te vw A SURE MENT OF FAULTS: 
AccorDING to the definition given by Dana, “ faults are dis- 
placements along fractures.”” Whenever the rocks of the earth’s 
crust are subjected to strain, fractures take place in them as in 
any other body under similar conditions, and the different parts 
of the rock tend to move past one another along the fracture- 
planes, seeking to obtain relief from the strain and to accommo- 
date themselves to new conditions. In this movement one part 
of the fractured rock-mass may move upon the other in any 
direction, up, down, sidewise or obliquely, according to the 
conditions, which are different in each instance. There is, so 
far as I know, no law governing the direction of movement in 
faults which is of any use in geological diagnosis. Naturally, 
when there is any preéxisting plane of weakness of the rock 
which is subjected to strain the movement takes place by prefer- 
ence along this plane; and, hence, in sedimentary beds, it is 
probable that movements along the stratification planes consti- 
tute the commonest variety of faults. Inasmuch, however, as 
the beds in disturbed districts lie in every conceivable position, 
the probability just stated does not give any clew to the average 
attitude of faults. 
The movement in faults can be completely ascertained only 
by the aid of independent and accidental phenomena. In homo- 
geneous rock-masses (leaving out of consideration fault scarps, 
fault gulches, and other topographic phenomena, and treating 
the faulted mass as a solid without boundaries), the amount of 
movement cannot be ascertained or even approximately esti- 
mated; although the exzstence of a fault can be determined by 
the records left on the slipping surface or surfaces in the shape 
of ground-up rock or fault-breccia, in polished and striated rock 
faces, and so on. It is certain, however, that the amount of 
friction as displayed by trituration and polishing is not neces- 
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