HISTORY OF THE SACRAMENTO RIVER. 327 



During the Pliocene, when the coast region stood 2,000 to 2,500 feet above 

 its present level, the Bay of San Francisco did not exist, except as a valley 

 between mountain ridges. The waters of the united Sacramento and San 

 Joaquin rivers at that time flowed through the region now occupied by the 

 bay, then through the valley continuations of the same southward and into 

 the Pacific ocean by the submarine channel of Monterey bay. About the be- 

 ginning of the Quaternary, the orogenic changes which submerged its lower 

 course cutoff also its mouth and opened another through the Golden Gate.* 



Comparison of Eastern and Western Elver Beds. — Nothing can be more 

 interesting than a comparison of the river beds of the two coasts of the 

 United States, and the changes they have undergone, both in their upper 

 and in their lower courses. On the eastern coast the old or Pliocene river 

 beds underlie the present river beds ; the new are in the same places as the 

 old, but at a higher level, because by continental subsidence the land is lower 

 now than then. In California, on the contrary, the Pliocene rivers were dis- 

 placed from their beds by lava streams ; and the new beds have been cut far 

 below the old beds, because by mountain elevation the height and slope of 

 the Sierra have been greatly increased since that time. An exactly similar 

 difference is found in the case of the lower courses, and is due also to similar 

 causes. In the east the submarine channels are a direct continuation of the 

 old river beds, because the changes were continental, not orogenic. In Cali- 

 fornia, on the contrary, in addition to these continental movements which 

 submerged their lower courses, there were also orogenic changes in the Coast 

 ranges, by which the places of discharge of the rivers into the sea were 

 entirely changed. These two events, viz., the displacement of the upper 

 courses of the rivers in the Sierra and the displacement of the river-mouths 

 on the coast, were probably coincident, or nearly so. 



Brief History of the Elvers and Eanges of California. — The Sierra was 

 formed, as we now know, by lateral crushing and strata-folding at the end 

 of the Jurassic. But during the long ages of the Cretaceous and Tertiary 

 this range was cut down to very moderate height, with gentle slopes east- 

 ward and westward from a crest which was probably situated along a line 

 just above Jhe Yosemite and Hetch-hetchy valleys ; for there the erosive 

 biting into the granite axis seems to be deepest. The rivers by long work 

 had finally reached their base-levels and rested. The scenery had assumed 

 all the features of an old topography, with its gently flowing curves. The 

 continental elevation of the Pliocene did not affect greatly the river slopes 

 of this part. At the end of the Tertiary came the great lava streams run- 

 ning down the river channels and displacing the rivers; the heaving up of 

 the Sierra crust-block on its eastern side forming the great fault-cliff there 

 and transferring the crest to the extreme eastern margin ; the great increase 



* Whitney concludes on other grounds that the break forming the Golden Gate was made during 

 or at the end of the Pliocene.—" Auriferous Gravels," 1880, p. 26. 



