34-6 



PHYSICAL GEOLOGY 



1 



the deformation resulting from the crushing of such rocks as con- 

 glomerates, granites, or basalts. (3) In contact metamorphism the 

 development of minerals, especially mica, along the stratification 

 planes of sedimentary rocks also produces a schist. 



Schists are given various names, depending upon their most 

 conspicuous mineral. Mica schist is composed principally of mica 

 and quartz and is the most common type of metamorphic rock. 

 Mica schists are usually metamorphosed, fine-grained sandstones 

 and shales. Hornblende schist consists largely of hornblende, and 

 varies from green to black in color. In some cases, the characteristic 

 needle or blade-like crystals are readily recognized, but in others 

 the grain is so fine that the individual crystals cannot be seen. Horn- 

 blende schists are derived from diorites, gabbros, etc., by pressure, 



and it is probable 

 that impure lime- 

 stones containing 

 sand, clay, and iron 

 oxides also produce 

 hornblende schists 

 when subjected to 

 metamorphism. 



Gneiss. — This is 

 a banded, crystalline 

 rock (Fig. 339) in 

 which feldspar is 

 present. It is a rock 

 with the composition 

 of granite, but with 

 a banded structure. 

 Gneiss may be con- 

 sidered for conven- 

 ience as intermediate 

 between an igneous rock, such as granite or diorite, and schist. It 

 will readily be seen from the above that a gneiss may, on the one 

 hand, so closely resemble a schist that one will be in doubt as to 

 its classification, and on the other hand, that it may be confused 

 with a granite. Typically, however, gneisses are easily recogniz- 

 able and may be considered for convenience as banded granites. 

 As in the case of schists, various qualifying adjectives are used in 

 describing gneisses, as biotite gneiss, hornblende gneiss, garnet biotite 



FlG. 339. — Gneiss, showing banding. 

 Museum.) 



(U. S. National 



