302 VAN HISE—METAMORPHISM OF ROCKS AND ROCK FLOWAGE. 
quartz of a given flat granule may be largely the same quartz as that of 
the original grain, but it has been dissolved and redeposited, perhaps re- 
peatedly in part. 
Mica, and especially biotite and muscovite, are very abundant in the 
crystalline schists. Moreover, in proportion as the rocks approach typi- 
cal schists, the particles of these minerals are large, of approximately 
uniform size, and oriented crystallographically. In the original sedi- 
mentary rocks from which the mica-schists most extensively form, the 
micas are rare constituents, except in the coarse arkoses, and where they 
occur the particles are large, more or less irregularly arranged, often 
somewhat decomposed, and are readily discriminated from the regularly 
arranged, fresh micas of the crystalline schists. These facts are so well 
known that nearly all petrographers who have studied thin sections of 
the crystalline schists have regarded such micas as authigenic. Chem- 
ical analyses show that soils, muds, clays, and shales contain the ele- 
ments out of which mica may develop.* Many of these elements occur 
in hydrated compounds, such as kaolinite, zeolite, chlorite, limonite, 
etcetera. In the crystalline schists which develop from such sediments 
these hydrated minerals may be altogether absent, their places being 
largely taken by micas. It is clear that during the metamorphism 
of the rocks these hydrated minerals are taken in solution, and from 
such solutions the mica molecules, containing little water, are deposited. 
The solution and deposition give the material a less hydrated and more 
compact form; therefore the original material as compared with the re- 
sultant material contains potential energy, which is liberated during the 
process of crystallization. During the process at numerous places mica 
leaflets oriented by the differential stresses (see pages 324, 325) begin to 
form. The minute leaflets once formed serve as nuclei upon which the 
material which is continuously taken into solution may be deposited. 
The mineral particles grow somewhat uniformly, being subject to the 
same laws in this respect as original mineral particles (see pages 297, 298). 
By studying a series of thin sections from any of the districts in which 
the rocks of a formation vary from little altered to completely metamor- 
phosed rocks all stages of the process may be seen, from that in which the 
original hydrated minerals are abundant and mica is absent to that in 
which the former are absent and mica is abundant. 
In the foregoing we apparently have the explanation of the large aver- 
age size of the mineral particles which constitute the crystalline schists 
formed at considerable depth by dynamic action. They are continuous 
growths during deformation by solution and redeposition. 
*Analyses of rocks and analytical methods, by F. W. Clarke and W. F. Hillebrand: Bull. U.S. 
Geol. Survey, no. 148, 1897, pp. 277-301. 
