326 J. LE CONTE — ATLANTIC AND PACIFIC COAST CHANGES. 



^reat lava-flows ; similar flows are found also in abundance in the region 

 south of the Bay of San Francisco. None of these flows are older than the 

 Pliocene, and many of them undoubtedly belong to the Quaternary. There 

 is, therefore, abundant evidence of orogenic changes in late Pliocene and 

 early Quaternary times sufficient to divert the lower courses of the rivers. 



In brief, then, the general character of the changes in the Coast range 

 region was as follows : During the Miocene the coast line was somewhere to 

 the east of the Coast range, and the place of that range was marginal sea- 

 bottom. At the beginning of the Pliocene the Coast range was formed, and 

 the coast line was transferred westward beyond its present position to the 

 border of the submarine plateau. During the Pliocene, continental eleva- 

 tion commenced, and culminated at its end or perhaps in the early Quater- 

 nary. All the islands bordering the coast, especially the high islands off* the 

 coast of southern California, Avere added to the continent. Meanwhile the 

 rivers, whether rising in the Coast range or breaking through gaps in that 

 range, cut their channels deeper and deeper. At the beginning of the Qua- 

 ternary, coincident with the great orogenic changes and lava-flows of the 

 Sierra, there occurred also great lava-flows in the coast region which modified 

 the orographic forms of the Coast range and changed the lower courses of 

 the rivers. Soon after these orogenic changes the coast region went down to, 

 or indeed considerably below, its present level and the deserted lower chan- 

 nels were submerged. The fact that they were deserted, and that therefore 

 they were unmodified by subsequent sedimentation, is the reason they run 

 in so near shore and are so distinct. From this subside4 condition, abun- 

 dantly shown by elevated sea margins both on the mainland and on the high 

 islands off the southern coast, the land was again raised to its present level. 

 This, however, is far below its former position, and therefore the channels 

 remain submerged. 



Changes in Elvers. — Concerning the courses of the Pliocene rivers which 

 cut these channels we know nothing and it is vain to speculate ; this must 

 be left to future investigation. But there is one river, and that the greatest 

 in California, concerning which some words may be not wholly profitless. 



At the present time the tributaries of the Sacramento and San Joaquin 

 pour their united waters into the Bay of San Francisco and through the 

 Golden Gate into the Pacific. But this outlet certainly did not exist in 

 Pliocene times, for there is no submarine channel off the Golden Gate. 

 Where then did the river empty at that time? Probably far southward 

 into the Pacific, off" the Bay of Monterey. Professor Davidson tells me that 

 a depression of 100 feet at the divide between the bays of San Francisco 

 and Monterey would now empty the waters of the former into the latter. Is 

 it not probable, then, that the deep channel running in close to shore in 

 Monterey bay may be the submerged Pliocene outlet of this great river? If 

 so, then the history of this river may be as follows : 



