134 Ol > (-KOLOGY IAKI i 



that they indicate the position of a fault, and we may 

 use them in confirmation of other evidence bearing 

 on the existence of that fault. In unravelling the 

 geological structure of a country, the observer may 

 thus often be able, by means of springs, to localise a 

 line of fracture, the existence of which he can demon- 

 strate otherwise. 



FIG. 38. Geological section showing how a prominent feature at the surface 

 may be caused by toe outcrop of a hard rock (a) intercalated among softer strata. 



In the same way, a marked and abrupt change in the 

 form of the ground along a definite line may serve to 

 show the position of a fault It is true that here again 

 the junction of two rocks or two groups of rock of 

 different durability, such as sandstone upon shale, or 

 limestone upon sands and clays, may give rise to a 

 long straight or curving escarpment or slope. The mere 

 existence of such a long line of bank would not of itself 

 justify any conclusion or inference as to a fault In Fig. 

 38, for example, we see how a steep declivity is produced 

 by the intercalation of a hard bed between others of a 

 much softer nature. The underlying strata (c) being 

 more easily worn away have, by their removal, deprived 

 the thick, solid, overlying mass of its proper support. 

 Hence slices of that mass from time to time slip off and 

 cumber the base of the cliff with ruined blocks, which 

 tend to arrest the progress of decay until they themselves 

 are gradually split up by the weather and removed. 



