This section is from the book "An Introduction To Geology", by William B. Scott. Also available from Amazon: An Introduction to Geology.
The foliated or schistose rocks are those which are divided into rudely parallel planes, with rough or undulating surfaces, due to the flakes and spangles of some mineral. The planes of foliation may coincide with the original bedding-planes or they may intersect the latter at any angle, just as do the planes of cleavage and fissility. The foliated rocks represent the most advanced stage of what we can confidently call metamorphism, and may be derived from either sedimentary or igneous originals; it is not always possible to say which.
Gneiss is a term of wide significance, which includes a number of rocks of different modes of origin and different mineralogical composition. It is " a laminated metamorphic rock that usually corresponds in mineralogy to some one of the plutonic types." (Kemp.) The varieties of gneiss are ordinarily named in accordance with the most conspicuous dark silicate present, as biotite gneiss', hornblende gneiss, etc.; but this system of nomenclature gives an imperfect notion of the character of the rock. A better method has been suggested by Dr. C. H. Gordon and adopted by Professor Kemp, though the older scheme is still in general use. This is to name the varieties in accordance with the igneous rocks to which they correspond in mineralogical composition; as granitic gneiss, syenitic gneiss, dioritic gneiss, etc. The commonest variety is granitic gneiss, with mica or hornblende; the orthoclase and quartz are mingled together, with conspicuous laminae and folia of the dark mineral.
Fig. 225. - Plicated gneiss, Montgomery Cotmty, Pa. (U. S. G. S.) This figure clearly displays the characteristic foliation.
Most gneisses were generated by the dynamic metamorphism of granite, either before its consolidation or after it had cooled and hardened. Some authorities deny that gneiss has ever been formed from sedimentary rocks, but there is good reason to believe that it sometimes has such an origin, and in certain instances the crushed pebbles of the parent conglomerate are still distinctly visible, especially on a weathered surface. Still another series of these rocks are of complex origin, granitic magmas being injected along the foliation planes and into all the crevices of metamorphosed sediments.
Gneisses are widely spread in ancient formations, especially in the most ancient of all, and they cover vast areas in the northern part of North America.
The Crystalline Schists are more finely foliated than gneiss, into which they often grade imperceptibly, having very similar mineralogical composition. They have very diverse modes of origin arising from both sedimentary and igneous rocks. Slates, impure sandstones and limestones, as well as felsites, andesites, diabases, tuffs, etc., may all give rise to crystalline schists by contact or dynamic metamorphism. The varieties are named from their most important ferro-magnesian mineral.
Fig. 226. - Boulder of gneiss, displaying its conglomeratic nature on weathered surface. (International Boundary Survey).
Quartz Schist is a foliated quartzite in which cleavage or fissility has developed into schistosity. The mashing and cementation of the original sandstone may take place at the same time, or the • quartzite may be produced by the latter process and subsequently converted into schist by compression.
Mica Schist is principally composed of quartz, muscovite, and biotite, with more or less felspar. By an increase in the quantity of felspar present, and a coarser foliation, it grades into gneiss, and by an increase of quartz it may pass into quartzite and thence to sandstone. Through the phyllites mica schists are connected with the slates, and in another direction, by increase of lime they pass into argillaceous limestones. Mica schists are very largely exposed in New England and southward along the eastern flank of the Appalachian Mountain system.
Fig. 227. - Mica schist with garnets. Nearly natural size.
Hornblende Schist is a foliated rock, consisting of hornblende with a varying proportion of felspar and less quartz. The hornblende schists are, for the most part, derived from the dynamic metamorphism of various basic igneous rocks, the augite being readily converted into hornblende by crushing, but in rare instances they are believed to have bad a sedimentary origin. The hornblende schists occur as belts or bosses in metamorphic areas and are largely exposed around Lake Superior. The schists already described are much the most abundant members of the group, but there are several others. Thus, we have talc and chlorite schists, both of which are due to alteration, chiefly of hornblende schist, and graphite schist, which has quantities of that carbon mineral along its foliation planes.