396 



Mr. J. A. Phillips on the Chemical [Mar. 12, 



most auriferous portions of veins, the cavities are numerous but exceedingly- 

 small, and are often so opaque, apparently rendered so by being internally- 

 coated with a lining of clay, that no vacuities can be distinguished. 



Out of more than fifty sections of veinstone examined, only some six 

 or seven -were found to contain fluid-cavities of sufficient size to admit of 

 any attempt at accurate measurement by means of ordinary appliances ; 

 but in all cases there appeared to be considerable differences in the rela- 

 tive dimensions of the vacuities and the enclosing cavities, and the tem- 

 peratures at which they severally became filled were consequently ascer- 

 tained by direct experiment. In every instance they were found to require 

 very different degrees of heat to become full, since in the same specimens 

 some of the vacuities disappeared at 180° Fahr., others filled at tempera- 

 tures slightly above that of boiling water, whilst many, though much re- 

 duced in size, remained perfectly visible at 365° Fahr. 



Alluvial Deposits. — Although a very large amount of the gold annually 

 obtained was no doubt originally derived from auriferous veins, not more 

 than about one-third of the precious metal collected is procured directly 

 from that source. The larger proportion of the gold now brought into 

 the market is derived from alluvial diggings, in which it is separated by 

 washing from the clay, sand, and gravel with which it is associated. 



This gold-bearing drift belongs to at least two distinct geological epochs, 

 both comparatively modern — although the latter period is distinctly sepa- 

 rated from the earlier, its materials being chiefly derived from the disinte- 

 gration and redistribution of the older deposits. 



In Cahfornia the more ancient deposits or " deep placers " are referable 

 to a river-system different from that which now exists, flowing at a higher 

 level, and frequently nearly at right angles to the direction of the main 

 valleys of the present period. 



The deep placers are in many localities covered by a thick capping of 

 lava ; and the eruptive matter covering them often occurs in the form of 

 basaltic columns, beneath which are found the layers of sand, gravel, and 

 boulders with which the gold is associated. The wood which occurs in 

 these gravel-beds is either beautifully silicified, or is replaced by iron 

 pyrites. 



In the more clayey strata of these 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 they probably belong to the 

 later Pliocene epoch. Water-worn gold is disseminated throughout the 

 whole mass of these deposits, not, however, with uniformity, but always in 

 greater abundance near the bottom, and more particularly in direct contact 

 with the "bed rock," which is invariably grooved and worn by the action 

 of water. 



The materials of which these deep placers are composed are frequently 

 consolidated into a sort of hard concrete, by being firmly bound together 



