130 OUTLINES OF FIELD-GEOLo PART i 



one side, being harder than those on the other, stand up 

 as a long steep bank or sloping hill-front, above the lower 

 ground, which is underlain by the more destructible 

 materials. Yet in all these and other instances, the 

 actual dislocations are seldom to be seen. 



When we consider, however, the vast number of 

 faults that traverse the crust of the earth, as is so im- 

 pressively brought home to the mind by mining opera- 

 tions, it must be admitted that in the vast majority of 

 cases, they produce little or no visible change on the 

 contour of the surface. Had they been left uncontrolled 

 by any other influence, they might have been expected 

 to cut up the ground into innumerable irregular segments, 

 standing at different levels and bounded by steep walls 

 of rock. That such a topography does not exist proves 

 how extensively the ground has everywhere been planed 

 down by denuding agencies. In the foregoing section 

 (Fig. 35), for example, five faults are shown ; yet in no 

 single case does the line of dislocation betray itself by 

 any marked surface feature. 



In the consideration of faults, therefore, two questions 

 obviously arise. How does a geologist recognise faults 

 when he sees them ? and how does he prove their exist- 

 ence when he does not, and cannot, see them ? 



I need not enter into any detailed answer to the first 

 of these questions. The inspection of the section of a 

 fault in nature will tell more in a few minutes than could 

 be learnt from description in an hour, and the lesson so 

 received will be better remembered. A fault is not 

 usually vertical but inclined at a high angle. The rocks 

 are commonly somewhat shattered on one or on both 



