876 STUDIES FOR STUDENTS 



7. Observation of slickensides. — While offering contributary 

 evidence in many instances, observation of slickensides is one of 

 the less important methods of locating a fault, and it could 

 hardly be used without other evidence. 



8. Disappearance of outcrops alo?ig a rectilinear boundary. — As 

 already pointed out, little notice seems to have been taken of 

 the way in which outcrops and areas of near-lying outcrops ter- 

 minate, though in the opinion of the writer such boundaries are 

 full of meaning. When outcrops or groups of outcrops ter- 

 minate in right lines which are at the same time parallel to the 

 strike of the beds and to the topographic contour lines, no 

 special explanation is required. If they terminate in right lines 

 across the strike of the beds, but along the topographic con- 

 tour lines, their explanation may be found, either, first, in the 

 erosion history of the region with no dependence upon geologic 

 structure planes ; second, in the valley lines as fixed by pre- 

 existing fault planes ; or, third, in a fault scarp still in evidence 

 at the locality. If the outcrop boundary is a right line and 

 neither parallel to strike of beds nor to topographic contour 

 lines, it is pretty sure to represent the position of a fault plane. 1 

 In glaciated areas regard must, however, always be had to essen- 

 tially local and irregular accumulations of drift as affecting this 

 explanation. 



9. Occurrence of scarps in the more resistant rock. — In certain 

 areas, at least, of the crystalline schists in which there is accent- 

 uated topographic relief, scarps are among the commonest of 

 the physiographic features. In the mapping of the hard-rock 

 geology account has, however, rarely been taken of them, it 

 being perhaps rather generally assumed that their interpretation 

 is the special field of the physiographer and glacialist. If, 

 however, it can be shown that their directions conform to planes 

 of jointing and faulting, their study will furnish some of the most 

 fascinating and instructive lessons for the geologist in the crystal- 

 line areas. It is doubtless because their secrets lie hidden in 

 the structure of the underlying rocks that rock scarps as topo- 



1 See Hobbs, loc. cit., pp. 89-91 for special cases of faults fixed by rectilinear 

 boundaries of outcrop. 



