246 



KEMP'S ORE DEPOSITS. 



softer strata, breaking at times across the harder rocks. The 

 channel filling consists of gravel, sand, and clays, volcanic tuffs, 

 and firm basalt. With these are great quantities of silicified 

 trees, and even standing trunks project through some beds. The 

 gravel is oftenest formed of white quartz boulders, but may con- 

 tain all the metamorphic rocks of the neighborhood, and even 

 boulders brought from a great distance. The gravel at times is 

 cemented together by siliceous and calcareous matter, and then 

 requires blasting ; but loose gravel also occurs. The clays are 

 locally called "pipe clays," and are often interbedded with sand 

 layers. They are blue when unoxidized, giving rise to the term 

 " blue lead," but red oxidized clays are not infrequent. The clays 



FIG. 61. Generalized section of a deep gravel bed, with technical terms. 

 After H. E. Browne, Rep. Cal. State Mineralogist, 1890, p. 437. 



contain many leaf impressions of species thought by Lesquereux to 

 be late Tertiary. The gravels also contain bones of extinct verte- 

 brates, and have afforded some authentic human remains and 

 stone implements of good workmanship. The volcanic tuffs have 

 been strong factors in modifying the original drainage lines. 

 They have flowed into the ancient valleys in a state of mud and 

 have then consolidated. 



2.12.10. The richest gravels are those nearest the bed rock. 

 In these the distribution of the gold is governed more or less by 

 the character of the ancient channels. It favors the inside of 

 bends and the tops of steeper runs. The gradients of the old 

 channels were fairly high, often running 100 to 200 feet per mile. 

 Gold has also been found by assay in pyrite that has been formed 

 in the gravels since their deposition, and from this it is evident 

 that the precious metal does circulate in solution with sulphate of 



