ARID CYCLE IN A PLAINS REGION 595 



de Maya practically represents a Tertiary peneplain which existed at the 

 time of the general elevation of the region. At the town of Eaton its 

 surface is now 9,000 feet above tide. Were it not for the great protecting 

 lava field, the remnants of which constitute this plateau plain, there 

 would today remain no undoubted traces of the old peneplain in this part 

 of the country.^^ Probably not even a low rounded ridge would remain 

 to mark the position of the Eaton range. 



That the Mesa de Maya is a remnant of what is essentially a pene- 

 planation level which, perhaps, once extended over much, if not most, of 

 the present desert region around the southern end of the Eocky cordillera 

 is strongly supported by a number of facts : ( 1 ) The foundation strata, 

 both hard and soft beds, which alternate frequently, are evenly beveled, 

 indicating that the country at the time of planation must have been only 

 slightly above the level of the sea. (2) The principal orogenic deforma- 

 tion and faulting appears to have taken place in early or mid-Tertiary 

 times, and prior to the period of the general planing off. (3) The 

 numerous mountain ranges of 'New Mexico, outside of the Eockies, are 

 subequal in height, a fact irdicating, when taking into account the period 

 of principal deformation and faulting, the general alternation of hard 

 and soft belts of rock, and the extent of the subsequer^ denudation, that 

 the present cycle of erosion must have started with the country already 

 more or less a well defined plain. (4) The present bilateral symmetry 

 of the desert ranges on the whole, even in the cases of the so-called block 

 mountains, as the Jemez, Sandia, Franklin, Magdalena, and Caballos 

 ranges, for example, is suggestive of long continued attack by the ele- 

 ments upon hard mountain rock. In every one of the mountain ranges 

 just mentioned the major fault-line, if such really exists, is as far from 

 the crest of the mountain ridge as is the foot of the back slope. (5) 

 Plateau plains that lie far above the present general plains level, but still 

 below the Mesa de Maya surface, are beveled rock surfaces, protected, 

 usually, by lava flows or hard strata. (6) With all of the present ranges 

 of the so-called block type bordered on either side by soft beds of great 

 thickness and the very resistant mountain strata in monoclinal attitude 

 once extending such relatively long distances beyond the present moun- 

 tain crests, it does not seem likely that general lowering of the surface of 

 the country could have gone on so evenly without something of a plains 

 surface to begin with. (7) The postulation of a general mountainous 

 surface at the beginning of the present geographic cycle, as represented 

 by the Mesa de Maya planation surface, finds many incongruities which 

 need not be dwelt on at this time. 



^ Keyes : Proceedings of the Iowa Academy of Sciences, vol. xv, 1908, p. 221. 



