THE TERTIARY PERIOD 



447 



8. In Thibet, marine limestone of Eocene age has been found at 

 altitudes of 20,000 feet. How much of the history of Thibet may 

 be inferred from this 

 fact? 



9. On the west- 

 ern slope of the 

 Sierra Nevada range 

 there are flat-topped 

 ridges capped with 

 sheets of lava (Fig. 

 466). Beneath the 

 lava gold-bearing 

 gravels have been 

 found. Can you 

 suggest how the 

 present conditions 



FIG. 465. Conglomerate, 

 sandstone, and shale, in 

 contact with a mass of 

 granite (Oregon). 



FIG. 464. Alternate 

 shale, sandstone, and 



conglomerate (Colo- ^ere brought about ? 

 What would be the 



FIG. 466. Diagram of a 

 lava-capped ridge in Cali- 

 fornia. 



best method of mining the gold in these deposits ? 



