MAPPING OF THE CRYSTALLINE SCHISTS 877 



graphic features have received so little consideration at the hands 

 of the modern school of physiographers. 



The perfection of rock scarps will doubtless be dependent 

 primarily upon the resistance of the rock of which they are 

 formed to the agencies of erosion, and to the time that has 

 elapsed since their formation. In dense basalt their life is 

 probably the longest, and the present surfaces will most nearly 

 represent the original fracture planes. It is a fact of general 

 observation that scarps are usually interrupted in such a way as 

 to produce a step-like structure, which suggests in nearly all 

 cases that the throw was distributed over several parallel and 

 near-lying planes. The great cliffs of the Newark basalt of the 

 Connecticut valley produced in an unknown period of post- 

 Newark time, furnish striking illustrations of rock scarps in 

 great numbers and perfection. 



The evidence that precipitous rock walls are connected genet- 

 ically with the fault and joint system of a region — an inher- 

 ently probable supposition — must be drawn from a study of 

 the direction, not of a single cliff, but of a large number of 

 scarps embracing a considerable area. Accurate topographic 

 maps are most instructive in this regard, but they fail to tell the 

 full story. Photography here comes to the aid of the geologist, 

 not only in recording topographic peculiarities, but in the 

 exposition of them. To give to photographs their full sphere of 

 usefulness, however, it is important to indicate upon the map 

 from what points they were taken and the pointing of the 

 camera at each locality. A photograph common and uninter- 

 esting in itself becomes full of meaning in its relation to the 

 map. A character like Fig. 3 entered upon the map 

 and consisting of a circle which incloses the point at 

 which the view was taken, and proceeding from the Fig. 3. 

 circle a short arrow to show the camera pointing, will supply 

 the needful information. A figure within the circle refers to the 

 number of the photograph or to the plate of a report, as the 

 case may be. 



Scarps may, however, be produced in other ways than by 

 faulting. Other agencies most potent to produce them are : 



