594 C. R. KEYES — RELATIVE EFFICIENCIES OF EROSIONAL PROCESSES 



There is some of the faulting that is, no doubt, more recent and more 

 profound than elsewhere. Late orogenic movements are, perhaps, more 

 extensive. Evidences of much greater precipitation than now at no dis- 

 tant geologic date are manifest. Xo noteworthy streams traverse the 

 district to disguise the effects of typical desert-leveling. There are no 

 traces of the probable nature of the surface relief prior to the commence- 

 ment of the present dry cycle. The general conditions are such as to 

 present little critical evidence in support of any of the several hypotheses 

 that have been proposed concerning the genesis of the desert ranges. 



In other portions of the desert region there are, as already noted, many 

 features which are suggestive of structures and conditions which formerly 

 prevailed, but of which there is small hint to be derived in the Great 

 Basin. The most noteworthy of these characteristics are the mesas, or 

 plateau plains, many of which now stand high above the present level of 

 the interment plains or general plains surface of the region. As I have 

 shown recently,^ ^ these mesas manifestly represent former positions of 



Figure 5. — Geologic Cross-section of the Mesa de Maya, New Mexico 



the general plains surface. Their greater resistance to erosional influ- 

 ences and ty^e general lowering of the country is due mainly to the pro- 

 tection affoided by extensive lava flows, or to hard strata, which are now 

 the capping rocks of the remnant al levels. The surfaces on which the 

 lava sheets rest are true beveled rock floors, just as in the cases of the 

 present plains surface. 



The most noteworthy of these elevated plains is the Mesa de Maya, in 

 northeastern New Mexico. Its extension is the flat-topped Eaton range. 

 The greater part of this mesa is formed by a basalt plate 500 feet in 

 thickness, resting on the beveled edges of soft Laramie shales and sand- 

 stones. The surface of this mesa is gently inclined to the eastward and 

 extends from the Eocky Mountains, a distance of more than 100 miles, to 

 beyond the Texas line. (See figure 5.) It is 3,500 feet above the next 

 extensive plains level below, known as the Oc^te Mesa, which in turn is 

 500 feet above the general plains level of the region, in this part of the 

 territory called the Las Vegas plateau. It would appear that the Mesa 



w Bulletin of the Geological Society of America, vol. 19, 1908, p. 75. 



