GEOLOGIC PROCESSES OF THE ARID REGION 573 



COMPLETE ENCIRCLEMENT op MOUNTAINS BY PLAINS 



The mountains everywhere rise abruptly out of the plains, as do volcanic 

 isles out of the sea. The Jemez, Florida. Caballos, and Oscura ranges, which 

 finely display this feature, are only typical of the large majority of the moun- 

 tains of the southwest.' The view of the last mentioned range is from a point 

 on the Jornada del Muerto, 15 miles distant (plate 39. figure 1). 



CHARACTERISTIC ABSENCE OF FOOTHILLS 



This is one of the most surprising features of the arid region. One leaves 

 plain and plunges by way of deep canyon at once into heart of mountain. 

 There are no foothills about the Sandias. The Tijeras is a narrow and pro- 

 found canyon until it meets the plain.' The Socorro mountain well illustrates 

 this feature (plate 39, figure 2). A still more characteristic view, shown in 

 plate 40, figure 1, is from a photograph by W J McGee of the Coyote range of 

 southern Arizona, rising over 3,(X)0 feet above the plain. 



RESISTANT CHARACTER OF MOUNTAIN ROCKS 



Independent of the origin and original rock-makeup of the mountains it is 

 significant that it is the most resistant rocks which form them. The Sandia 

 block is chiefly ancient crystallines and hard Carbonic limestones ; the Ortiz 

 group is mainly mica-andesite, which was originally covered by great thick- 

 nesses of Cretacic sandstones ; Socorro mountain is made up of Tertiary lava 

 flows. All of these mountains are essentially arid monadnocks rising above 

 the plains. 



SOFT SUBSTRUCTURE OF THE PLAINS 



The Las Vegas plains are worn out on the soft Colorado shales ; the Estancia 

 plains have a substructure of friable Cretacic sandstones and shales ; the old 

 bolsons of the present Rio Grande valley above and below Albuquerque are 

 floored by Early Tertiary deposits. 



BEVELED ROCK-STRUCTURE OP PLAINS 



In the valley of the Rio Galisteo, north and east of Albuquerque, and near 

 the town of Los Cerrillos, the plains-surface is clearly shown in the field to 

 bevel the rock-structure (plate 40, figure 2). The Jornada also displays the 

 same phenomenon.^ as does the Chupadera mesa to the east of the Jornada. 

 The Rio Grande valley near Socorro gives many evidences of the old beveled 

 rock-tloor. 



PLAINS CHAnACTKR OP THE ROCK-PLOOR ITSELF 



In the valleys of La Jara and Arroyo San Pedro, east of the Sandia range, 

 the surface of the rock-floor is exposed for distances of many miles. Ever.y- 

 where the rock-floor is seen to be an even surface independent of the surface 

 deposits." 



" Journal of Geology, vol. xili, 1905, pp. 63-70. 



* Bulletin of the Geological Society of America, vol. 12, 1901, pp. 217-270. 

 '• V. S. Geological Survey, Water S\ipply and Irrigation Paper no. 123, 1905. 

 "Engineering and Mining .Journal, vol. Isviii, 1904, pp. 670-671. 



