STRUCTURE OF THE GOLD DEPOSITS. 
155 
distribution. That, the Assuring is in fact local and does not extend indefinitely 
and with undiminished intensity into the surrounding rocks of the prevolcanic 
plateau is indicated not only by the results of mining development as plotted in PI. 
XII, but by the behavior of underground water and by the distribution of basic 
dikes, as will subsequently be shown. 
* 
DIRECTION OF FISSURING. 
The major fissures, as appears in PI. XII, have a recognizable though irregular 
radial plan. In the western and southwestern parts of the district the prevailing 
strike is northeasterly; in the southern part it is northerly; and in the southeastern 
and eastern parts of the productive area a northwesterly strike predominates. 
There is much irregularity, and there are some fissures in all parts of the field that 
do not conform to this plan. When, however, account is taken of the heterogeneous 
character of the material filling the volcanic neck and the many dikes and irregular 
intrusive masses tending to deflect fissures from courses that they might follow in a 
homogeneous medium, it is less surprising that individual fissures should be eccentric 
than that the Assuring should on the whole so clearly exhibit a definite arrangement. 
While the general disposition of the fissures is radial, they converge rather to 
various parts of a central tract than to a single point. This tract is that already 
referred to as containing relatively few important fissures and as corresponding 
approximately with the drainage basin of Squaw Gulch above Anaconda. It 
includes the island-like mass of granite of Bull and Ironclad hills and the similar 
schist mass of Fairview. North of this tract lies a mass of breccia extending to 
Tenderfoot and Carbonate hills, in which the rather scanty known fissures suggest 
little systematic arrangement. The general radial plan thus prevails over a sector 
comprising about 270° of the rudely circular productive area. 
The radial arrangement of the lode fissures is shared to a certain extent by the 
dikes, particularly by the “basalt” dikes. It would thus seem that whatever the 
nature of the stresses which produced the lode fissures, similar stresses must have 
formed the fissures followed by the basic dikes. The ore deposition and some of the 
Assuring were later than the dike intrusions, but it is not certain that the fissures 
themselves can be separated into an earlier and a later group—that filled with 
“basalt,” this with ore. It is probable that the basic magma was injected into 
a part of the earlier fissures, the remainder being left empty and in part subse¬ 
quently filled with ore. 
The Cripple Creek lodes, as will be shown later, have a sheeted structure. As 
sheeting is a characteristic result of compressive stress and as the fissures produced 
by a generally simple stress, like the compression of a block of glass in a vice, tend 
to form two intersecting groups of parallel conjugate fissures, the occurrence of a 
radial system rather than of two or more conjugate systems is noteworthy. The 
latter feature is common to many districts, even where the lodes are not regular 
sheeted zones and are therefore not so clearly due to compression. A general radial 
grouping of the fissures over a whole mining district is comparatively rare, and 
where associated with well-defined sheeting indicates the operation of compressive 
stresses of rather unusual character. 
