PHILOSOPHICAL SOCIETY OF WASHINGTON. 75 



Mr. Powell then explained the effect of a fault oblique to the 

 axis of upheaval ; he then explained the efiPect of two oblique 

 faults crossing each other, in all these cases illustrating from 

 examples in nature. Then he further explained how a ridge that 

 was simple togopographically might be compound geologically, 

 being composed in different parts of beds of diffei*ent geological 

 horizons. 



In the examples considered, those changes in monoclinal ridges 

 were produced by faults transverse or oblique to the dip of the 

 beds or the axis of upheaval. Another series of changes are 

 produced by faults parallel or nearly so to the axis of upheaval. 

 On either flank of an anticlinal or in a monoclinal, there is a belt 

 of dipping rocks, which may be denominated as the zone of flex- 

 ure. It is often observed in nature that displacement proceeds 

 as flexure along such a zone for a time, and subsequently dis- 

 placement by flexure ceases, but is continued by faulting, so that 

 a plane of fracture is found to run along a zone of flexure. If 

 the plane of fracture is on the side of the zone of flexure nearer 

 the axis of upheaval, the ridges composed of beds of the lower 

 geological horizon are lost ; if the plane of fracture is on the side 

 of the zone of flexure farther from the axis of upheaval, the 

 ridges composed of the beds of the higher geological horizon dis- 

 appear ; if the plane of fracture is midway in the zone of flexure, 

 the middle ridges disappear. 



Often the plane of fracture meanders through the zone of 

 flexure where the ridges are more or less curved and broken, and 

 ridges that are simple geographically, are often compound geolo- 

 gically. 



All the monoclinal ridges thus far described, are Monoclinal 

 Ridges of Unequal Degradation as distinguished from Mono- 

 clinal Ridge of Displacement. The latter have a very difl'er- 

 ent genesis from the former. A long fracture is produced in the 

 sedimentary beds involved, and on one side of this line the beds 

 are upheaved — tilted up — so as to produce a ridge with the beds 

 all dipping in one direction. Often such upheavals are of great 

 magnitude, producing important ranges. Such a ridge is pro- 

 duced directly by upheaval, and ail ensuing degradation serves 

 but to obscure the ridge-like character of the mass. Many of 

 the ranges of the Desert System and Basin System are of this 

 character. 



The whole subject was illustrated by drawings on the black- 

 board. 



From the facts presented, the following deductions were 

 drawn : — 



I. Displacement by flexure is very slow, and where it appears 

 as upheaval in relation to the level of the sea is little faster than 

 atmospheric degradation. 



II. Displacement by faulting is very slow, and where it ap' 



