COUNTRY WEST OF THE HITH MERIDIAN OF LONGITUDE. 63 



nient of vegetable life. The plains exposed to the drifting sand as well as to climatic severities 

 *re almost wholly deprived of an alluvial coat. A few traces may be looked for at the so-called 

 "■ Playas," (depressions in the plains.) What little rain may fall collects here, and bringing 

 down with it the lighter particles of the surrounding soil, affords a foothold for vegetation, 

 which presents, however, more a mass of equals than a diversity of species and genera. Often, 

 apparently, this premature effort of nature to develop vegetation is sadly counterbalanced by 

 the saline character of the soil, which causes the prevalence of corresponding forms, as obione, 

 salicornia, salsola, chenopodium, and others, in the place of algarobia, prosopis, or even salix, 

 the usual types in analogous localities. 



The plains lying between the mountain ranges are formed of a more or less uniform deposit 

 of loose diluvial sand, its composition not differing essentially from that of the adjoining 

 mountains. This diluvial main may, therefore, be called the debris of the adjacent mountains 

 and of the underlying ma s. 



As to the formation of this diluvial main, we incline to the opinion that it is the residue of a 

 sea once a connecting link between the waters of the Atlantic and Pacific. Changes in the 

 constituents of this deposit certainly occur ; but they are of a local character, besides having a 

 certain uniformity. Fragments of quartz, mica, feldspar, and other similar elements of crystal- 

 line and igneous rock, associated with calcareous particles, constitute the formation of that vast 

 region of deserts stretching from the eastern foot of the California Cordilleras to the table-lands 

 bordering even the Kio Bravo. This section of country may thus be viewed as the bed of an 

 ocean variously intersected by numerous reef-like or dyke-shaped mountain ranges. 



In the immediate vicinity of the mountains, isolated beds of pebbles are sometimes seen, the 

 lithological character of which indicate their origin. These pebbly beds, however, must not be 

 confounded with similar ones occurring occasionally about the centre of the desert basins and 

 frequently along the dry water-courses. The former are the disintegration of the rocks, unmoved 

 from their original locality, whilst the latter are gatherings of an immense area. The latter 

 bear evidence of being brought from the most opposite and most remote geographical quarters ; 

 pieces of limestone representing both the carboniferous and cretaceous periods with tertiary and 

 even traces of lime recently precipitated ; these fragments are mingled with agate, chalcedony, 

 semi-opal, opal, jasper, slates, silicia, breccias, and crystalline and amorphous conglomerates ; 

 here, also, are silicified, agathized, or opalized fragments of wood side by side with pieces merely 

 incrusted — scarcely metamorphosed or entirely unchanged, and of quite a recent geological date; 

 semi-opal, formed entirely of shell, whose age is readily recognized by the numerous nummulites 

 associated with it ; agate, with neat fragments of encrinitic or coraline forms ; jasper or horn- 

 stone, which, under a common lens, discloses both the texture and grain of coniferous wood ; 

 opal, exhibiting traces of the structure of fossil-wood, with distinct annular concentric rings, 

 but no marks of the grain could be detected ; glass-opal, and hyalite, containing casts 

 of some forms of the coral age pisolites, in appearance like a toadstone, which are either 

 unchanged or metamorphic. Tlie deserts of both sides of the Colorado and along the Gila 

 abound with these pebbly beds, surrounded by and occasionally entirely buried in the sand. 

 To the scientific observer they are pearls of this vast terrestrial ocean, which once formed the 

 bottom of a sea, whose currents in all probability collected these pebbly deposits. Since the 



