GEOLOGICAL STRUCTURE OF THE DISTRICT. 
35 
The contact is usually sharp and in many cases minutely irregular. In places, 
particularly on the west side of the hill, the granite is only slightly brecciated at the 
contact. Elsewhere, as in the Gold Dollar mine, the two massive rocks are sepa¬ 
rated by as much as 15 feet of breccia composed of mingled fragments of phonolite 
and granite. In the open cut and stopes of the Prince Albert mine, on the east side 
of the hill, bodies of similar breccia are inclosed in the phonolite. 
The orifice now sealed by the phonolite was probably formed by a local explo¬ 
sive eruption and was partly filled with breccia, this breccia being subsequently 
forced out and replaced by the intrusion of the massive phonolite. It is possible 
that greater erosion of the district would expose similar plugs which are now capped 
by thick irregular sheets. 
DIKES OF THE VOLCANIC PERIOD. 
The breccia and latite-phonolite and the surrounding pre-Cambrian rocks are 
cut by abundant dikes of phonolite, ranging in width from a few inches to 400 feet. 
While many of these phonolitic dikes belong to late phases of the eruptions, much 
phonolite was erupted at earlier periods and these masses contributed their frag¬ 
ments to the final breccia produced by the volcanic explosions. The diversity in 
age of the phonolite intrusions is shown by one phonolite dike cutting another, as 
may be seen half a mile east of Galena Hill, or by phonolite dikes cutting breccia 
composed of phonolite fragments, as may be seen in the mines of Raven Hill. In 
a general way there is a rough radial arrangement of the phonolite dikes around the 
main volcanic neck; but the dikes are often exceedingly irregular and not all of 
them conform to this plan. 
The number of these dikes is vastly greater than can be indicated on a geolgo- 
ical map of the scale used in this report. They are found in practically all the 
mines of any size and in many cases do not appear at the surface. They can be 
studied to particular advantage in mines situated in the granite, such as the Gold 
Coin, Dead Pine, Granite, and Independence, their green-gray color and aphanitic 
texture contrasting much more strongly with the granitic rocks than with the 
volcanic breccia. In the Gold Coin mine the remarkable irregularity of some of 
the dikes is clearly shown. They branch and coalesce; they swell and pinch out; 
they change abruptly in course and dip; or they turn sharply into irregular, nearly 
horizontal sheets, in some cases resuming the vertical dike-like form a few feet 
away. 
The phonolite dikes are of much economic importance in the Cripple Creek 
district, as will be pointed out in the section devoted to the ore deposits. 
The last eruptions from the Cripple Creek volcanic center were the basic dikes. 
Like the phonolite dikes, they show a certain tendency to radiate from the central 
part of the district. They are less abundant, however, than the phonolite dikes 
and apparently nowhere extend more than 2,000 feet from the volcanic neck, 
while most of them are found within the breccia. Generally nearly vertical, few 
of them are over 5 feet wide and they are usually fairly regular in trend. They 
occur characteristically in zones, of which the individual members seldom continue 
for long distances. Usually as one dike of such a zone pinches, another appears 
in a parallel fissure a few feet away. In some places the two dikes are seen to be 
