512 extmons and merrill — sketch of lower california. 



General Conclusions. 



In comparing the thirtieth parallel section of the Lower California 

 peninsula, as indicated by the above noted observations, with that de- 

 scribed by Lindgren near the thirty-second parallel, the most striking 

 fact is the parallelism in structure of the older pre-Cretaceous mountain 

 mass. In either case there is an eastern uplift of granite and metamor- 

 phic slates, and a western one composed largely of eruptives probably of 

 more recent age than the granite. The sedimentary beds or metamor- 

 phics, with their associated intrusives, bear evidence of intense dynamic 

 action and consequent alteration, and of long continued erosion and 

 degradation. That they have been to a much greater extent removed 

 in the higher northern region, and that the underlying granite conse- 

 quently occupies a so much larger proportion of the exposed surfaces, is 

 in so far an evidence that the relative difference of level of the two regions 

 had existed prior to the Cretaceous transgression, which left the northern 

 region exposed to continuous degradation, while it protected the south- 

 ern region, so that its present conditions beneath the covering of more 

 recent beds represent those which might have prevailed in the northern 

 region before that time. 



The petrographical parallelism between both sedimentary and eruptive 

 rocks of the respective regions is also very close, except in the case of the 

 granite of the western uplift, but the examination of the southern granite 

 body was so superficial that one is hardly justified in considering this 

 apparent difference as essential. The metamorphic slates in either region 

 are, moreover, gold-bearing and the granite apparently not. The eastern 

 escarpment in either case represents a probable line of displacement, but 

 in the southern region it is less sharply defined and may have been 

 distributed over several lines of faulting. ' The evidence is here rather 

 opposed to Lindgren's assumption that the great movement is of very 

 recent date, since the present position of the recent beds on either side 

 of the divide indicates a relative change of level since their deposition 

 that is to be measured by hundreds rather than thousands of feet. 



On the other hand, the general deduction that the depressed area of 

 the gulf of California and its continuation in the desert valleys to the 

 north bears the same relation to the Peninsular Sierra that the Great basin 

 does to the Sierra Nevada appears to be borne out by what is known of 

 the general structure of the former; and if, like the Great basin, this at 

 present depressed area was once relatively higher than that which now 

 constitutes the mountain range, it would have formed a barrier in the 

 ancient ocean which would account for the faunal differences which 

 appear to prevail between the Cretaceous life as now known on the 

 Pacific coast, and in Mexico and Texas. 



