334 Mr. J. A. Phillips's Notes on the Chemical Geology 



in which it is associated with clay, sand, and gravel, from which 

 it is separated by washing. 



These gold-bearing drifts belong to at least two geological 

 epochs, both comparatively modern, although the later period is 

 distinctly separated from the earlier, its materials being chiefly 

 derived from the disintegration and redistribution of the older 

 deposits. 



In California the more ancient deposits, or deep placers, are 

 probably referable to a river-system different from that which 

 now exists, flowing at a higher level or over a then less elevated 

 continent, and frequently nearly at right angles to the direction 

 of the main valleys of the present period. 



The sources from which the supply of gold is derived are 

 therefore the following : — 



(1) Auriferous veins, most frequently enclosed in metamorphic 

 slates. 



(2) The distribution of gold by ancient river-systems, giving 

 rise to deep diggings. 



(3) The redistribution of placer gold by the present river- 

 system, forming accumulations known as shallow diggings. 



The deep placers are in many localities covered by a thick 

 capping of lava ; and in some places the eruptive matters cover- 

 ing the auriferous deposits occur in the form of basaltic co- 

 lumns, beneath which are found the layers of sand, gravel, and 

 boulders with which gold is associated. The wood which occurs 

 in these gravel-beds is either beautifully silicified, or is replaced 

 by iron pyrites. In such localities it not unfrequently happens 

 that a piece of wood will be observed of which one end had been 

 converted into lignite, whilst the other remained unaltered; but 

 the whole having subsequently become silicilied, now presents 

 the appearance of a combination of alabaster and jet, each por- 

 tion still retaining the structure of the original wood. 



In the more clayey strata of these sedimentary deposits, leaf- 

 beds and impressions of leaves are not unfrequently found; and 

 an examination of these made by Dr. Newberry authorizes the 

 conclusion that the auriferous deposits lying beneath the lava 

 are of tertiary age, and that in all probability they generally 

 belong to the later Pliocene epoch*. 



In many localities, and particularly between the south and 

 middle forks of the Yuba River, these auriferous gravels have 

 frequently, where exposed to denudation, a thickness of 120 feet, 

 and of more than 250 feet where they have been protected by a 

 volcanic capping. These vast auriferous beds are composed of 

 rounded masses of all the eruptive and metamorphic rocks which 

 occur above them in the Sierra. As a general rule, the lower 

 * Geological Survey of California, pp. 250 & 251. 



