
ROCKS ’ 81 
Mylonites.—This is the name given to more or less fine- 
grained cataclastic rocks. They are typically developed along 
the lines of great overthrusts or reversed faults, and are 
usually closely associated with crystalline schistose rocks, into 
which indeed they often pass. Most frequently they show 
well-developed “shear-structure”—the rock being composed 
mostly of minute fragments and particles with now and again 
larger fragments, set in a streaky groundmass of crushed 
materials. When the nature of the larger fragments (of 
minerals or rock) is obvious, not infrequently one is able to 
say what the original uncrushed rock may have been. 
Friction- or Crush-breccia.—This is an aggregate of 
angular and subangular fragments, varying in size up to one 
foot or even more in diameter. Breccias of this kind occur 
in the same way as mylonites, into which they often pass. 
All gradations, indeed, may be traced from coarse breccia 
into mylonites, and from the latter into schists. When there 
has been considerable movement of the rock-débris and the 
fragments have been rolled over and more or less rounded, the 
rock is termed a friction-conglomerate. Although friction- 
breccias are best developed in regions where the rocks have 
been subject to much compression—to folding and great 
dislocations and displacements—and where frequently meta- 
morphism is more or less pronounced, they are nevertheless 
not confined to such regions. Faults traversing strata of all 
kinds are not infrequently accompanied by breccias. Some- 
times these are confined to a line of fracture, filling up the 
space between the two walls of a fault ; while in other cases the 
rocks forming one or both walls of a fault have been jumbled, 
shattered, and brecciated. The stones in such fault-breccias, 
as they are termed, are not infrequently rubbed smooth and 
striated on one or more sides. 
Determination of Rocks in the Field 
The beginner who casts his eye for the first time over a good 
collection of rock-specimens is apt to be dismayed by the numerous 
species and varieties with which he is expected to become acquainted. 
The coarser grained rocks, whether they be derivative, igneous, or meta- 
morphic, do not appear to present much difficulty. He may feel hopeful 
that, with ordinary care, he will eventually learn to distinguish one from 
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