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RESUME AND THEORETICAL DISCUSSION. 



Kariges. 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, whde portions of the inorganic or mineral substances exposed to the 

 same reaction exhibit a tendency to a 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 

 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 



* Sec ante, pp. 220-231. 



regions. Hot 







