GEOTECTONICS OF ESTANCIA PLAINS 437 



Chupadera Mesa just south of Estancia there is an unusually 

 good exposure bearing directly upon this fact (Fig. 5). In some 

 other examples the geologic dates of the compressure are not 

 so clearly set off. In the Sierra de los Caballos where a thrust-plane is 

 shown to advantage (Fig. 4), the movement may have been very recent. 



Basin-range type of mountain structure, — In marked contradis- 

 tinction to the type just described the basin ranges are the result of 

 faulting on an enormous scale. The mountain blocks, rising 3,000 

 to 5,000 feet above the general level of the high plains which itself 

 is 6,000 feet above the sea, appear tilted as ice cakes in a stream. 



There are several instructive features relating to the structure of 

 the Desert ranges that are shown better in and about the region under 

 consideration than anywhere else in the Southwest. Recently a new 

 interest has been awakened in the tectonics of the Great Basin by 

 the publication of a number of more or less suggestive articles. The 

 main structural features about which discussion centers appear to 

 be whether the basin ranges are the result of normal faulting and 

 form ** block mountains;" or whether the "block" aspect is only 

 apparent, the monoclinal "blocks" originally being, in reality, 

 sharp asymmetric folds in which subsequent erosion has worn off the 

 steeper limb faster than the other. 



In the elucidation of the arguments by specific example, it is 

 unfortunate that many of the illustrations selected have not been 

 chosen with greater discernment. It is now well understood that 

 some of the instances noted furnish the most conclusive proofs directly 

 contrary to the purposes for which they were cited. Without entering 

 into details in regard to many of these cited examples from other 

 parts of the Great Basin region it seems pertinent at this time to caU 

 attention briefly to certain features displayed in the New Mexican 

 part of the field. These may help to explain similar phenomena in 

 other districts. 



As has recently been noted, the geologic sequence in central New 

 Mexico is especially noteworthy on account of the almost complete 

 absence of the Paleozoic rocks and the enormous development of 

 Mesozoic strata. The important member of the sequence above 

 the Azoic metamorphics is the Mid Carbonic limestone which attains 

 a normal thickness of over 2,000 feet. 



