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 that 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. 



i 



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- 

 eanic in character. At Chalk Bluffi, 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 



* Sec ante, pp. 235 - 239. 



