STRUCTURES OF BASIN RANGES 69 



In the late account of the Humboldt region in Nevada, Lauterback 1 

 dissents somewhat from the view of the simple tilted block idea, and 

 is inclined to believe that the mountain-block and the valley-block 

 are distinct. This appears often to be the correct interpretation in 

 the New Mexican part of the Basin region. 



The physiographic development of the New Mexican region 

 appears, briefly to be as follows: About the beginning of Tertiary 

 time the area lying between what is now the Gulf of California and 

 the Gulf of Mexico must have been a vast low-land plain elevated 

 but slightly above sea-level, and having faint relief features. A 

 large part of the plain was a surface worn out on the beveled edges 

 of Cretaceous and older strata, as is, even at the present day, still 

 clearly discernible in its remnants. The Las Vegas plateau, the 

 Llano Estacado, the bolson plains of central New Mexico, and some 

 of the less-broken plains of eastern Arizona appear to belong genetic- 

 ally together. To the east and west of the vast area thus outlined 

 there had been formed, from the sediments derived from the planing 

 off of the central land area, a broad submarine platform. When 

 later in Tertiary times the general bowing up of the region began to 

 take place, the great plain that had been formed was partly a pene- 

 plain of destructional land origin and partly a constructional plain 

 of marine origin. 



During the period of uprising folds were extensively developed, 

 and the compression was so intense that in many cases thrust-faults 

 were formed. Many low mountain ridges were probably produced 

 at this time. Subsequently, as if the upward movement had been 

 too extensive, the compressive force gave way to one of tension. 

 Normal faulting on a grand scale occurred, producing the numerous 

 short monoclinal or "block" mountains of the region. 



Several important points bearing upon Basin Range structures 

 appear to be reached in the present connection: 



1. The determination of the most obvious structures of any par- 

 ticular mountain region is not sufficient and is not critical evidence; 

 the time of the formation of the structures is an all-important considera- 

 tion. When the younger strata are removed from the mountains, the 

 structures of the older rocks may tell a very different story. In some 



1 Bulletin of the Geological Society 0} America, Vol. XV (1904), p. 343. 



