68 CHARLES R. KEYES 



faulting'is on a gigantic scale — a scale very much larger than is usually 

 met with. When the hade is steep, the strata on the down-throw 

 side to a greater or less degree commonly lag, until a considerable 

 zone is produced in which the strata become highly inclined, and in 

 many cases stand even nearly vertically. A typical case is represented 

 below : 



In a smaller way we find the same lagging structures accompany- 

 ing the faulting in coal mines. It is a fact well known to the miners 

 that when the dip lessens a down-slip is soon to be expected. 



/ 



/ 

 / 



/ 



Fig. s 



There is a theoretical reason for believing that in the region under 

 consideration normal faulting, instead of folding, is to be expected 

 to explain the origin of the New Mexican mountains of the Basin 

 district. The principle was clearly set forth by LeConte 1 as long ago 

 as 1889, when discussing the Nevada region between the Sierra 

 Nevada and the Wasatch Mountains. 



As considered there and in his textbook of geology 2 this author 

 appears to have regarded the area in question as a region which was 

 subjected to slow and general uprising, but continually adjusting 

 itself through normal faulting in great blocks. By the tilting which 

 these blocks suffered mountain ranges were produced on the elevated 

 edge, and on the depressed side were formed valleys which were 

 subsequently filled with sediments. 



1 American Journal of Science (4), Vol. XXXVIII (1889), p. 259. 



2 Elements 0} Geology, 5th ed. (1904), p. 242. 



