328 RESUME AND THEORETICAL DISCUSSION. 
Ranges. It would appear that the wood has been best preserved when 
surrounded by eruptive materials, in which the trees may perhaps have 
grown, or which may have been swept over the trunks, after they had been 
prostrated by age or other causes, so as entirely to envelop them. Many 
fragments of silicified wood are found in the coarse gravel; these, however, 
usually have a more or less water-worn appearance, and may have been 
carried by currents of water far from the place in which they grew. 
The lava itself frequently exhibits signs of having been acted on by silici- 
fying agents after its deposition. Some of the beds of white ashes have 
evidently undergone a change since they reached the place where they now 
lie; the whole mass, which, in all probability, was originally light and inco- 
herent, has acquired more or less completely the texture and appearance of 
semi-opal, as if acted on by silicifying waters. The abundance of infusorial 
silica in portions of the volcanic rocks occurring in close connection with 
the gravels, as already described,* is another proof of the frequent presence 
of this element in such condition that it could be readily assimilated by the 
organisms requiring it for their development. 
The legitimate inference, then, from what has been stated above, is that a 
large part if not the whole of the series of beds included within the gravel form- 
ation has been at some period since its deposition quite thoroughly permeated 
with water holding silica in solution in sufficient quantity and in the right 
condition to bring about extensive chemical changes of such a kind as to 
result in the replacement of such organic matter as was present by that ele- 
ment, while portions of the inorganic or mineral substances exposed to the 
same reaction exhibit a tendency toa similar replacement. There are, indeed, 
some proofs of the presence of fluorine as well as of lime in the vapors or 
waters which have permeated the formation, but the evidences of silicification 
are far more abundant. 
The reactions thus indicated are decidedly those of volcanic regions. Hot 
springs, the water of which is charged with silica, are a very common form 
of volcanism, as may be seen on a grand scale in the geyser districts of Ice- 
land, and on a still grander one in the neighborhood of the Yellowstone 
Park. We may assume, then, that the gravel deposits of the Sierra have 
been more or less subjected to those peculiar chemical influences which 
accompany volcanic action. This is by no means a matter to excite surprise, 
since, as has already been described, the Sierra Nevada was the theatre of 
* See ante, pp. 220-231. 
