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Along the sides of fissures, through which, no doubt, the gasesascended, the granite is converted into schorl rock for a distanceranging from a fraction of an inch to several feet, and vein-likemasses of grey schorl rock branching and uniting are thus produced.In other places considerable areas of granite are changed in thisway, principally near the margin of the granite, and an interruptedbelt of this kind of rock encircles some of the larger outcrops ofgranite in Cornwall. A similar origin must be ascribed togreisen(q.v.), the aggregate of quartz and white mica commonly found inassociation with tin-bearing granites; there are complete gradationsbetween schorl rock and greisen, according to the varyingproportions of white mica and tourmaline which may be present in eachspecimen. Another mineral which is produced by the pneumatolyticalteration of granite is topaz (a silicate and fluoride ofaluminium); an aggregate of quartz and topaz is called topaz-fels ortopaz rock, and is largely developed in some of the tin-miningdistricts of Germany, though not found in Cornwall.

As might be expected every stage of the conversion of graniteinto schorl rock can be found. Tourmaline may have been to someextent an original constituent of the granite, but most of it is ofnew formation and must have resulted from the alteration of thebiotite and the felspar of the original rock, both of these mineralshaving disappeared when the metamorphosis was complete. It iscommonly found that the schorl is of a brown colour in the interiorof the crystals but blue at the edges; probably the brown is primaryor has been derived from biotite, but the blue principally from thereplacement of felspar. The rock known as luxullianite, obtainednear Luxullian village in Cornwall and used as an ornamental stonefor the sarcophagus of the duke of Wellington's monument inSt Paul's Cathedral, is a tourmaline granite in which the replacementof biotite and felspar by quartz and tourmaline can be seen inprogress. The new tourmaline is in fine pointed needles which havea stellate or divergent arrangement, and is embedded in quartz:often these needles are planted on the surface of corroded crystalsof primary brown schorl. This rock still contains a good deal offlesh-coloured felspar in large porphyritic crystals which contrastwell with the dark matrix and give polished specimens a veryhandsome appearance. In the completely altered schorl rocksthere are rarely needles of tourmaline, but this mineral occurs asirregular grains mingled in varying proportions with small crystalsof quartz. In nearly all cases the structure of the granite hasvanished, but at Trevalgan, St Austell, and other places in Cornwallthere are schorl rocks which contain white pseudomorphs of quartzafter porphyritic crystals of orthoclase.

In porphyries of “elvans” tourmalinization also is frequent,though not so common as greisening. Veins of quartz with stellateschorl needles may be seen spreading through the groundmass orwhen this has been previously converted into an aggregate of quartzand fine scaly white mica, the porphyritic crystals of felspar alonemay be replaced by bunches of tourmaline embedded in quartz.Tinstone often makes its appearance in these rocks either in smallcrystals enclosed in quartz or lining fissures and cavities left by theremoval of a portion of the rock in solution.

The same process goes on also in sedimentary rocks; a felspathicsandstone may yield a schorl rock which can hardly be distinguishedfrom one derived from a fine-grained granite. In shales browntourmaline is often deposited in the vicinity of fissures, and the wholemass may be converted into a hard splintery aggregate of quartzand schorl (often containing also rutile and tinstone). But theserocks are always banded, like the original slate; their originalstructures (bedding and cleavage) are probably never completelyeffaced and the ultimate product has been called schorl-schist(tourmaline hornfels, cornubianite).

The stanniferous veins which in large numbers intersect thegranites of Devon and Cornwall and the slates around them, andhave yielded a large part of the world's supply of tin consist mostlyof quartz, tourmaline and chlorite (with varying proportions ofcassiterite). The veinstones are typically very fine grained, hardand dark blue or dark green in colour. The green varieties containmuch chlorite, the blue are richer in tourmaline, and both kindsare known to the miners as “peach.” Essentially aqueous depositsin lines of fissure, these rocks show that quartz and tourmaline werecarried up in hot solutions at a late period in the cooling of thegranite, and the changes above described are due to the operationof these solutions as they spread outwards through the surroundingrocks. Their tourmaline crystals are very small and usually ofdark-blue shades, but owing to repeated movements of the wallsof the veins the ore deposits have sometimes an intricate history, asmicroscopic studies show that the first infillings of the fissures havebeen broken up and cemented together again by a later material ofslightly different character.