ransome.] RELATION OF FISSURES TO KIND OF COUNTRY ROCK. 53 
intersecting systems of vertical fractures, with their obtuse angles of 
intersection bisected by the direction of the applied stress. Thus 
systems of parallel fissures flatly dipping in opposite directions and 
systems of nearly vertical intersecting fissures may both result from 
compressive stress, and may both be called conjugated fissures. Fur- 
ther, all gradations are possible between these two extreme cases, 
dependent upon the relation of the present surface to the original 
direction of least resistance when the fracturing took place. It is 
quite possible that some of the steeply dipping fissures of the Silver- 
ton quadrangle which intersed nearly at right angles form conjugate 
systems in the sense of Daubree. The test of this lies in determining 
I whether the intersecting fissures are of the same or different ages. 
RELATION OF FISSURES TO KIND OF COUNTRY ROCK AND TO ROCK 
STRUCTURE. 
It has already been pointed out thai fissures carrying ore in greater 
or smaller amounts occur in practically all the rocks found in the 
quadrangle. Opportunities are rare, however, for observing directly 
the character of the fissures as they pass from rocks of one kind to 
another, and when such observations are possible no marked change 
is apparent. It has been stated by at least one observer, 1 that the 
lodes of this region occupy contraction fissures in the volcanic rocks, 
and do not extend down into the underlying schists or sediments. 
Nothing could be much farther from the facts than this assertion. 
Not only does the distribution of the fissures disprove it, but at the 
i Green Mountain and Highland Mary mines, in Cunningham Gulch, 
the lodes ma} 7 be seen passing from the Algonkian schists up into the 
overlying volcanic rocks of the San Juan and Silverton series. In the 
case of the Green Mountain mine no difference was noted in the fissure 
itself, either in width, direction, or general character, in the older 
schists and the younger rhyolite flow-breccia. The exact point at 
which the fissure passed from one rock into the other was not seen. 
The Highland Mary lode, where originally worked in Algonkian 
hornblende-schist, is a simple vein filling a clean-cut fissure 2 or 3 
feet in width. Toward the northwest the lode passes into the vol- 
canic rocks of the San Juan and Silverton series, and becomes a rather 
irregular zone of Assuring about 12 feet wide where observed at a 
point some 1,500 feet in elevation above the mine. In general it is 
apparently true that the fissures in the Algonkian rocks tend to be 
simpler and narrower than those in the volcanic rocks of later age. 
The fissures as a rule cut the schists cleanly at various angles with 
the schistosity, which apparently exercised no influence upon the 
direction of the fracturing. 
No very definite differences can be observed in the character of the 
1 Philip Argall: Proc. Colo. Sci. Soc, Vol. IV, 1891-1893, p. 353. 
