568 



CENOZOIC ERA— AGE OF MAMMALS. 



of the country about the upper Yuba, and Fig. 937 is an ideal section 

 across the river-beds along the line N S, Fig. 936. It is seen that there 



Fig. 937.— Section along the Line, north and south: r' r', old river-beds; r r, present river-beds; L, 



lava; si, slate. 



are remnants of old lava-flows on the divides between the deep river- 

 channels. Beneath these lavas there are river-gravels, and beneath these 

 trough-shaped river-beds, with their smoothly and variously eroded bed- 

 rock. The section shows, moreover, that the present rivers have com- 

 menced their beds on the old divides (shown by dotted lines), but have 

 cut much deeper, leaving the old beds high up on the present divides. 



Such are the facts The history of the process is briefly as follows : 

 At the end of the Tertiary there was an outburst of lava near the crest 

 of the Sierras.* The lava flowed down the river-beds, filled them up, 

 and displaced the rivers. These immediately commenced cutting new 

 beds on the old divides, because the lava was thinner or absent there. 

 Now, coincidently with the lava-flow, there was an elevation of the 

 Sierra crest by tilting of the Sierra crust-block (p. 265), and therefore 

 increase of the Sierra slope. Such tilting was attended with enormous 

 displacement or faulting on the eastern side, as already explained 

 On account of the increased slope, the rivers seeking their base- 

 level cut down far below their previous level. This increase of slope 

 is shown, if possible, still more plainly, in some of the rivers of the 

 southern part of the State, beyond the limits of the lava-flow. Here 

 also the tilting and the increased slope took place, but the rivers were 



Pig. 938.— Ideal Section across a River-bed in Southern California beyond the Region of the Lava- 

 flow. 



not displaced. They, therefore, retained their beds, but deepened their 

 channels, leaving the old river-gravels high up on their sides (Fig. 938). 



* The great lava-flooding of the Tertiary commenced probably at the end of the Mio- 

 cene, and continued through the Pliocene. These flows, in California, seem to have been 



the last. 



