STUDIES FOR STUDENTS. 
DEFORMATION OE ROCKS: 
i GENERATE 
Rock units under thrust act very differently, depending upon 
their thickness, strength, and other characters, upon the charac- 
ter and thickness of the rock units above and below, and upon 
the closeness of the folding. 
It is believed that the outer part of the earth may be divided 
into three zones: (1) An upper zone of fracture; (2) a middle 
zone of combined fracture and plasticity; (3) a lower zone of 
plasticity. 
(1) Rocks under less weight than their ultimate strength when 
vapidly deformed are in the zone of fracture. Vhat is, when rocks 
under such conditions are deformed they break, and crevices small 
or great separate the broken parts. The fractured rocks may be. 
jointed, faulted, or brecciated in a simple or complex manner. 
The fractures may be far apart and of great size and extent, or 
near together and of small size and extent. Innumerable parallel 
fractures may occur in the same direction when, as shown in a sub- 
sequent paper, the rocks develop a parting or fissility. In extreme 
cases of fracture the rocks become autoclastic or are broken into 
innumerable fragments by the forces of deformation. These 
t Published by permission of the Director of the United States Geological Survey. 
In the preparation of this series of papers I have been greatly assisted by PROFESSOR 
L. M. Hoskins. In the Sixteenth Annual Report of the United States Geological Sur- 
vey, where my entire paper will appear, PROFESSOR HoskKINs’s work, from which 
extracts are below made, will also be published. To WILLIs’s paper upon the Appala- 
chians I am also greatly indebted. (‘The Mechanics of Appalachian Structure,” by 
BAILEY WILLIs, Thirteenth Ann. Rept. U. S. Geol. Surv., pp. 211-281.) I should also 
mention HEIm’s great work, ‘‘ Mechanismus der Gebirgsbildungen,” from which I have 
absorbed many ideas. 
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