_Mica schist_, the most common of schists, and in fact of all metamorphic rocks, is composed of mica and quartz in alternating wavy folia. All gradations between it and phyllite may be traced, and in many cases we may prove it due to the metamorphism of slates and shales. It is widespread in New England and along the eastern side of the Appalachians. _Talc schist_ consists of quartz and _talc_, a light-colored magnesian mineral of greasy feel, and so soft that it can be scratched with the thumb nail. _Hornblende schist_, resulting in many cases from the foliation of basic igneous rocks, is made of folia of hornblende alternating with bands of quartz and feldspar. Hornblende schist is common over large areas in the Lake Superior region. _Quartz schist_ is produced from quartzite by the development of fine folia of mica along planes of shear. All gradations may be found between it and unfoliated quartzite on the one hand and mica schist on the other. Under the resistless pressure of crustal movements almost any rocks, sandstones, shales, lavas of all kinds, granites, diorites, and gabbros may be metamorphosed into schists by crushing and shearing. Limestones, however, are metamorphosed by pressure into _marble_, the grains of carbonate of lime recrystallizing freely to interlocking crystals of calcite. These few examples must suffice of the great class of metamorphic rocks. As we have seen, they owe their origin to the alteration of both of the other classes of rocks--the sedimentary and the igneous--by heat and pressure, assisted usually by the presence of water. The fact of change is seen in their hardness arid cementation, their more or less complete recrystallization, and their foliation; but the change is often so complete that no trace of their original structure and mineral composition remains to tell whether the rocks from which they were derived were sedimentary or igneous, or to what variety of either of these classes they belonged. [Illustration: Fig. 256. Contorted Gneiss, the Ottawa River, Canada] [Illustration: Fig. 257. Quartz Veins in Slate] In many cases, however, the early history of a metamorphic rock can be deciphered. Fossils not wholly obliterated may prove it originally water-laid. Schists may contain rolled-out pebbles, showing their derivation from a conglomerate. Dikes of igneous rocks may be followed into a region where they have been foliated by pressure. The most thoroughly metamorphosed rocks may sometimes be traced out into unaltered sedimentary or igneous rocks, or among them may be found patches of little change where their history maybe read. Metamorphism is most common among rocks of the earlier geological ages, and most rare among rocks of recent formation. No doubt it is now in progress where deep-buried sediments are invaded by heat either from intrusive igneous masses or from the earth's interior, or are suffering slow deformation under the thrust of mountain-making forces.