168 GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
The areal distribution of the fissures, their radial plan, and their connection with 
the basic dikes show that they were produced by local rather than regional stresses. 
It is true, as Penrose has shown, that the granite of the prevolcanic plateau is con¬ 
spicuously sheeted at points distant from the productive area, as in Eightmile Can¬ 
yon, but this sheeting is associated with no known ore deposits and is not clearly 
related to the Assuring of the Cripple Creek district. It seems most reasonable to 
regard the Cripple Creek fissures as having some genetic connection with the local 
volcanic center. 
The nature of the stresses that fissured the rocks is not easily determined. The 
character of the Assures shows that the stresses were of a kind that could be relieved 
by comparatively slight strain. The rocks were in most places merely fractured and 
not noticeably displaced. Had the tangential movement along the Assures been 
greater it might have supplied a clue to the directions and character of the forces that 
produced the faulting. In the present case, however, the tacts do not enable us to 
decide with certainty which of a number of hypotheses is applicable. The incon¬ 
spicuous character of the Assures, their radial disposition, and the absence of notable 
faulting suggested at one period in their study earthquake shocks, emanating from 
some point under the central part of the district, as a possible explanation. It is 
diAicult, however, to account by this hypothesis for the characteristic sheeted struc¬ 
ture of the lodes, which seems to demand an explanation involving compressive 
stresses. The hypothesis which on the whole seems most in harmony with the facts 
is that the entire mass of breccia and volcanic rocks, after the phonolitic eruptions 
had ceased and the breccia had become Armly cemented, settled down very slightly 
within the steep-walled volcanic funnel in the ancient rocks of the plateau. As the 
walls of the volcanic neck are somewhat irregular and as in general they converge 
downward, such slight sinking, by forcing a rigid mass to adjust itself to a slightly 
smaller space of different shape, would produce compressive stresses in the subsiding 
mass and to some extent in the inclosing granitic rocks. The stresses would be 
relieved by fractures characterized by their number and by diversity of trend rather 
than by great size or by conspicuous faulting. The amount of settling necessary to 
produce Assuring of the kind found in the district would be extremely slight and need 
call for no greater faulting along the general line of the granite-breccia contact than 
is known to exist. 
This hypothesis also affords an explanation of the observed relation between the 
Assuring and the basic dikes. The dikes may be regarded as the Anal magmatic 
residuum of the volcanic reservoir, squeezed quietly into some of the Assures formed 
by the slight settling of the solidiAed products of earlier eruptions. Fissuring and 
the intrusion of the basic dikes, according to this hypothesis, are genetically related 
and represent the dying out of the volcanic forces. They were the Anal structural 
manifestations of volcanism and were succeeded by gaseous and aqueous emanations 
and by ore deposition. 
It is possible that the removal by erosion of the volcanic cone that in late Ter¬ 
tiary time covered the central part of the Cripple Creek district may have resulted in 
some readjustment of the rocks relieved of this local load. While it does not seem 
that such unloading could initiate the formation of the ore-bearing Assures, it may 
have increased their width. 
