QUARRY MATERIALS OF NEW YORK 1 5 



their appearance varies with the direction of the surface with respect 

 to the structure. This parallehsm is brought about by the Hnear 

 arrangement of certain constituents hke mica or hornblende which 

 have tabular or elongated forms ; or it may be produced by the 

 separation of unlike minerals in layers. There is some analogy 

 between such structure and that of stratification in the sediments. 

 But it is no criterion as to the origin of the rock for it is quite 

 prevalent among those of igneous derivation. This structure is 

 commonly called foliation or schistosity. It denotes usually 

 weakened cohesion between the minerals ; and rocks split more 

 evenly along the foliation than in other diredtions. 



The changes accomplished by metamorphism are not limited 

 ordinarily to a physical rearrangement of the constituents. In 

 many instances there results also a breaking up of the mineral 

 compounds and their crystallization in new forms, more stable 

 under the conditions. The degree to which the chemical alteration 

 may be carried depends upon the nature of the rock and the agencies 

 at work upon it. An igneous rock like granite under the same 

 influences is more resistant to chemical changes than a sediment 

 like shale. In fact, granite undergoes little alteration beyond the 

 crushing down of the quartz and feldspar crystals and possibly a 

 certain amoimt of recrystallization, producing a parallel appearance. 

 The basic igneous rocks (those with low percentages of silica) in 

 which the iron, magnesia and lime compounds are well represented, 

 are more prone to chemical change ; they form readily such rocks 

 as amphibolite, serpentine and various schists. Among sediments, 

 the limestones are recrystallized into marbles, but in the presence of 

 silica and other compounds existing as original impurities or later 

 introduced, they may be converted into garnetiferous, tremolitic or 

 micaceous schists or amphibolites. Sandstones are hardened by 

 secondary growth of the quartz grains or by deposition of silica 

 cement so as to form quartzites. Shales are converted into slates, 

 with microscopic mica and feldspar crystals ; or by further meta- 

 morphism into schists and gneisses. Inasmuch as the agencies of 

 metamorphism are mainly restricted to the deeper zones within the 

 earth, the rocks which bear widespread evidence of their effects 

 must at some time in their history have been buried far below the 

 surface. It is only through removal of many thousands of feet 

 • of overlying rock by erosion that they are now exposed to view. 

 They are found, therefore, among the older geological formations 

 and include the very earliest members of which we have knowledge. 



