CHEMICAL CHANGES IN THE GRAVELS. 327 
the more likely it is to be loosely held together. Thus most Tertiary 
conglomerates and sandstones are of very inferior value as building stones; 
while, in the case of some of the older pudding-stones, the forms of the origi- 
nally water-worn fragments can be distinguished by the eye, but the whole 
mass has become so thoroughly welded together tuat it has no more ten- 
dency to separate into its original components than to break in any other 
direction. In such cases, that peculiar form of chemical action called 
metamorphism appears to have made over the mass so completely that 
the original surfaces of contact of the components have been entirely 
obliterated. 
The case of the Tertiary gravels of the Sierra seems in some respects a 
peculiar one, as regards the original compacting of the mass and its subse- 
quent local disintegration. To understand it more fully, it will be necessary 
to inquire what other evidences the formation presents of chemical action as 
having taken place at any period since the deposition of the mass. Proofs 
of such action present themselves in abundance, when the gravel and 
the organic bodies which it encloses at numerous localities come to be 
examined. 
The most conspicuous of the chemical changes wrought in the gravel, 
as evidenced by the known change in substances imbedded in it, is silicifica- 
tion. As has already been stated in a previous chapter, the quantity of 
wood buried in the detrital masses of the Sierra is very large, and much the 
larger portion of it has become converted into opal, the amorphous form of 
silica.* Occasionally the fragments of trunks of trees have been slightly 
charred before being silicified, as is apparent from their color. This “ char- 
ring”? seems to have been the first step of a passage into coal, or, more 
properly, lignite. Indeed, there have been and are occasionally pieces of 
wood found of which the organic matter was so well preserved that they 
could be used as fuel. 
So far as the writer’s observations extend, the largest quantity of best 
preserved silicified wood is found in connection with deposits chiefly vol- 
canic in character. At Chalk Bluffs, for instance, where the quantity of 
prostrate silicified trunks of trees which have been washed out is very large, 
the material in which the leaves are imbedded is a white pulverulent sub- 
stance, apparently almost entirely made up of rhyolitic ash. The same is 
the case in the well-known “ fossil forest” near Calistoga, in the Coast 
* See ante, pp. 235-239. 
