6 GEOLOGY. 



gently toward the main bed of the valley. This plain, in receiving the drainage from the ad- 

 joining mountain ranges, is variously cut up hy deeply-trenched arroyos, and terminates on the 

 alluvial tracts below in gravelly bluffs of variable height. 



This table-land is encountered wherever the course of the Kio Grande is not hemmed in by 

 precipitous rocky cliffs, and is seen forming a belt of variable width on both sides of the river, 

 extending to the base of the adjoining mountains. In all these situations it presents very uni- 

 form features. 



An obvious analogy will be at once perceived between the latter formation and the wide- 

 spreading upland alluvial plains, before noticed ; in fact, a direct continuous connexion between 

 them may be often traced. They evidently belong to the same general formation, representing 

 basins filled up with alluvial and diluvial depositions, concealing, it may be, older tertiary 

 strata below. 



The pebbles contained in this formation can readily be traced to their original sources in the 

 adjoining mountains, being of larger size and more angular near the base of the mountains, and 

 smaller and more rounded at a greater distance. The earthy medium is generally a coarse sand 

 or fine marl, argillaceous matter being less frequent. Occasionally the exposed bluffs show de- 

 posits of gypsum, which in some localities forms extensive beds. The most usual form of this 

 material is in confused crystalline and fibrous masses, imbedded in loose marl. At other places 

 a calcareous chalklike deposition is met with, occupying usually the upper stratum of the table- 

 land. 



A general saline character, pertaining to this formation, is also evidenced in the growth of 

 saline plants or direct salt efflorescence in the lower depressions of valleys. 



SKCTION OF EARTHY TABLE-LAND FORMING THE BLOFFS OF THE RIO BRAVO ABOVE EL PASO, CORRESPONDINS WITH THAT FORMING THE 



"JORNADA DEL MUERTO," TO THE NORTH. 



A. Highly calcareous marl, chalklike, with occasional pebbles. 



B. Brownish gray sand, with nodules of clay. 



C. Yellow ferruginous marl. 



D. Debris of drifted sand and washed clay. 



As seen from any high mountain elevation, this table-land sweeps with all the exactness of a 

 sheet of water, encircling as with a shore-line the bases of distant mountains, frequently com- 

 pletely insulating peaks and ridges, and everywhere masking the true connexion of the various 

 formations. 



The progress of subsequent drainage is also plainly seen in the various terraced elevations 



