572 PROCEEDINGS OP THE ALBUQUERQUE MEETING 



tion that the passage from one to another is very soon noted. In the present 

 connection it is to be remembered that of the four provinces only the Mexican 

 tableland and the High, or Colorado, plateau have typically arid climates ; the 

 Gi-eat Plains province represents semi-arid conditions ; while the Rocky Moun- 

 tains province possesses a semi-humid or humid climate. 



The special workings of the geological processes to which attention is soon 

 directed are the extent, rapidity, and vigor with which eolian erosion or defla- 

 tion goes on, the feebleness and infrequency of normal water-action, and the 

 remarkable effects of sheetflood transportation. 



In the light of a distinct geographic cycle for an arid climate recently recog- 

 nized, some of the most striking peculiarities of the region about us must be 

 viewed quite differently from what they were before. Brief allusion is here' 

 made to some of the most important and also to some of the best local exam- 

 ples. The real significance of the commonest features now possess a new in- 

 terest and their origin a new interpretation. 



It may be noted in passing that the Rio Grande valley does not exhibit most 

 of these features in typical form, for the reason that this stream is not a char- 

 acteristic desert drainage-way, but a great river snow-fed and flowing through 

 to the sea. Although it receives no perennial tributaries for a distance of a 

 thousand miles it is a typical humid-land stream, with high gradient and great 

 corrading powers. The old original plains-level, or bolson surface, over which 

 it once flowed, is 500 to 1,000 feet above the present channel. Throughout its 

 entire course of 300 miles in New Mexico it receives only sporadic and torren- 

 tial waters from the lofty mountains on either side. Nevertheless, in the 

 neighborhood of Albuquerque illustrations of all of the phenomena mentioned 

 are well displayed, and especial attention may appropriately be called thereto. 



Characteristic geographic Features 



vastness and evenness of ixtermont plains 



Such plains as the Estancia plains, the Hueco bolson, and the Jornada del 

 Muerto appear to owe their leading characteristics directly to the plains- 

 forming effects of wind-action ; and to be the immediate product of local 

 desert-leveling without base-leveling. The last mentioned plain is, perhaps, as 

 typical as any that could be named. The view (plate 38, figure 1) shows an 

 expanse of 30 miles in front of the highland rim in the background; the 

 length of the plain is over 200 miles.' 



ISOLATION OF THE MODNTAINS 



Ortiz laccolith, the Magdalena block, Sandia range, and Socorro volcano are 

 examples. It is to be noted that this isolation is perfectly independent of 

 mountain origin. With few exceptions, the mountains rise monadnock-like 

 out of the even plains. The lofty and steep-sided Ortiz, Tuertos, and San 

 Tsidro groups, to the east of Albuquerque, are separated only by five or six 

 miles of flat valley-plains, yet preserve their characteristic isolation.' The 

 Franklin block, north of El Paso, finely displays tnis same characteristic (plate 

 38, figure 2). 



1 U. S. Geological Survey, Water Survey and Irrigation Paper no. 123, 1905. 

 * Journal of Geology, vol. xvi. 1908, pp. 434-451. 



