In the scheme of classification it is not yet practicable to separate the metamorphic rocks of igneous origin from those which are transformed sediments, for it is often impossible to distinguish one from the other.
These represent the less advanced stages of metamorphism, in which the forces of compression may have produced cleavage or fissility, but not foliation. The more important rocks of this class are of sedimentary origin, and it will be unnecessary for us to consider the igneous rocks which have been changed, though not to the extent of producing foliation.
Quartzite is derived from the metamorphosis of sandstone, and between the two kinds of rock are found such complete transitions, that the separation of them seems almost arbitrary. In a typical quartzite the rock is crystalline, and the quartz deposited around the sand-grains is in crystalline continuity with those grains, though the microscope still reveals the original fragmental nature of the rock. Quartzites also result from the metamorphism of conglomerates, and the pebbles are sometimes much flattened by compression. If the sandstone or conglomerate contained impurities, other minerals besides quartz are generated; if any considerable quantity of clay was present, mica will be produced and, it may be, in such abundance that the rock passes into mica schist (see below).
Fig. 224. - Fissile quartzite, California. (U. S. G. S).
Quartzites are formed both in contact and regional metamorphism, but the change is principally due to cementation, large amounts of silica (estimated as one-sixth of the original quantity present in the sandstone) being brought in and deposited from solution, though this cementation may be effected by ordinary percolating waters bearing Si02 in solution, so that some quartzites should hardly be regarded as metamorphic. Many quartzites do not appear to have been subjected to great compression, while others are cleaved or fissile (Fig. 224).