AURIFEROUS GRAVELS—INDEPENDENCE HILL FLORA 8S3 



niickllc and lower slopes of ihis range did the gravels accumu- 

 late. 



In the principal broad Neocene river valleys one may gener- 

 ally distinguish the following formations : 



A. The Ante-Volcanic Deposits. 



1. The deep gravels. — These, usually hard and compact, 

 coarse gravels fill the deepest trough-shaped depressions to a 

 maximum depth of about 200 feet, usually lOO to 150 feet. 



2. The boicli gravels.- — Covering the deep gravels and attain- 

 ing a maximum depth of 300 feet, the bench gravels are spread 

 out often to a width of one or two miles on the sloping shelves 

 on both sides of the deepest trough. They are frequently very 

 quartzose and more admixed with finer sediments than the deep 

 gravels. 



B. The Volcanic and hiter-Volcanic Deposits. 



3. The rhyolitic tuffs. — Sweeping down the main river chan- 

 nels from the vents in the high Sierra, the flows of white rhyo- 

 lite, accompanied by large masses of rhyolitic tuff, of clayey and 

 sandy character, covered the bench gravels. These rhyolitic 

 flows attain on the middle slopes a maximum depth of about 

 200 feet, while higher up they are much heavier. Much of 

 this tuff is in the mining region designated as pipe-clay or chalk. 



4. The gravels of the rhyolitic period. — The effects of the 

 rhyolitic flows were to dam many lateral streams, thus causing 

 immediate accumulations of gravels, clay, and sands. During 

 the intervals between the rhyolitic eruptions the streams cut 

 down new channels in the soft material and accumulated masses 

 of gravel in their new beds. All these detrital masses of gravel, 

 sand, and clay, generally of a finer character than the bench 

 gravels, and usually containing rhyolitic pebbles, are designated 

 as the ''gravels of the rhyolitic period." In many places, such as 

 Nevada City, Nevada county, and the Long Canyon divide, 

 Placer County, the rhyolitic orravels attain a thickness of several 

 hundred feet. 



