APPENDIX. 
MEASUREMENT OF FAULTS. 
According to the definition given by Dana, “faults are displacements 
along fractures.” Whenever the rocks of the earth’s crust are subjected 
to strain, fractures take place in them as they would in any other body 
under similar conditions, and the different parts of the rock tend to move 
past one another along these fractured planes, seeking relief from the 
strain and accommodating themselves to new conditions. In this move- 
ment 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 preexisting plane of 
weakness of the rock which is subjected to strain the movement generally 
takes place along this plane, and hence in sedimentary beds it is probable 
that movements along the bedding planes constitute 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 
clue to the average attitude of faults. 
The amount of movement in faults can be completely ascertained only 
by the aid of independent and accidental phenomena. In homogeneous 
rock masses (leaving out of consideration fault scarps, fault gulehes, and 
other topographical phenomena, and treating the faulted mass as a solid 
without boundaries) the amount of movement can not be ascertained or 
even approximately estimated. The existence of a movement 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 necessarily proportionate to 
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