CHEMICAL REACTIONS IN THE GRAVELS. 331 
which they occur. Were this not the case, the lodes would be so faulted 
and disturbed the working them would be almost or quite an impossibility. 
Faulting of the lodes, however, even in a slight degree, is not common, and 
the existence of any very extensive breaks is something unknown to the 
writer; such must, at all events, be of rare occurrence. There is also abun- 
dant evidence that the volcanic epoch was not inaugurated in the Sierra 
until the range had approximately its present form. This, however, must 
not be taken as excluding the possibility of the presence of eruptive masses 
in the bed-rock itself. These do exist, and probably in great quantity, al- 
though ina much metamorphosed condition, so that it is not easy to recog- 
nize their real nature, except by the aid of the microscope. But the era of 
volcanic action belonging to the bed-rock series is widely separated from 
that of the gravel formation, as shown by abundant facts: the one seems to 
have been the result of massive eruptions; the other was undoubtedly that 
of the ordinary crater ejections of the present day. 
We have, then, the two sets of phenomena — the formation of the quartz 
veins in the bed-rock, and the covering of the gravel deposits with eruptive 
materials — brought very near each other in geological time. That these 
occurrences were absolutely synchronous cannot, of course, be maintained, 
because we have rolled fragments of quartz under the lava in many places, 
showing that veins or masses of this material existed before the opening of 
the volcanic epoch. The epoch of metamorphism proper seems to have 
been the first stage in the process which was to culminate in the pouring 
forth from the crest of the Sierra of the immense mass of eruptive material 
which now rests upon its flanks. That during or soon after the meta- 
morphism of the slates the quartz veins began to be developed, is also a 
reasonable supposition, and we know that these phenomena were followed by 
the opening of a line of orifices from one end of the range to the other, 
through which vents came the eruptive matter in its various forms, as 
already indicated. That these various stages of chemical action are geneti- 
cally connected cannot, in the present state of our knowledge of the myste- 
ries of volcanic activity, be positively affirmed; but certainly the evidence 
pointing in that direction is very strong. 
It might appear that what has been said above, in regard to the formation 
of the quartz veins after the range of the Sierra had assumed its present 
development, was in contradiction to what had been previously stated in this 
volume in reference to the peculiar form of the “Great Quartz Vein,” as 
