STRUCTURES OF BASIN RANGES 67 



Attention is especially called to these facts for the reason that in 

 some instances cited relative to the structure of Basin ranges it is 

 quite manifest that the proper local interpretations have not been 

 made. It is probable that a large number of other cases will be found 

 to be illy chosen for purposes of type illustration. The observations 

 made in the Sierra de los Caballos are suggestive of .similar phe- 

 nomena occurring in other districts. Critical evidence on the points 

 emphasized is much desired from many other ranges. When once 

 secured, it may do much towards correcting some very erroneous 

 present interpretations. 



A generalized geological cross-section of a part of the Basin region 

 of New Mexico more clearly illustrates the type of mountain struct- 

 ure under consideration (Fig. 4), 



Sacramento Mrs. 



Xaballos Mts. 



Fig. 4 



In all of these ranges there are abundant evidences of marked 

 compression producing the phenomena of folding and thrusting. 

 Yet in every instance thus far observed the period of this movement 

 is found to be long prior to the elevation of the present mountains. 



There is another very deceptive feature connected with the forma- 

 tion of the block-like mountains of the New Mexican portion of the 

 Basin region. At the foot of the steeper slope the strata are often 

 found tilted at a high angle, and inclined away from the range. The 

 attitude of the beds easily suggests, at first consideration, the possibility 

 of the mountain ridges being a sharp anticline, with the center com- 

 pletely removed through erosion, leaving the limbs of the arch 

 unequally exposed. This condition might be readily fancied because 

 of the fact that the greater part of the height of the mountains, 3,000- 

 4,000 feet, is usually composed of massive crystallines and schists, 

 and the crest and backslope of the limestones. 



There are strong reasons for believing that this phenomenon, 

 instead of being ascribed to folding of the asymmetrical type, should 

 be considered an accompaniment of normal faulting. Only the 



