| Sedimentary rock | |
 A piece of black argillite fromHaida Gwaii, Canada | |
| Composition | |
|---|---|
| induratedclay particles |
Argillite (/ˈɑːrdʒɪlaɪt/) is a fine-grainedsedimentary rock composed predominantly ofinduratedclay particles. Argillaceous rocks are basicallylithified muds andoozes. They contain variable amounts ofsilt-sized particles. The argillites grade intoshale when thefissile layering typical of shale is developed. Another name for poorly lithified argillites ismudstone.[citation needed] These rocks, although variable in composition, are typically high inaluminium andsilica with variablealkali andalkaline earthcations. The termpelitic orpelite is often applied to thesesediments and rocks.Metamorphism of argillites producesslate,phyllite, and peliticschist.
TheBelt Supergroup, an assemblage of rocks of latePrecambrian (Mesoproterozoic) age, includes thick sequences of argillite, as well as other metamorphosed or semi-metamorphosed mudstones.[1] It is exposed primarily in westernMontana, including theBitterroot Valley andBitterroot Mountains, theMissoula area,Flathead Lake, andGlacier National Park, and in northernIdaho. There are also minor occurrences in northeasternWashington and westernWyoming.[2] Excellent outcrops of deep purple, wine red, red, blue, turquoise, and green argillites of the Belt Supergroup can be seen in Glacier National Park in northwestern Montana and in Wolf Creek Canyon alongInterstate 15 in west-central Montana.[3]
TheHaida carvings ofHaida Gwaii along the coast ofBritish Columbia are notable aboriginal art treasures created from a type of a hard, fine black silt argillite, sometimes called "blackslate". The black slate occurs only at a quarry on aSlatechuck Mountain in the upper basin ofSlatechuck Creek, near the town ofSkidegate onGraham Island. At one time, around 1900, it was shipped to Victoria for manufacturing; today theHaida have a monopoly on use of the argillite. Argillite carvings are synonymous withHaida artwork and are one of the few art forms on theNorthwest Coast that is the exclusive right of one cultural group. This artwork has been of high quality and prized around the world since the Haida first began carving it to trade and sell to sailors around 1800. Contemporary Haida carvers continue the tradition.
