Davis.] 340 [Dec. 15, 



cannot be ascribed to the eruption of the trap ; for the greater part 

 of it was poured out conformably on the bedded rocks during their 

 accumulation, and hence passively suffered disturbance at some 

 later date : nor can the intrusive sheets have been the cause of the 

 monoclinal ; for the dip of the strata does not vary significantly on 

 approaching the intrusions ; and the topography of the intrusive 

 trap ridges is so closely like that of the overflow trap ridges that 

 the structure of both must be referred to a single cause ; this im- 

 plies that the intrusions as well as the overflows took place while 

 the formation still lay horizontal, and that the whole mass of beds, 

 igneous as well as aqueous, was afterwards deformed by some exter- 

 nal force. 



It is therefore suggested that the schists and gneisses, on which 

 the triassic strata lie uncouformably, may have suffered distortion 

 by a lateral compression from east to west. In such case, one of 

 the easiest ways of yielding to the crush would be by a slipping of 

 slab on slab, whereby their inclination should steepen and their 

 horizontal measure decrease. As slab slips on slab, the formerly 

 horizontal bevelled surface of every one is canted so as to dip 

 in one direction at an angle equal to the change of the inclination 

 of the slabs ; and the surface of every slab is separated from that of 

 its neighbors by faults with upthrow on the side of the direction 

 of dip. The triassic cover is not strong enough to bridge across 

 from ridge to ridge of the uneven surface thus produced ; its weight 

 is much greater than its strength can bear, and it perforce follows 

 the deformation of its foundation, and thereby acquires a faulted 

 monoclinal attitude. The explanation of the triassic monoclinal 

 may therefore be included in the following general statement. 

 Wherever unconformable masses are deformed together, the struct- 

 ure given to the lesser, relatively superficial mass must depend in 

 great part on the changes in the surface shape of the greater, deeper 

 mass below. Since this explanation came to mind, the author 

 finds that its essential feature has been distinctly stated by Gil- 

 bert, as affording some clew to the origin of the faulted mountain 

 ranges of the Great Basin (Wheeler's Surveys West of 100th 

 meridian, vol. in, 1875, 62). 



The tests thus far applied to this hypothesis are satisfactory, in- 

 asmuch as they correlate structures that were not before perceived 

 to be dependent on one another. The strike of the surface faults 

 corresponds with the strike of the schists beneath, as far as has yet 



