale. Organic matter is frequently mingled with thefiner-grained sediments.

These three types have been named the psephitic (or pebbly;Gr.ψῆφος, pebble); psammitic (or sandy, Gr.ψάμμος, sand), andpelitic (or muddy: Gr.πηλός, mud).

Two groups of clastic sediments deserve special treatment.The pyroclastic (Gr.πῦρ, fire, andκλαστός, broken) rocks of volcanicorigin, consist mostly of broken pieces of lava (bombs, ash, &c.)(Pl. IV. fig. 2), and only accidentally contain other rocks or fossils.They are stratified, and may be coarse or fine, but are usually muchless perfectly sorted out, according to their fineness, than ordinaryaqueous or aeolian deposits. The glacial clays (boulder clays),representing the ground moraines of ancient glaciers and ice sheets,are characterized by the very variable size of their ingredients andthe striated, blunted sub-angular form of the larger rock fragments.In them stratification is exceptional and fossils are veryrare.

The crystalline sedimentary rocks have been deposited from solutionin water. The commonest types, such as rock-salt, gypsum,anhydrite, carnallite, are known to have arisen by theevaporation of enclosed saline lakes exposed to a dryatmosphere. They occur usually in beds with layers of red clay andmarl; some limestones have been formed by calcareous watersCrystalline.containing carbonate of lime dissolved in an excess of carbonicacid; with the escape of the volatile gas the mineral matter is precipitated(sinters,Sprudelstein, &c.). Heated waters on coolingmay yield up part of their dissolved mineral substances; thus siliceoussinters are produced around geysers and hot springs in manyparts of the world. There seems no reason to separate from thesethe veinstones which fill the fissures by which these waters rise tothe surface They differ from those above enumerated in beingmore perfectly crystallized and in having no definite stratification,but only a banding parallel to the more or less vertical walls of thefissure. Another subdivision of this class of rocks is due to recrystallizationor crystalline replacement of pre-existing sediments. Thuslimestones are dolomitized or converted into ironstones, flints andcherts, by percolating waters which remove the lime salts andsubstitute for them compounds of iron, magnesia, silicon, and so on.This may be considered a kind of metamorphism; it is generallyknown asmetasomatism (q.v.).

The rocks of organic origin may be due to animals or plants. They are of great importance, as limestones and coals belong to this group. They are the most fossiliferous of all rocks; but elastic sediments are often rich in fossils though crystalline sediments rarely are. They may be subdivided, according to their dominant components,Organic.into calcareous, ​carbonaceous, siliceous, ferruginous, and so on. The calcareousorganic rocks may consist principally of foraminifera, crinoids,corals, brachiopoda, mollusca, polyzoa, &c. Most of them, however,contain a mixture of organisms. By crystallization and metasomaticchanges they often lose their organic structures; metamorphismof any kind has the same effect. The carbonaceous rocks areessentially plant deposits; they include peat, lignite and coal.The siliceous organic rocks include radiolarian and diatom oozes;in the older formations they occur as radiolarian cherts. Flintnodules owe their silica to disseminated fossils of this nature whichhave been dissolved and redeposited by concretionary action.Some kinds of siliceous sinter may be produced by organisms inhabitinghot silicated waters. Calcareous oolites in the same waymay have arisen through the agency of minute plants. Bog ironores also may be of organic rather than of merely chemical origin.The phosphatic rocks so extensively sought after as sources of fertilizingagents for use in agriculture are for the most part of organicorigin, since they owe their substance to the remains of certainvarieties of animals which secrete a phosphatic skeleton; but mostof them no longer show organic structures but have been convertedinto nodular or concretionary forms.

All sediments are at first in an incoherent condition (e.g. sands,clays and gravels, beds of shells, &c.), and in this state they mayremain for an indefinite period. Millions of years haveelapsed since some of the early Tertiary strata gatheredon the ocean floor, yet they are quite friable (e.g. theLondon Clay) and differ little from many recent accumulations.Cementation.There are few exceptions, however, to the rule that with increasingage sedimentary rocks become more and more indurated, andthe older they are the more likely it is that they will havethe firm consistency generally implied in the term “rock.” Thepressure of newer sediments on underlying masses is apparentlyone cause of this change, though not in itself a very powerfulone. More efficiency is generally ascribed to the action ofpercolating water, which takes up certain soluble materials andredeposits them in pores and cavities. This operation is probablyaccelerated by the increased pressure produced by superincumbentmasses, and to some extent also by the rise of temperature whichinevitably takes place in rocks buried to some depth beneaththe surface The rise of temperature, however, is never verygreat; we know more than one instance of sedimentary depositswhich have been buried beneath four or five miles of similar strata(e.g. parts of the Old Red Sandstone), yet no perceptible differencein condition can be made out between beds of similar compositionat the top of the series and near its base. The redepositedcementing material is most commonly calcareous or siliceous.Limestones, which were originally a loose accumulation of shells,corals, &c., become compacted into firm rock in this manner; andthe process often takes place with surprising ease, as for examplein the deeper parts of coral reefs, or even in wind-blown masses ofshelly sand exposed merely to the action of rain. The cementingsubstance may be regularly deposited in crystalline continuity onthe original grains, where these were crystalline; and even in sandstonessuch as Kentish Rag) a crystalline matrix of calcite oftenenvelopes the sand grains. The change of aragonite to calcite and ofcalcite to dolomite, by forming new crystalline masses in theinterior of the rock, usually also accelerates consolidation. Silicais less easily soluble in ordinary waters, but even this ingredientof rocks is dissolved and redeposited with great frequency. Manysandstones are held together by an infinitesimal amount of colloidor cryptocrystalline silica; when freshly dug from the quarry theyare soft and easily trimmed, but after exposure to the air for sometime they become much harder, as their siliceous cement sets andpasses into a rigid condition. Others contain fine scales of kaolinor of mica. Argillaceous materials may be compacted by merepressure, like graphite and other scaly minerals. Oxides andcarbonates of iron play a large part in many sedimentary rocks andare especially important as colouring matters. The red sands andColoration.limestones, for example, which are so abundant, containsmall amounts of ferric oxide (haematite), which in afinely divided state gives a red hue of all rocks in which it ispresent. Limonite, on the other hand, makes rocks yellow orbrown, oxides of manganese, asphalt and other carbonaceoussubstances are the cause of the black colour of many sediments.Bluish tints result sometimes from the presence of phosphates or offluorspar; while green is most frequently seen in rocks which containglauconite or chlorite.

When a rock is contact altered by an igneous intrusion it veryfrequently becomes harder, more crystalline and more lustrous,owing to the development of many small crystals in itsmass. Many altered rocks of this type were formerlycalled hornstones, and the termhornfelses (Ger.Hornfels) is often used by geologists to signify thoseThermo-metamor­phism.fine grained, compact, crystalline products of thermal metamorphism.A shale becomes a dark argillaceous hornfels, full of tinyplates of brownish biotite; a marl or impure limestone changes toa grey, yellow or greenish lime-silicate-hornfels, tough and splintery,with abundance of augite, garnet, wollastonite and other mineralsin which lime is an important component. A diabase or andesitebecomes a diabase hornfels or andesite hornfels with a largedevelopment of new hornblende and biotite and a partial recrystallizationof the original felspar. A chert or flint becomes a finelycrystalline quartz rock; sandstones lose their clastic structure andare converted into a mosaic of small close-fitting grains of quartz.

If the rock was originally banded or foliated (as, for example, alaminated sandstone or a foliated calc-schist) this character may notbe obliterated, and a banded hornfels is the product; fossils even mayhave their shapes preserved, though entirely recrystallized, and inmany contact altered lavas the steam cavities are still visible, thoughtheir contents have usually entered into new combinations to formminerals which were not originally present. The minute structures,however, disappear, often completely, if the thermal alteration is veryprofound; thus small grains of quartz in a shale are lost or blendwith the surrounding particles of clay, and the fine ground-mass oflavas is entirely reconstructed.

By recrystallization in this manner peculiar rocks of very distincttypes are often produced. Thus shales may pass into cordieriterocks, or may show large crystals of andalusite (and chiastolite,Pl. IV., fig. 9), staurolite, garnet, kyanite and sillimanite. A considerableamount of mica (both muscovite and biotite) is simultaneouslyformed, and the resulting product has a close resemblance to manykinds of schist. Limestones, if pure, are often turned into coarselycrystalline marbles (Pl. IV., fig. 4); but if there was an admixtureof clay or sand in the original rock such minerals as garnet, epidote,idocrase, wollastonite, will be present. Sandstones when greatlyheated may change into coarse quartzites composed of large cleargrains of quartz. These more intense stages of alteration are notso commonly seen in igneous rocks, possibly because their minerals,being formed at high temperatures, are not so easily transformed orrecrystallized.

In a few cases rocks are fused and in the dark glassy productminute crystals of spinel, sillimanite and cordierite may separateout. Shales are occasionally thus altered by basalt dikes, and felspathicsandstones may be completely vitrified. Similar changesmay be induced in shales by the burning of coal seams or even byan ordinary furnace.

There is also a tendency for interfusion of the igneous with thesedimentary rock. Granites may absorb fragments of shale orpieces of basalt. In that case hybrid rocks arise which have notthe characters of normal igneous or sedimentary rocks. Such effectsare scarce and are usually easily recognized. Sometimes an invadinggranite magma permeates the rocks around, filling their jointsand planes of bedding, &c., with threads of quartz and felsparThis is very exceptional, but instances of it are known and it maytake place on a large scale.

Rocks which were originally sedimentary and rocks which wereundoubtedly igneous are converted into schists and gneisses, and iforiginally of similar composition they may be very difficult to distinguishfrom one another if the metamorphism has been great.A quartz-porphyry, for example, and a fine felspathic sandstone,may both be converted into a grey or pink mica-schist. Usually,however, we may distinguish between sedimentary and igneousschists and gneisses. Often the metamorphism is progressive, andif the whole district occupied by these rocks be searched traces ofbedding, of clastic structure, unconformability or other evidencemay be obtained showing that we are dealing with a group of alteredsediments. In other cases intrusive junctions, chilled edges, contactalteration or porphyritic structure may prove that in its originalcondition a metamorphic gneiss was an igneous rock. The lastappeal is often to the chemist, for there are certain rock types whichoccur only as sediments, while others are found only among igneousmasses, and, however advanced the metamorphism may be, it rarelymodifies the chemical composition of the mass very greatly. Suchrocks, for example, as limestones, calc-schists, dolomites, quartzitesand aluminous shales have very definite chemical characters whichdistinguish them even when completely recrystallized.

The schists and gneisses are classified according to the mineralsthey consist of, and this depends principally on their chemicalcomposition. We have, for example, a group of metamorphiclimestones, marbles, calc-schists and cipolins, with crystallinedolomites; many of these contain silicates such as mica, tremolite,diopside, scapolite, quartz and felspar. They are derived fromcalcareous sediments of different degrees of purity. Another groupis rich in quartz (quartzites, quartz schists and quartzose gneisses),with variable amounts of white and black mica, garnet, felspar,zoisite and hornblende. These were once sandstones and arenaceousrocks. The graphitic schists may readily be believed to representsediments once containing coaly matter or plant remains; thereare also schistose ironstones (haematite-schists), but metamorphicbeds of salt or gypsum are exceedingly uncommon. Among schistsof igneous origin we may mention the silky calc-schists, the foliatedserpentines (once ultrabasic masses rich in olivine), and the whitemica-schists, porphyroids and banded halleflintas, which have beenderived from rhyolites, quartz-porphyries and acid tuffs. Themajority of mica-schists, however, are altered clays and shales, andpass into the normal sedimentary rocks through various types ofphyllite and mica-slates. They are among the most common metamorphicrocks; some of them are graphitic and others calcareous.The diversity in appearance and composition is very great, but theyform a well-defined group not difficult to recognize, from the abundanceof black and white micas and their thin, foliated, schistosecharacter. As a special subgroup we have the andalusite-, staurolite-kvanite- and sillimanite-schists, together with the cordierite-gneisses,which usually make their appearance in the vicinity ofgneissose granites, and have presumably been affected by contactalteration. The more coarsely foliated gneisses are almost asfrequent as the mica-schists, and present a great variety of typesdiffering in composition and in appearance. They contain quartz,one or more varieties of felspar, and usually mica, hornblende oraugite, often garnet, iron oxides, &c. Hence in composition theyresemble granite, differing principally in their foliated structure.Many of them have “augen” or large elliptical crystals, mostlyfelspar but sometimes quartz, which are the crushed remains ofporphyritic minerals; the foliation of the matrix winds around theseaugen, closing in on each side. Most of these augen gneisses aremetamorphic granites, but sometimes a conglomerate bed simulatesa gneiss of this kind rather closely. There are other gneisses, whichwere derived from felspathic sandstones, grits, arkoses and sedimentsof that order; they mostly contain biotite and muscovite,but the hornblende and pyroxene gneisses are usually igneous rocksallied in composition to the hornblende-granites and quartz-diorites.The metamorphic forms of dolerite, basalt and the basic igneousrocks generally have a distinctive facies as their pyroxene and olivineare replaced by dark green hornblende, with often epidote, garnetand biotite. These rocks have a well developed foliation, as theprismatic hornblendes lie side by side in parallel arrangement. Themajority of amphibolites, hornblende-schists, foliated epidioritesand green schists belong to this group. Where they are leastaltered they pass through chloritic schists into sheared diabases,flaser gabbros and other rocks in which remains of the originaligneous minerals and structures occur in greater or less profusion.

Bibliography.—Most text-books of geology treat of petrology in more or less detail (seeGeology: § Bibliography). Elementary books on petrology include F. H. Hatch,Petrology (5th ed., London, 1909); L. V. Pirsson,Rocks and Rock-minerals (New York, 1908); J. D. Dana,Handbook of Mineralogy and Petrography (12th ed., New York, 1908); A. Harker,Petrology for Students (4th ed., Cambridge, 1908); G. A. J. Cole,Aids to Practical Geology (6th ed., London, 1909). For rock minerals consult J. P. Iddings,Rock Minerals (New York, 1906); A. Johannsen,Determination of Rock-forming Minerals (New York, 1908); E. Hussak and E. G. Smith, Determination of Rock-forming Minerals (2nd ed., New York, 1893); N. H. and A. N. Winchell,Optical Mineralogy (New York, 1909). On the classification and origin of rocks see A. Harker,Natural History of Igneous Rocks (London, 1909); J. P. Iddings,Igneous Rocks, (New York, 1909); Cross, Iddings, Washington and Pirsson,Quantitative Classification of Igneous Rocks (Chicago, 1902); C. Van Hise, Metamorphism (Washington, 1904); A. P. Merrill, Rocks,Rock-weathering and Soils (London, 1897); C. Doelter,Petrogenesis (Brunswick, 1906); J. H. L. Vogt,Silikatschmelzlösungen (Christiania, 1903); F. Fouqué and A. Michel Lévy,Synthèse des minéraux et des roches (Paris, 1882). The principal authorities on the analysis and chemical composition of rocks are J. Roth,Beiträge zur Petrographie (Berlin, 1873–1884); A. Osann,Beiträge zur chemischen Petrographie (Stuttgart, 1903);H. S. Washington,Manual of the Chemical Analysis of Rocks (New York, 1904) andChemical Analyses of Igneous Rocks (Washington, 1904); F. W. Clarke, Analyses of Rocks (Washington, 1904); Max Dittrich,Anleitung zur Gesteinsanalyse (Leipzig, 1905); W. F. Hillebrand,Analysis of Silicate and Carbonate Rocks (Washington, 1907).

The great systematic treatises on Petrology are F. Zirkel,Lehrbuch der Petrographie (2nd ed., Leipzig, 1894, 3 vols. ); H. Rosenbusch,Mikroskopische Physiographie (4th ed., Stuttgart, 1909, 2 vols.)

Useful German handbooks include E. Weinschenk,Polarisationsmikroskop,Gesteinsbildende Mineralien andGesteinskunde (2nd ed., Freiburg, 1907, &c); R. Reinisch,Petrographisches Praktikum (2nd ed., Berlin, 1907); H. Rosenbusch,Elemente der Gesteinslehre (3rded., Stuttgart, 1909); A. Grubenmann,Die krystallinen Schiefer (Berlin, 1907); F. Loewisson Lessing,Petrographisches Lexikon(1893 and 1898, also a Fr. ed., 1901); F. Rinne,PraktischeGesteinskunde(2nd ed., Hanover, 1905).

The principal French works are E. Jannettaz,Les Roches (3rd ed., Paris, 1900); F. Fouqué and A. Michel Lévy,Minéralogie micrographique (Paris, 1879); A. Michel Lévy and A. Lacroix,Les Mineraux des roches (Paris, 1888); A. Lacroix,Minéralogie de la France (I., II., Paris, 1893); andLes Enclaves des roches éruptives (Macon, 1893).

British petrography is the subject of a special work by J. J. H.Teall (London, 1888). Much information about rocks is containedin the memoirs of the various geological surveys, and inQuart. Journ.of the Geol. Soc. of London,Mineralogical Magazine,GeologicalMagazine, Tschermak’sMineralogische Mittheilungen (Vienna),Neues Jahrbuch für Mineralogie (Stuttgart),Journal of Geology(Chicago), &c. (J. S. F.) 

1. ↑Idiomorphic, having its own characteristic form, Gr.ἴδιος,belonging to one’s self, (αὐτός),μορφή, (form), allotriomorphic, fromGr.ἀλλότριος, belonging to another (ἄλλος), a stranger (ξένος).
2. ↑The term “propylite” (Gr.πρόπυλον, a gateway) was givenby Richthofen to a volcanic rock which is supposed to have markeda new epoch in volcanic geology (seeAndesite).