108 ECONOMIC GEOLOGY OF SILVERTON QUADRANGLE. [Buix.ld 
normal andesitie breccia of the Silverton formation, which covers hun- 
dreds of square miles of the San Juan region, and is in no sense an 
agglomerate filling a volcanic neck. 
Another common explanation is that the Red Mountain stocks 
occupy the throats of extinct geysers and hot springs. The supposi- 
tion that the ascending heated waters which were primarily instru- 
mental in forming ore bodies may have issued as hot springs, or even 
geysers, from a former surface, is of course perfectly tenable and 
difficult to disprove. But to argue that the knolls of silicified and! 
impregnated andesitic breccia conspicuous in the topography of to-day 
are really mounds of siliceous sinter is to be carried away by an 
analogy of the most unessential and superficial kind. The knolls 
exist merely because the bleached and altered rock of which they are 
composed resists erosion better than the crumbling, more easily 
weathered breccia, which has been less affected by the mineralizing 
solutions and is more readily disintegrated and carried away by rains 
and streams. 
It seems most reasonable to regard the ore spaces of the Red 
Mountain district as a local modification of the general Assuring of 
the region. It is possible, however, that much of the minor, very 
irregular Assuring which is characteristic of this region may be due 
to contraction within the rock mass consequent upon, the prevalent 
alteration of the volcanic rocks to aggregates of quartz, kaolin, and 
pyrite, with the removal of certain constituents, as more fully dis- 
cussed on pages 114-131. Not only was the Assuring locally complex, 
but the ascending thermal waters had more chemical and probably 
more physical activity than elsewhere within the quadrangle, whereby 
solution played an important part in enlarging zones or aggregations 
of fractures into ore spaces and in metamorphosing the country rock 
to an extent not elsewhere observed in this region. The cause of 
this local intensity of chemical and physical activity is not known, 
although it is probable that the circulating intratelluric waters were 
here hotter and more copious than elsewhere. Some indication of 
the latter is afforded by the abundant strongly mineralized springs 
which issue from the surface in this district at the present day. 
There is, moreover, some geological grounds for believing that the 
Red Mountain region may have been formerly a local center of. 
volcanic activity. 
There has probabl} 7 been fracturing of the rocks, or at least move- 
ment along preexisting fissures, since the ore bodies were formed, but 
there are not at present facilities for studying the relation of the 
older to the later fissures. It may even be considered doubtful 
whether the so-called fault found below the seventh level in the 
Guston and the sixth level in the Yankee Girl is entirely a post- 
mineral dislocation, or whether it is in part a fracture antedating 
the deposition of the ore. 
