
- Parr IL. § vi] CRYSTALLINE ROCKS—SCHISTOSE. 127 
- Quartz-rock, Quartzite, though not properly a schistose rock, 
_ may be most conveniently considered here, as it is so constant an 
accompaniment of the schists, and, like them, can often be directly 
traced to the alteration of former sedimentary formations. It is a 
granular to compact mass of quartz, generally white, sometimes 
yellow or red, with a characteristic lustrous fracture. It occurs in 
thin and thick beds in association with schists, sometimes in con- 
tinuous masses several thousand feet thick. In Scotland it forms 
ranges of mountains, and is there frequently accompanied with sub- 
ordinate beds of limestone, which in Sutherlandshire contain Lower 
Silurian fossils. - | 
Kyen to the naked eye, the finely granular or arenaceous structure 
of quartz-rock is distinctly visible. Microscopic examination shows 
this structure still more clearly, and leaves no doubt that the rock 
originally consisted of a tolerably pure quartz-sand, which has been 
metamorphosed by pressure and the transfusion of a siliceous cement 




Fic. 20.—Microscoric STRUCTURE OF QUARTZ-ROCK. 
mto an exceedingly hard mass. This cement was probably pro- 
duced by the solvent action of heated water upon the quartz grains, 
which seem to shade off into each other, or into the intervening 
silica. It is owing, no doubt, to the purely siliceous character of 
the grains that the blending of these with the surrounding cement 
is so intimate, that the rock often assumes an almost flinty homo- 
geneous texture. That quartzite as here described is an original 
sedimentary rock, and not a chemical deposit, is shown not only by 
its granular texture, but by the exact resemblance of all its leading 
features to ordinary sandstone—false-bedding, alternation of coarser 
and finer layers, worm-burrows, and fucoid-casts. The lustrous 
fracture which distinguishes this rock from sandstone is due to the 
exceedingly firm cohesion of the component grains which break 
across rather than separate, and to the consequent production of 
innumerable minute clear vitreous surfaces of quartz. A sandstone, 
on the other hand, has its grains so loosely coherent, that when the 
