ABSTRACTS OF PAPERS 89 



Tertiary deposits been found in areas affected by tbe laccolitlis. Except tliat 

 tlie intrusions and accompanying deformation followed most of the Cretaceous, 

 it is not possible definitely to determine the age relations of the movement. 



The faults, like the monoclinal flexures, have a predominant north-south 

 trend. They are of the normal type, with the downthrow persistently to the 

 west. The displacements are comparable in size to the monoclines, involving 

 movements with a vertical component ranging from a few hundred to more 

 than two thousand feet. In all cases observed, these faults are more recent 

 than the Tertiary formations of the region, and they belong, therefore, to a 

 different chapter in the history of the province than the monoclines. 



Analysis of the monoclinal structures indicates that the displacement in- 

 volved has been dominantly vertical rather than horizontal, but the presence 

 of horizontal compressive stresses at the time of the deformation is indicated 

 by the occurrence of some lateral movement and by the fact that the adjust- 

 ment to the stresses was accomplished by folding rather than faulting. On 

 the other hand, the forces which produced the large faults at a subsequent 

 epoch appear to have been of a tensional nature. Instead of being squeezed 

 together, blocks of the plateau tended to pull apart, one slipping downward 

 on another. Both types of deformation are probably a response to isostatic 

 adjustments, but in the one case horizontal compression, in the other tension, 

 appears to modify the structural expression of the vertical stresses. The 

 deformation associated with the laccolithic intrusions is evidently caused by 

 the forces producing the intrusion. 



Presented in abstract extem23oraneonsl3\ 



Brief remarks were made by Prof. A. C. Lawson. 



STRUCTURE OF THE SPRING MOUNTAIN RANGE, SOUTHERN NEVADA 



BY D. F. HEWETT 



(Abstract) 



Spring Mountain is a crescentic range, convex toward the northeast, about 

 75 miles long, in southern Nevada, west of Las Vegas. The highest point, 

 Charleston Peak, 11,910 feet high, rises nearly 10,000 feet above the nearby 

 valleys. Detailed mapping of the southern third of the range during 1921-22 

 yields the following conclusions : The section exposed ranges from the lower 

 part of the Upper Cambrian to the Jurassic. The Cambrian, Devonian. Missis- 

 sippian, and Pennsylvanian sections include about 6.500 feet of dolomite and 

 limestone with traces of shale and sandstone. The Permian, Triassic, and 

 Jurassic sections include 1,000 feet of red and buff sandstone, 400 feet of lime- 

 stone, 50 feet of red shale, 600 feet of thin-bedded limestone. 1,000 feet of red 

 shaly sandstone, and 2,200 feet of buff sandstone. 



In late Cretaceous time (?) the beds were gently folded along" axes trending 

 west of north ; then three distinct blocks were successively thrust from the 

 southwest. The lowest thrust is the flattest, the dip ranging from 9 degrees 

 to 15 degrees ; the higher thrusts are steeper. In each block there are local 

 steep thrusts with small displacement. The thrust faults were closely followed 

 by a group of early normal faults that trend north and dip west. Solutions 



