352 RESUME AND THEORETICAL DISCUSSION, 
there is no doubt in the writer's mind—and this statement is not here made 
without many years of careful examination of important mining regions in 
various countries — that the occurrence of metalliferous ores is rather a 
surface phenomenon than a deep-seated one. The conditions favorable to 
the formation of veins and vein-like or segregated masses are more likely 
to have existed near the surface than deep down below it. Such conditions 
would be —in part at least — diminution of temperature, relief from pres- 
sure, and, in the case of true veins, the existence of fissures. The history 
of mining operations shows beyond dispute that bodies of ore ovcurring in 
the segregated form are, on the whole, not to be depended on for persistence. 
And even true fissure veins must eventually give out in depth, if for no 
other reason than the change in the character of the enclosing rock brought 
about by intense heat. Neither would fissures be likely to continue to 
exist, nor the materials filling them to retain a distinct form, where the 
temperature was above the melting point. Thus, whatever theories we may 
adopt for the formation of mineral veins, we are led to the conclusion that 
they must, as a general rule, be better developed near the surface than at 
great depths. The fact that in some important mining regions the very 
upper portion of the veins has been oxidized, and then dissolved away by 
water, Is very easy of comprehension, and not in conflict with what has been 
stated above. . 
The fact must be admitted that the quartz veins and masses were the chief 
source of the gold contained in the gravel, but at the same time it is true 
that gold does exist in the bed-rock where no quartz is found associated with 
it. As far as the writer's observations go, however, such occurrences are 
pretty much limited to the immediate vicinity of veins of quartz. The Great 
Quartz Vein, or Mother Lode of California, is in places associated with slaty 
rocks which carry considerable gold, although this appears to be very irregu- 
lar in its occurrence. Such localities have never, so far as known to the 
writer, been found persistent enough in their yield to pay for working* As 
far, however, as the special theory of the gravels is concerned, it is a matter 
of but little consequence whether the gold they contain came exclusively 
from the quartz veins, or in part also from the adjacent rock. 
* In regant to such localities as those of Quail Hill and the Harpending Claim, where the slates are reported to 
have been highly productive in gold, but little can be said, except that they have proved, on working, to be con- 
spicuous failures. How far the reported richness of the rock is to be accounted for by previous “‘salting” of the 
places from which the specimens were taken for assay, it is not easy to say. It is very clear, however, that there 
was more or less of frand mixed up with the transactions at beth of these lecalities. 
