ILLUSTRATIONS OF GEOLOGICAL STRUCTURE. 165 



for the narrowness of the block ci;t out by the double dislocation ; this 

 double fault is one of a pair, as is shown in the following tigure, which is 

 drawn to the same vertical and horizontal scale, and represents with con- 

 siderable accuracy the exposure observed in the caiion wall. 



J 10 ;o 4g eo ao loe 



Fig. *.iO. — F..iiU.s iu l;unMti;il clayN, Uiiiiibuldl Caiiuii, Xeviula. 



This section includes the upper portion of the medial oravels, the 

 upper clays, and the subaerial accumulations forming the surface of the 

 desert. 



In the walls of the Walker River Canon, between Mason Valley and 

 Walker Lake, there are many examples of faults which shear the lacustral 

 deposits of the Lahontan series; two illustrations of the displacements there 

 observed are given in Figs. A and B, Plate XXVIII. In the former, the 

 actual fault is concealed by an alluvial slope, but the dip of the strata 

 proves that it was formed previous to the deposition of the upper clays and 

 probably before the medial gravels were accumulated. In the latter 

 instance (Fig. B) the faulting occurred after the last rise of the ancient lake, 

 and affected both the medial gravels and the upper clays. The inclination of 

 the strata in the lower portion of this section is mainly due to their having 

 been deposited in an inclined position. In this instance, as is usually the 

 case in the faults we are considering, the general inclination of the beds 

 due to deposition is but little disturbed. On Plate XVII, Fig. E, a number 

 of faults belonging to the class we are considering are represented, which 

 cut the stratified lapilli composing the ancient craters now occupied by the 

 Soda Lakes near Ragtown, Nevada. 



The faults noticed in the preceding paragraphs can only be studied 

 to advantage when fresh sections of the Lahontan beds are exposed, and in 



