86 Ore- Deposit of the Treadwell Mine. 
development in it of chloritic minerals or reddish, and, micro- 
scopically examined, shows two feldspars with some quartz. In 
general aspect it in fact resembles the varieties of fine grained 
granite which are frequently met with near the junction of an 
ordinary granite with other older rocks through which it has 
broken. 
The impression formed from such examination of this 
remarkable deposit as I was able to make is, in fact, that it 
represents the upper portion, or "feather edge" of a granitic 
intrusion, probably contemporaneous and connected with the 
characteristic granites of the neighboring Coast Ranges, but 
which, owing to peculiar conditions, has become decomposed 
and silicified by solfataric or hydrothermal action, to which 
the concentration of gold in it and the deposition of pyrites, 
are also due. To what extent the presence of gold may 
depend on the occurrence of the adjacent slaty argillites 
(elsewhere known to contain auriferous quartz-veins) it is im- 
possible to say, but it appears not improbable that the deeper 
portions of these rocks may, under the action of such heated 
solvent waters, have afforded both the gold and the pyrites. 
It is conceivable that the hydrothermal action which has 
affected this part of the original granitic magma may have 
been due to the water included by the mass itself while in a state 
of "aqueo-igneous" or "granitic" fusion, the escape of such 
water through the substance of the upper part of the intrusive 
mass being rendered possible by the relief from pressure con- 
sequent on the approach of the intrusion to the actual surface. 
It may, however, perhaps with greater probability, be supposed 
that the water included in the adjacent sedimentary deposits, 
became vaporized by the heat of the intrusive mass, and 
found its way to the surface in the form of steam through the 
substance of that mass. It will be noticed that Mr. Adams finds 
evidence in the microscopical character of the rock of much 
crushing and fracture, so that in any case it must have afford- 
ed a convenient channel for the passage of heated waters or 
steam, and this appears to have been one of the more impor- 
tant circumstances leading to its mineralization. 
The slaty rocks themselves in the vicinity of the ore-deposit 
are traversed by numerous small veins of quartz ; and at the 
distance of a few miles (on the mainland opposite Douglas 
island in "Silver Bow Basin") similar slaty rocks are found 
