330 



RESUME AND THEORETICAL DISCUSSION. 



Such being the facts, it would seem that the influences by which the 

 chemical changes in the gravels have been brought about, must have come 

 from below ; and when we inquire more closely what these influences must 

 have been, and in what way they were exercised, we find ourselves com- 

 pelled to fall back on the volcanic epoch as that of the reactions in question, 

 being obliged to admit, however, that it is through the bed-rock that they 

 have been propagated, and not through the overlying volcanic masses. This 

 may, at first thought, seem an unwarranted hypothesis ; but the following 

 explanations may perhaps remove a part of the difficulties. 



It will be admitted by all, that the mass of slaty and other strata — d 

 nated for convenience by the term " bed-rock " — has been at some time the 

 scene of very extensive chemical changes. Not to speak of the variety of 

 those included in the commonly used term " metamorphic," we have ex- 

 hibited, all through the formation, on the grandest scale those proofs of a 

 former chemical activity, the result of which has been the development of 

 the very numerous quartz veins by which it is everywhere intersected, and 

 many of which are of great size, as already described. Here, then, are evi- 



esur- 



dences of silicification on a grand scale 



the same phenomenon in another 



form which is presented to our notice so frequently in the gravels them- 

 selves. And the quartz veins are not exclusively confined to the bed-rock ; 

 the writer has seen at least one cutting the gravel itself, which was well- 

 defined, although of small dimensions* 



The inquiry naturally suggests itself, then, At what epoch did the forma- 

 tion of the quartz veins in the bed-rock take place ? Was it contempora- 

 neous with the grand exhibition of volcanic forces which took place in the 

 Sierra, during the latter part of the gravel epoch ; and, if so, are the phe- 

 nomena genetically connected ? To the writer's mind it seems clear that 

 both these questions can be answered in the affirmative, and the reasons for 

 the belief will now be set forth, although space permits it to be done only in 

 the briefest possible manner. Some further references to the same subject 

 will, however, be found in the section of this chapter relating to the distri- 

 bution of the gold in the gravel. 



All the phenomena of occurrence in the quartz veins indicate most clearly 

 that these were not formed until the chain of the Sierra had been uplifted 

 into pretty nearly its present position, and that since their formation there 

 have been but kw, if any, important orographic disturbances of the region in 



* See Geology of California, Vol. I. p. 276. 



