











342 



RESUME AND THEORETICAL DISCUSSION. 



Before making any comparisons between the dimensions of the channels 

 of the gravel period and the river beds of the present epoch, it will be necessary 

 to revert to the fact, already clearly established, that the area drained by the 

 Pliocene rivers was not, on the whole, essentially different in size from that 

 which the present ones drain. Were this otherwise, it would be impossible 

 to draw any trustworthy conclusions with reference to the special points 

 before us for consideration; for if we could conceive the topography of the 

 Pacific Coast so altered that the character of the drainage area should be 

 essentially modified, we could institute no comparisons of value between 

 the ancient and the present rivers. But it has been already abundantly 

 shown that the chain of the Sierra was essentially, in Tertiary times, what it 

 now is, so far as concerns its general elevation and the position and direction 

 of the principal river basins into which its western slope is divided. Hence, 

 the existence in former geological times of very much larger channels than 

 any which the present streams now occupy is sufficient proof of the flowing 

 of a much larger amount of water from the same area. To be sure, we can- 

 not be certain that the number of streams reaching the Great Valley in 



* 



Pliocene times was exactly the same as now; there may formerly have been 

 a more general concentration of tributaries into main rivers than now exists. 

 For instance, it is quite probable that the Pliocene equivalent of Bear River 

 joined the Pliocene American at an elevation of 2,000 feet or more, instead 

 of debouching in the Sacramento Valley, as it now does. This would account 

 in part for the .magnificent dimensions of the ancient river-course along 

 the divide between the Middle and North Forks of the American River, the 

 width of which channel Mr. Goodyear estimates at 4,000 feet, or possibly 



a mile.* 



Tw r o grand rivers flowed down the western slope of the Sierra in Pliocene 



times, one representing the present American with its tributaries, and with 

 which Bear River was probably connected, as suggested above ; the other 

 was the equivalent of the present Yuba. Both of these streams were of such 

 dimensions that their now existing representatives are but rivulets in com- 

 parison. Both the Pliocene Yuba and the Pliocene American rivers can be 

 traced far down into the Great Valley by their broadly spread deposits of 

 travel and the usual accompanying volcanic masses.! The width of the 



* See ante, p. 106. 



t The extension down into the Great Valley of the gravels and volcanic deposits 



belonging to the Yuba 



system is well seen in the General Gravel Map : they reach nearly as far as Marysville. The shape of the map 

 does not admit of tracing the Pliocene American in the same way. 



