64 GEOLOGY. 



water has receded, an ocean of a more subtile character sweeps over this area, ^rial currents 

 are now driving the shifting sand from place to place as the waters of the sea once did. 



Besides the general inclination of the western slope of the Sierra Madre towards the Gulf of 

 California, an increased inclination of stratum is perceptible around the bases of the intersecting 

 sierras This does not, however, aifect the mean ascent of the main land, and may be ascribed 

 solely to the deposition of debris, as the angle formed by the inclination of the diluvial deposits 

 was observed (particularly in the valley of the Santa Cruz river) to be =: 2.5°. 



The general ascent already referred to is conclusive proof of the action of upheaval forces 

 since the deposition of the quaternary or diluvial drift. A straight line over its surface from the 

 valley of the Colorado to the foot of the Sierra del Pajarito, where it ceases, gives a grade of 

 12.44 per mile, or 0.23 ft. in one hundred. In some valleys heading on the slopes of this sierra 

 this deposit may be seen ; but its occurrence in such localities being exceptionable, does not 

 affect the mean angle of inclination of the stratum. 



The height to which this diluvial main rises, in its approach to the Sierra Madre, gives a 

 striking peculiarity to the features of the country. But for it the rugged crests of the sierras 

 would be scarcely accessible. 



If the climatic conditions were favorable, these now bleak and forbidding mountains would 

 present a region teeming with vegetable and animal life. Instead of that, this country now lies 

 an arid waste. 



The few periodical streams descending from the mountain sides share a similar fate, for no 

 sooner do they reach this drift than they disappear from the surface, sinking to unknown depths, 

 and leaving only in the vicinity of the mountains slight marks of rudimentary drainage, clumps 

 of shrubbery bordering dry water-courses. 



The vegetation peculiar to the diluvial main is similar to that of the corresponding localities 

 on the eastern side of the Sierra Madre and west of the Colorado. Besides smaller and more 

 inconspicuous forms, are the Larrea, Fouquiera, Obione, and other chenopodiaceous shrubs ; 

 there are also a variety of leguminous plants, numerous members of the Cacti family, and some 

 few bushes and trees, all well known to the traveller whose fortune has led him through these 

 desert regions. 



In passing to the consideration of the underlying strata — those upon which the diluvial 

 deposits rest — a deep step is made at once ; constituents of the secondary age seem to be wanting. 

 Crystalline rocks of primary and transition age — more or less metamorphic — constitute the bed 

 upon which the diluvial deposit lies. This bed does not occur as an even or slightly inclined 

 plane, for its surface is variously broken by eruptive masses. These upheavals have not only 

 disturbed and protruded through the primary and metamorphic strata, but carried with them- 

 selves masses of the latter above the level of the supercumbent deposits. Thus are formed the 

 mighty sierras now representing the frame-work of our geological edifice^ most of which may be 

 distinguished as Pluto-volcanic. 



With the hypsometrical features of these sierras, better called Cordilleras, three important 

 peculiarities are connected. These are, 1st. Parallelism among themselves, with the Gulf of 

 California, and with the Pacific coast. 2d. Articulation. 3d. General petrographic relation- 

 ship. 



