STRUCTURE OF THE COLD DEPOSITS. 167 
fault, show no noticeable displacement. The net throw of the fault must be very 
small and the dislocation may be older than the fissure zone of the Coin lode. 
ORIGIN OF FISSURES. 
It has been shown that the principal fissures are confined to an area little larger 
than that of the Cripple Creek volcanic neck ; that they have a general radial arrange¬ 
ment, and that their formation was accompanied by very slight faulting. It appears 
further that the basic dikes have a similar radial grouping; in fact, the fissures 
occupied wholly or in part b} r dikes and those occupied by ore are so similar in 
character and so closely related in direction and distribution that they were probably 
formed in the same manner, if not at the same time. 
The basic dikes and the ore-bearing fissures are younger than any of the phono- 
lite intrusions and originated after the volcanic breccia had been cemented into a 
firm rock. The sheeted zones were certainly formed in part before the intrusion 
of the basic dikes. This is clearly shown in the Granite mink, where a “ basalt ” dike 
turns from one fissure to follow the Bobtail lode, and in the Elkton mine, where 
parts of the Raven lode consist of a zone of sheeted breccia from which the “ basalt’’ 
dike is locally absent. Had this sheeting been produced after the intrusion, the 
dike should be fissured like the breccia. This, however, is not the case. The 
sequence of events seems to have been as follows: (1) Sheeting of the indurated 
breccia, (2) intrusion of basic dikes along parts of the sheeted zone, (3) slight 
Assuring of the dikes, probably in part by mere contraction in cooling, whereby 
they were divided near the walls into thin plates, and (4) ore deposition. That 
the breccia had been much fissured and even shattered before the basic intrusions 
and that all of the resulting fissures were not fdled by the dikes is well shown in 
the Moose mine. It is probable that some sheeting followed the basic intrusions, 
but it is rarely possible to distinguish this from the earlier Assuring. We may 
conclude, then, that some of the productive Assures were formed before the basic 
intrusions, that some were formed after the intruded rock had solidiAed, and that 
Assuring and intrusion were closely associated in point of geological time. 
The character of the Assures is suggestive of fracturing under light or moderate 
load. Facts supporting this suggestion are (1) the greater number and width of 
the Assures near the surface, (2) the branching of the upper parts of some Assures, 
such .as the Bobtail veins in Stratton’s Independence mine, and (3) the change in 
dip of some Assures, the superAcial portion having usually the Aatter dip. Though 
at Arst glance the generally small size of the openings produced by the Assuring 
might be considered as indicative of fracturing under heavy load, yet the explana¬ 
tion of this feature is thought to be that the fracturing stresses were relieved by 
comparatively slight deformation of the rigid rock mass. Had the stresses been 
regional instead of conAned to the volcanic neck the initial fracturing might have 
been followed by considerable faulting along the Assures before equilibrium was 
restored. 
The conclusion that the Assures were formed under relatively light load does 
not rest entirely upon the study of the Assures, but is supported by general geological 
considerations relating to the development of the present topography, as shown 
on page 36, and by the distribution of pay shoots, as suggested on page 215. 
