GEOLOGY AND NATURAL HISTORY OF LOWER CALIFORNIA. 983 



earlier date than the lake beds. The top of the mesa saudstones as 

 thus determined is about 3,000 feet above preseut sea level, and their 

 maximum observed thickness 800 feet. Augite andesite flows, appar- 

 ently of more recent date, are found capping intermediate portions of 

 the divide. The contrast in topographical structure between the region 

 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 considerable distance out on to the desert. 



"South of the thirtieth parallel the summits of the buried range rise 

 gradually, and east of the divide are completely denuded of any cov- 

 ering of recent beds that they may have had. They also spread out to 

 the eastward, approaching more and more closely to the gulf coast, and 

 south of the limits of the iield 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 valley and effectually cutting oft 

 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 1, 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 sand- 

 stone, 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 100 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. 



"The winding bed of the modern stream cuts into the travertine 

 deposit, exposing at one place a cliff of over 20 feet in height, showing 

 three distinct layers of " Mexican onyx," one of which is over 3 feet thick, 

 interstratified with the travertine, while for a distance of nearly a mile 

 down the ravine sheets of the more resisting onyx cap the little traver- 

 tine mesas on either side. The occurrence of the onyx, which is a ther- 

 mal spring and surface deposit in successive layers, separated by traver- 

 tine and resting on conglomerate, indicates a probable successive rise 

 and fall of the waters of the lake where the travertine was deposited, 

 which would have admitted of some slight erosion of the deposit in 



