602 



CEXOZOIC ERA— AGE OF MAMMALS. 



ent river-beds, R'. In this locality G represents the deep placers, and 

 G' the superficial placers. 



The history of changes shown in these sections is sufficiently obvi- 

 ous. In the time of the old river-system, R was a river-bed, doubtless 

 with a ridge on either side represented by the dotted lines. In this 

 bed accumulated gravel, containing gold. Then came the lava-flow, 

 which of course ran down the valley, displacing the river and covering 

 up the gravels. The displaced rivers now ran on either side of the 

 resistant lava, and cut out new valleys, 2,000 feet deep, in the solid 

 slate, leaving the old lava-covered river-beds and their auriferous gravels 

 high up on a ridge. The deeper cutting was the result of the higher 

 slope. In other cases the convulsion which ejected the lava also 

 changed greatly the direction of the slope of the country, and there- 

 fore the direction of the streams. In such cases of course the present 

 river-system cuts across the old river-beds and gravels, and their cover- 

 ing lavas, as shown in Fig. 966. 



Fig. 966.— Lava-Stream cut through by Rivers: a, a, basalt; b, b, volcanic ashes; c, c. Tertiary; 

 d, d. Cretaceous rocks: i?, i?, "direction of the old river-bed; R', R', sections of the present 

 river-beds (.from Whitney). 



Age of the River-Gravels. — The age of the old river-gravels is still 

 doubtful ; that of the newer river-gravels is undoubtedly Champlain or 

 early Terrace. Below we give a list, taken from Whitney, of the re- 

 mains found in these gravels : 



f Great mastodon. 



| Mammoth. 



x - , Bison. 



Is ewer placers. <j Taph . modern> 



Horse, modern. 

 [_ Man's works. 

 f Great mastodon.* 



Mammoth. 



Deep placers. 



Mylodon. 

 Tapir, modern. 

 Rhinoceros (ally). 

 Hippopotamus (ally). 

 Camel (ally). 

 Horse, extinct species. 



It will be seen that the fauna of the deep placers unite Pliocene 

 and Quaternary characters. The great mastodon, the mammoth, the 



* Whitney states (Geological Survey of California, vol. i, p. 252) that neither the 

 mastodon nor the mammoth is found in deep placers ; but both have since been found 



there. 



