GEOLOGICAL STRUCTURE EASTERN RANGE. 507 



the case may be, indicate that these rocks are to be found in place near 

 by and not far from the surface. In some cases the ground is whitened 

 over considerable areas by the abundant small fragments of vein quartz 

 resulting from the disintegration of the underlying slates. 



The relative age of the interior lake beds may be assumed to bear 

 some relation to that of the calcareous conglomerates already mentioned 

 which partially filled the earlier canyons of the Pacific slope. After the 

 de2:>osition and subsequent elevation of the mesa sandstones, which are 

 assumed to be of late Tertiary age, there must have been a long period 

 of erosion, during which the interior valley was carved out and drained 

 through the deeper canyons running to the Pacific ocean. This was 

 apparently followed by an extensive submergence of 2,000 feet or more, 

 since which time the whole peninsula has been gradually rising by 

 periodic movements, with considerable baseleveling in the intervals.' 



The present elevation of the mesa-topped ridges of the western range 

 indicates a baseleveling of the region at an elevation of about 2,000 feet 

 above present sealevel. This might have filled up all the outlets of the 

 interior region across the western range and admitted of the enclosure of 

 a body of water up to that level, but to account for the present position 

 of the deposits on the eastern side of the valle}^ it is necessary to assume 

 a subsequent differential movement by which that side has been raised 

 a few hundred feet more than the eastern side. 



Eastern Range. — The older or buried eastern range is made up of gran- 

 ite and gneiss, with highly altered sedimentary strata flanking it on the 

 northeast which stand either vertical or with a steep dip to the eastward 

 and strike about northwest, or somewhat more to the west of north than 

 the general trend of the peninsula. The present divide, on the other 

 hand, follows the general trend of the coastline at a distance of 10 to 15 

 miles from it, and is marked in general by abru'pt escarpments along the 

 eastern edge of the desert plain. 



To the north of the limits of the field of ol)servation, beyond the thir- 

 tieth parallel, the summits of the older range have been planed off and 

 their depressions so evenly filled up by the more recent deposits that 

 they play no part in the present topography of the countr3^ To the 

 south, however, where east of the present divide they have been denuded 

 of the more recent deposits, or still further south, where they were never 

 completely covered by these deposits, they form conspicuous and strik- 

 ing topographical features, in marked contrast with the prevailing hori- 

 zontal lines and broad, shallow valle3^s of the western portion of the 

 peninsula. 



In the northern region the desert plains and flat topped ridges of mesa 

 sandstones rise very gently from the west to the divide line, which al- 



