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. 8. P. Sharples for 
analysis by the writer, was found to have the following composition :— 
Silica . : ; : ; : é . 90.00 
Oxide of iron . ; : “ - ; 3.27 
Water, and organic matter . f é ee 5) 
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 
