64 ECONOMIC GEOLOGY OF SILVERTON QUADRANGLE, [bull. 182. 
volcanic eruption " referred to as the possible sources of the compressive 
stresses supposedly effective in Assuring the rocks of the Telluride 
quadrangle. There is, however, ground for believing that there may 
have been local manifestations of volcanism in the Red Mountain 
region. It is not impossible that the plug-like masses of porphyry 
mapped by Mr. Cross (PI. IV) ma} 7 occupj 7 conduits through which 
volcanic materials reached a former surface some thousands of feet 
above the present one. The character of the ore deposits, the intensit y 
of the associated metamorphism, and the geological distribution of 
the Tertiary volcanic rocks all strongly suggest that one or more cen- 
ters of former volcanic activity underlie the Red Mountain region. 
This, of course, does not mean that any trace of volcanic form remains 
in the present topography. Cones or craters, if they existed, have 
been obliterated in the evolution of the erosional forms of to-day. 
In the Silverton, however, no less than in the Telluride area, the 
greater part of the Assuring is not only post- volcanic, but is later 
than the monzonitic intrusions which cut the volcanic series. Thus, 
although the vast volcanic accumulations of the San Juan must have 
issued from local vents, the reference of the forces which produced the 
fissures to one or more centers of eruption immediately east or north- 
east of the Telluride quadrangle as their source is as yet scarcely 
warranted by the facts. In this connection it may be noted that the 
intrusion of the great stocks of mozonite into the rocks of both quad- 
rangles brings up the rather perplexing problem of the mechanics of 
such intrusions and the disposition of the rocks which formerly occu- 
pied the space now filled by the intrusive mass. He Avould be rash 
who would assert that the intrusion of these stocks was not accom- 
panied by strains and fissures in the surrounding rocks, and yet the 
fact is clear that the principal ore-bearing fissures of the region were 
formed after the monzonite had solidified, since they cut the latter as 
well as the adjacent formations. 
But leaving aside for the present the question of their source, it is 
necessary to investigate more fully the extent to which tangential 
stresses of the kind indicated may be regarded as an effective cause 
of the Assuring of the Silverton quadrangle. Normally, in homoge- 
neous rocks, such stresses might be expected to produce systems of 
conjugated fissures of parallel strike, with dips not far from 45°, and 
accompanied by some, although not necessarily great, thrust faulting; 
i. e., reversed faulting. The actual fissures do not conform even 
approximately to these conditions. Assuming that the direction of 
least resistance was vertical, it is inconceivable that fissures so nearly 
vertical and showing so little evidence of thrust could have been 
formed by tangential stress. Moreover, such evidence of faulting as 
Purington was able to obtain in the Telluride quadrangle indicated 
normal faulting and not thrust faulting. 
If, on the other hand, the direction of least resistance was not ver- 
