ABSTEACTS OF PAPERS 139 



the faulting which was the prime agent in producing the present scarps oc- 

 curred in late Geological time, probably Pliocene or early Pleistocene. It was 

 further pointed out that this was the prevailing type of origin of the present 

 ranges throughout this and adjacent areas. Physiographic evidence also serves 

 to indicate that the present mountain fronts were generated by a continuous 

 series of elevations, and that it is improbable that they were produced by two 

 or more widely separated uplifts, with long periods of rest between. 



The forms of the scarps were discussed and terracing of the scarps shown, 

 in such cases as were examiued, to be due to step faulting. The indications 

 are that these scarps have not suffered great denudation, and that the upper 

 portions have not migrated far back from their original positions. In a num- 

 ber of important occurrences cited there has been practically no recession of 

 the base. 



The evidence at hand indicates that the faulting was in general essentially 

 normal faulting, and it was pointed out that the occurrence of occasional 

 thrusts ill a large area of normal faults or the passage of normal faults into 

 flexures is to be expected. Flexures and occasional thrusts do uot necessarily 

 mean general compression action, and the idea of deformatiou with expansion 

 is accepted as the best interpretation of the phenomena of the northwestern 

 Basin region. 



An extension of the results to certain other parts of the Great Basin region 

 was also made. 



Presented without notes and illustrated by maps, lantern slides, and 

 blackboard diagrams. 



Discussion by Branner, Merriam, Lawson, and Weaver. 



At the conclusion of this paper the section adjourned sine die. 



