ORIGIN OF THE ORES. 
227 
a late Tertiary age to the close of direct volcanic activity, there is some reason for 
believing that the ore-forming epoch belonged to the close of that period. 
SOURCE OF THE WATER. 
The ore-depositing water was derived either from the atmosphere or from 
magmas under diminishing pressure and temperature or from both of these sources. 
In other words, it may have been a part of the ground water descending through the 
pores and fissures of the rocks from the surface on which it once fell as rain or snow, 
and possibly ascending, charged with dissolved material, from the lowest levels 
reached under the driving force of the volcanic heat encountered there. Or it may 
have formed part of the original molten phonolitic rocks and may have escaped 
from its bond during the ascent of this molten rock to levels of less temperature and 
pressure. Both hypotheses are plausible, though at first glance the former view 
seems much more natural and simple. Either may be difficult to prove, but it may 
be profitable to consider the probabilities involved. 
It will be shown in Chapter XII that the conditions of underground drainage 
are unusual. The porous, shattered volcanic mass is deeply sunk in much more 
massive and impervious granite and metamorphic rocks. It therefore holds water 
much as would a sponge in a cup. The circulation of 'the ground water in this 
volcanic plug is exceedingly slow; in fact, the«water is practically stagnant. The 
cold dilute sulphate solutions which constitute the ground water are evidently 
wholly impotent to deposit ores like those in the veins or to cause abundant pyritiza- 
tion of the rocks. They fill many open fissures in the rocks, but nowhere have they 
given the least indication of depositing telluride ores. 
If surface waters were present they must have constituted currents under- the 
influence of the heat of the volcanic rocks. This circulation was only a temporary 
phase, ceasing when the rocks had cooled sufficiently. 
Such waters ascending vigorous^ throughout the volcanic mass could not 
reasonably have been derived from the very limited surface area of that mass itself. 
They must have been derived chiefly from the surrounding granitic plateau. 
They must have percolated through the granite to great depths near the volcanic 
mass, and finally have been driven up by the volcanic heat still existing in it. 
Considering, however, the almost impermeable character of the granite, as demon¬ 
strated by the mining operations, it becomes very difficult, if not impossible, to 
conceive how a ‘sufficient amount of water could penetrate the porous volcanic 
mass from the surrounding granite to give rise to the strong ascending current 
which evidently streamed upward in every available fissure in this old volcano. 
The second hypothesis of the derivation of the vein-forming waters is that 
they were originally an integral part of the intrusive phonolitic magmas and were 
given off by release of pressure or by cooling and crystallization after the magma 
had ascended to higher levels. 
According to the general laws of solutions, pressure increases the solubility of 
water in magmas, and conversely, if all magmas contain more or less water which 
is just as much a part of them as is the silica, for instance, it follows that a portion 
of the water will be given off during the eruption. 
