THE PACIFIC SLOPE. 



247 



iron, but on this slender foundation some quite unwarranted 

 chemical hypotheses for the origin of nuggets have been based. 

 Substantially all the gold has been derived by the mechanical 

 degradation of the quartz veins in the slate. 



2.12.11. The depths to which the modern streams have cut out 

 their channels below the old drainage lines have received con- 

 siderable attention. Whitney avers that no disturbance has taken 

 place since the old gravels were laid down, but Leconte thinks 

 that there has been a tilting or elevation of the higher parts of 

 the range, all moving as a block. Becker has recently shown in the 



5000 



19 ZO l\ ZZ 23 Z.4- 5 2627 



FIG. 62. Section of Forest Hill Divide, Placer County, California, to il- 

 lustrate the relations of old and modern lines of drainage. After 

 R. E. Browne, Rep. Cal. State Mineralogist, 1890, p. 444. 



high portions a great series of small north and south faults with 

 uniform downthrow on the western side or upthrow on the east- 

 ern. (See paper below, cited from Geological Society of America.) 

 This is of varied intensity in different portions and is limited to 

 the strip just west of the summit. It occurred in the Pliocene 

 and increased the gradient of the streams where the present deep 

 canons occur, but had no effect near the plains, where the old and 

 new channels are nearly on the same level. 



2.12.12. After the formation of the deep gravels and after the 

 volcanic flows, glaciation took place in great extent over the 

 mountain sides, but it was doubtless later in time than the glacial 

 period of the East. References to the similar great development 

 of the ice in Washington have already been made. Many hypoth- 

 eses were early advanced to explain the deep gravels. They 



