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236 



THE AURIFEROUS GRAVELS OF THE SIERRA NEVADA. 



elevation, large masses of fossil wood are found in the volcanic deposits ; and 

 in Plumas County the same occurrence has been noticed on several of the 

 highest mountains in the region, as Penman's Peak and Clermont, peaks from 

 7,000 to 8,000 feet high. The impressions of leaves, of course, are chiefly 

 limited to the finer clays and tufaceous beds, and in these are often preserved 

 in the greatest perfection ; except that while small fragments are abundant, 

 whole leaves are very difficult to procure. Fragments and often large masses 

 of wood are found, both in the gravels and the associated clayey and tufa- 



■ 



ceous beds. In the gravel they frequently bear the marks of transportation 

 from a distance, as would be expected. 



Much the larger portion of the wood found in the volcanic and gravel 

 deposits is almost or quite completely silicified. This process has usually 

 had, as its result, the conversion of the woody fibre into what is called by 

 mineralogists, opal, — that is, quartz in the amorphous condition; for, although 

 water appears to be almost or quite invariably present, yet it is regarded 

 as not essential to the composition of the mineral. Specimens of opal, on 

 analysis, are usually found to contain from six to ten per cent of water ; 

 rarely does the percentage fall as low as three. A piece of fossil wood from 

 the Sierra Nevada, which was placed in the hands of Mr. S. P. Sharpies for 

 analysis by the writer, was found to have the following composition : 



Silica . 



Oxide of iron . 



Water, and organic matter . 



90.00 

 3.27 

 6.29 



99.56 



The opalized wood is often translucent, and sometimes almost transparent. 

 It has usually a vitreous or almost resinous lustre, and is of various shades 

 of gray or whitish-gray. Not unfrequently, however, it is light-green or 

 even yellow, or of varying tints, the colored portions contrasting beautifully 

 with darker bands of brilliant lustre. These latter seem to be the result of 

 an incipient carbonization of the wood previous to silicification. This car- 

 bonization was evidently the beginning of a series of changes which, car- 

 ried far enough, would have converted the mass into lignite or even coal, as 

 has happened on so grand a scale in the older formations, but hardly at all 

 in rocks so new as the Pliocene. Sometimes, however, in the gravel region, 

 the wood is black throughout, and would appear to a casual observer to be 

 considerably carbonized. As far as the writer's observations extend, however, 

 most of these specimens are really silicified, and contain but little organic 



