PHILOSOPHICAL SOCIETY OP WASHINGTON. 195 



ropean palasoutologists as valves of chitons), all Paleozoic chitons 

 belong to the group without laminae of insertion to the valves, 

 which are typified by the existing Leptochitons of Carpenter^ 

 which are conhned to arctic and temperate seas so far as known. 



Mr. G. K. GiLBEP.T made a communication entitled 



THE WASATCH A GROWING MOUNTAIN. 



(abstract.) 

 In a general way the structure of the Wasatch range is easily 

 described, for although it is complex in detail there is one feature 

 which prevails through its entire leugtli, and is always the impor- 

 tant feature. Everywhere it is bounded on the west by a pro- 

 found fault, and the rocks constituting it have a general dip to 

 the east. The rocks eastward of the fault plane have been uplifted 

 at several epochs, and have been continuously subjected to erosion 

 so that their remnant, which forms the mountain, exhibits but a 

 small fraction of the entire uplifted mass. Their revealed section 

 includes, according to Mr. King, ten miles in thickness of con- 

 torming strata. The rocks westward of the fault plane are not 

 in sight, being buried at an unknown depth beneath the debris 

 worn from the eastward mass. The maximum displacement or 

 throw of the fault is therefore more than ten miles. 



Along the plane of this great fault there have been recent 

 movements, and the bluffs or escarpments to which they have 

 |iven rise bear such relation to the water-marks produced by 

 Lake Lonneville (probably a feature of the Glacial Epoch) that 

 their date is established as post-glacial, or at least post-Bonne- 

 ville. ihe movements were not coincident, but were clearly 

 separated by intervals of time, and the latest one was so rece'nt 

 that the escarpment it produced has not yet been thrown down 

 by^frost, but stands a grassless cliff of earth. 



The total post-Bonneville erosion has not been sufficient to 

 destroy or even greatly impair the Bonneville terraces Thou"-h 

 chiefly composed of gravel and earth, they have not been de-raded 

 more than one or two feet at the utmost, and it is safe "to say 

 that the rocky summits of the range itself, exposed to fiercer 

 stonus but opposing them with harder material, have been de- 

 graded on an average not more than five feet. The total post- 

 ±;onneville displacement, examined for a distance of 125 miles 

 averages about fifty feet. The range has therefore been lifted' 

 with reference to the adjacent valley, an amount ten times as 

 great as the altitude lost by erosion ; and this has taken place in 

 the atest epoch of geological time—an epoch continuous with 

 the historic._ We may fairly say that at the present time the 

 causes to which the mountain is due not merely continue, but are 

 more potent than the causes which tend to obliterate it. It is a 

 growing mountain. 



