508 EMMONS AND MERRILL — SKETCH OF LOWER CALIFORNIA. 



most invariably presents an abrupt escarpment to the east, overlooking 

 a region deeply scored by narrow gorges several hundred feet in depth, 

 with almost vertical walls. Here the divide line is marked by occasional 

 isolated table-topped buttes, capped by rhyolite, which rise 500 to 1,000 

 feet above the desert level and serve to mark the original level of the 

 mesa sandstones, which have been protected from erosion by the cap of 

 more enduring rock. These rhyolites are generally of earlier date than 

 the lake beds. The top of the mesa sandstones as thus determined is 

 about 3,000 feet above present sealevel, and their maximum observed 

 thickness 800 feet. Augite-andesite flows, apparently of more recent 

 date, are found capping intermediate portions of the divide. The con- 

 trast in topographical structure between the regions east and west of the 

 divide is here less marked than in the region to the south, as on both 

 sides approximately horizontal lines prevail. The surface of the mesa- 

 topped ridges slopes upward toward it from either direction, but the 

 slope is much greater on the eastern side, and the ridges descend toward 

 the Gulf in a series of step-like terraces, while the whole eastern region is 

 deeply scored by narrow steep sided ravines from a few hundred to a 

 thousand feet in depth. The upturned beds of the metamorphic series 

 are well exposed along the walls of these ravines, often reaching the 

 surface of the intervening mesas. They are also seen in the shallow 

 stream beds of the desert plains on the west, and, as already remarked, 

 often outcrop through the thin covering of the lake beds for a consider- 

 able distance out on to the desert. 



South of the thirtieth parallel the summits of the buried range rise gradu- 

 ally, and east of the divide are completely denuded of any covering of 

 recent beds that they may have had. They also spread out to the east- 

 ward, approaching more and more closely to the Gulf coast, and south 

 of the limits of the field of observation, or 20 miles south of the thirtieth 

 parallel, they constitute a high granite range extending 10 or 15 miles 

 westward into the interior valle}^ and effectually cutting off any view of 

 the country beyond. 



The region in the vicinity of the New Pedrara onyx deposits, a few 

 miles south of the thirtieth parallel, shows well the general structure of 

 the eastern range as presented in generalized form in the section on plate 

 19 and will hence be described in some detail. 



The principal onyx deposits are situated in a shallow ravine or eastern 

 arm of the interior valley, between two ridges of mesa sandstone, at an 

 elevation of about 2,300 feet. Since the denudation of the granite bed 

 of this ravine of its former covering of. mesa sandstone it has been filled 

 to a depth of about a hundred feet by alternate beds of travertine and 

 calcareous conglomerate, which were probably contemporaneous and at 

 one time continuous with the lake beds of the interior valley. 



