440 CHARLES R. KEYES 



A number of observations lately made emphasizes the great impor- 

 tance of the unconformity at the base of the Cretacic strata of the 

 region. For example, in the Chupadera Mesa in eastern Socorro 

 County, there are found Carbonic limestones highly inclined on 

 either side of huge trachyte dikes, over the whole of which recline 

 nearly horizontally the Cretacic sandstones (Fig. 5). This surface 

 of unconformity represents extensive land erosion. During the inter- 

 val for which it stands the strata of the region were folded and planed 

 off long before the later Cretacic sediments were laid down. 



In nearly all of the basin ranges there are abundant evidences of 

 marked compression producing the phenomena of folding. Yet in 

 every instance personally observed the period of these movements 

 is manifestly long prior to the elevation of the present mountains. 

 The ancient tectonics of the basin ranges is a theme of very great 

 interest. 



There is another very deceptive feature connected with the for- 

 mation of the blocklike mountains of some portions of the Mexican 

 tableland. At the foot of the steeper slope the strata are often 

 found to be tilted at a high angle and inclined away from the range. 

 This attitude of the beds readily suggests at first the possibihty of the 

 mountain ridge's being a sharp anticHne with the center completely 

 removed through erosion, leaving the limbs of the arch unequally 

 exposed. This condition might be easily fancied because of the fact 

 that the greater part of the height of the mountains, 3,000 to 5,000 

 feet, is usually composed of massive crystallines and schists, while 

 the crest and the backslope are of limestone. 



There are many good reasons for believing that such phenomena 

 as these instead of being ascribable to folding of the asymmetrical type 

 are to be considered merely as an accompaniment of normal faulting. 

 The displacement, however, is on a gigantic scale and under condi- 

 tions not usually met with. When the hade is not vertical, or nearly 

 so, the strata on the down-throw side to a greater or less degree lag, 

 until a considerable zone is produced in which the beds become highly 

 inclined and in many cases stand even nearly perpendicular. A 

 typical instance is the Sandia Range, east of Albuquerque, as repre- 

 sented below (Fig. 6). 



There are strong theoretical grounds for thinking that faulting 



