ortion of the earth's crust (and which have resulted in
depositing in thousands of cracks and cavities the ores we now mine),
portions of the old table-land were in places set up at high angles
forming mountain chains, and doubtless extending to the zone of fusion
below. Between these blocks of sedimentary rocks oozed up through the
lines of fracture quantities of fused material, which also sometimes
formed mountain chains; and it is possible and even probable that the
rocks composing the volcanic ridges are but phases of the same materials
that form the sedimentary chains There is, therefore, no _a priori_
reason why the leaching of one group should furnish more ore than the
other; but, as a matter of fact, the unfused sediments are much the
richer in ore deposits. This can only be accounted for, in my judgment,
by supposing that they have been the receptacles of ore brought from a
foreign source; and we can at least conjecture where and how gathered.
We can imagine, and we are forced to conclude, that there has been a
zone of solution below, where steam and hot water, under great pressure,
have effected the leaching of ore-bearing strata, and a zone of
deposition above, where cavities in pre-existent solidified and
shattered rocks became the repositories of the deposits made from
ascending solutions, when the temperature and pressure were diminished.
Where great masses of fused material were poured out, these must have
been for along time too highly heated to become places of deposition; so
long indeed that the period of active vein formation may have passed
before they reached a degree of solidification and coolness that would
permit their becoming receptacles of the products of deposition. On the
contrary, the masses of unfused and always relatively cool sedimentary
rocks which form the most highly metalliferous mountain ranges (White
Pine, Toyabe, etc.) were, throughout the whole period of disturbance, in
a condition to become such repositories. Certainly highly heated
solutions forced by an irresistible _vis a tergo_ through rocks of any
kind down in the heated zone, would be far more effective leaching
agents than cold surface water with feeble solvent power, moved only by
gravity, percolating slowly through superficial strata.

Richthofen, who first made a study of the Comstock lode, suggests that
the mineral impregnation of the vein was the result of a process like
that described, viz., the leaching of deep-seat