GEOTECTONICS OF ESTANCIA PLAINS 441 



instead of folding is to be invoked to explain the structure of 

 the so-called block mountains of the Great Basin region. The 

 principle was clearly set forth by Le Conte^ as long ago as 1889, 

 when discussing the district between the Sierra Nevada and the 

 Wasatch Mountains. As considered in his textbook^ the same 

 author has regarded the area in question as a region which was sub- 

 jected to slow and general uprising but continually adjusted itself 

 through normal faulting in great blocks. By the tilting of these 

 blocks mountain ranges were produced on the elevated edge while 

 on the depressed side were formed valleys which were subsequently 

 filled with sediments. Lauterback^ is inclined to modify this view 





Fig. 6.— Fault-scarp of the Sandia Mountains. 



somewhat by regarding the mountain block and the valley block as 

 distinct. 



The present relief features of the region are, however, mainly the 

 product of general desert leveling, that is the result of eolian erosian 

 under conditions of an arid climate, and the mountains are to be 

 looked upon as remnantal ranges which are essentially monadnocks. 



Laccolithic type of mountain structure. — In the northwestern part 

 of the Estancia Plains there rise four isolated groups of lofty peaks. 

 The several groups are five to six miles from one another and lie in a 

 straight line trending nearly in a northeast and southwest direction. 

 The southernmost group is known as the San Ysidro, or South 



1 American Journal 0} Science (3), Vol. XXXVIII, p. 259, 1889. 



2 Elements of Geology, sth ed., p. 242, 1904. 



3 Bulletin of the Geological Society of America, Vol. XV, p. 343, 1904. 



