74 BULLETIN OF THE 



Further remarks were made by Mr. Henry. 

 Mr. J. W. Powell made a communication on 



MONOCLINAL RIDGES, 



of which the following is an abstract. v 



(abstract.) 



Throughout the Rocky Mountain region there are many con- 

 spicuous topographic features found on the flanks of great ranges 

 and elsewhere, known as monoclinal ridges. Such a ridge is 

 composed of beds dipping in one direction. On the face of the 

 ridge the escarped edges of the beds are exhibited. On the back 

 of the ridge, the highest geological bed is found, and often the 

 slope of the back conforms more or less to the dip of the beds. 

 But where the dip is great, the higher beds are bevelled toward 

 the summit of the ridge ; where the dip is small the higher beds 

 are cut through at the foot of the slope, so that lower beds are 

 revealed, especially in the gulches. 



In any region where the beds involved are stratified and dis- 

 placement is by flexure, and upheaval is faster than atmospheric 

 degradation, monoclinal ridges appear; such ridges being com- 

 posed of harder beds, while the valley spaces are excavated in 

 more friable material. Manifestly, all such ridges must face the 

 axis of upheaval. In somewhat symmetric anticlinals the series 

 of ridges appearing on one flank is also seen on the other. 



Here the genesis of such ridges both in anticlinal and mono- 

 clinal flexures was explained, and the following law was stated. 

 As upheaval progresses pari passu with degradation, monoclinal 

 ridges recede from the axis of upheaval with the inci'ease — in the 

 amplitude of the flexure, and new ridges may appear near the 

 axial region. 



Mr. Powell then explained the eff'ect faults have in giving 

 position to the ridges. He first considered the effect of a fault 

 at right angles to the axis of upheaval, and hence at right angles 

 to the monoclinal ridge. In such a case, upheaval is arrested on 

 one side of the fault, but continues on the other, both in anticlinal 

 and monoclinal flexures. In the arrested porticin the ridges cease 

 to recede from the axis of upheaval, while in the other portion 

 where upheaval goes on, the ridges continue to recede. By the 

 fracture the ridge is broken, and by the recession of one part 

 and non-recession of the other, the ridge is faulted in such a 

 manner as to give it the appearance of a " lateral displacement," 

 and such phenomena have thus been erroneously explained. 



The following law was stated. Where a monoclinal ridge is 

 broken and one portion carried farther back from the axis of up- 

 heaval than the other, the faulting is by vortical upheaval, though 

 the ridges appear to be horizontally displaced. 



