Qti ECONOMIC GEOLOGY OF SILVERTON QUADRANGLE, [bull. 182. 
account of the preponderance of nearly north-and-south fractures 
over nearly east-and-west fractures, it is thought that the effective 
stress producing the principal Assuring of the quadrangle may have 
acted in a north-and-south direction. Further, since there is no 
known source of such stress exterior to the quadrangle and imme- 
diately adjacent region, it is thought probable that the stresses were 
generated chiefly within the quadrangular area, by slight vertical 
movements following the enormous transfer, in Tertiary time, of vol- 
canic material from an intratelluric to a superficial position, and that 
the surrounding country merely acted as a relatively passive buttress 
against which the thrust was directed. In other words, it is believed 
that the stresses were due principally to local gravitative readjust- 
ment. Some genetic connection between the volcanism and the sub- 
sequent Assuring, mineralization, and veining can scarcely be doubted, 
as these phenomena rapidly diminish in intensity away from the vol- 
canic district of the San Juan Mountains. It does not seem possible 
at present to do much more than suggest that hypothetical relation 
which seems to adhere most closely to the known facts. 
In addition to the principal stress outlined, there were doubtless 
many minor directions of stress set up at various times, some contem- 
poraneous with the principal north-and-south stress and others ear- 
lier or later, frequently leading to the production of minor local fissure 
systems. These also were probably in part compressive tangential 
thrusts, and it is the existence of these widely differing directions of 
effective stress that furnishes one of the strongest arguments against 
seekingvsources for the latter outside of the region itself. It is exceed- 
ingly improbable that there should have been set up at various widely 
separated points in the relatively undisturbed regions inclosing the 
volcanic area, stresses which were to find their most energetic expres- 
sion in the Assuring of the area itself. It seems likely that torsional 
strains also had their place in the readjustments following volcanism. 
Such a complex of fissures as that around Lake Como seems hardly to 
admit of any other satisfactory explanation. The probability that 
torsion has been a factor in the minor Assuring of this region has been 
suggested by S. F. Emmons. 1 It seems not unlikely that earthquake 
shocks, to which there is good reason to suppose the region was sub- 
jected during the faulting and principal Assuring accompanying post- 
volcanic readjustments, may have, in some instances, transformed 
torsional strain into actual Assuring, as suggested by Crosby. 2 
In conclusion, it may be well to recall the complex forces and results 
that enter into the Assuring under simple pressure of a block of what 
is commonly called homogeneous material. The geological conditions 
under which rocks are fractured are vastly more complex, and, as Dr. 
Becker 3 has remarked, " it is entirely safe to presume that every pos- 
1 Structural relations of ore deposits: Trans. Am. Inst. Min. Eng., Vol. XVI, 1888, p. 804. 
2 The origin of parallel and intersecting joints: Am. GeoTogist, Vol. XII, 1898, p. 368. 
3 Tke torsional theory of joints: Trans. Am. Inst. Min. Eng., Vol. XXIV, 1894, p. 130. 
