334 GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
coarsely porphyritic Pikes Peak type, such as forms Squaw Mountain. North of 
the Thompson the granite-breccia contact runs nearly east-west, while east of the 
shaft it runs nearly north-south. The Thompson shaft is thus situated within a 
local granite promontory which juts northeastward into the breccia. The contact 
is very irregular in detail, but it is clear from its position on successive levels that 
the promontory in general steeply overhangs the breccia. The granite, as a rule, 
shows considerable shattering near the contact, which, however, is usually fairly 
sharp. Fragments of granite are also abundant in the breccia, in many places for 
distances, of more than 300 feet from the contact. 
The Thompson fault, already referred to, is chiefly in the breccia, though 
nowhere, so far as the present workings show, very far from the granite, except 
where the latter turns southward, east of the Thompson shaft. It is in some 
places a simple fissure containing a foot or more of soft slickensided gouge, in 
others two or more narrower parallel fissures filled with similar material. The 
Thompson fault slightly offsets the basalt dike and the north-south fissures of the 
Walter zone. The net displacement is probably not over 100 feet, though no 
actual measurement of the movement could be made with the exposures available 
at the time of visit. The throw is apparently normal. At a few points, as on 
level 4, the Thompson fault locally marks the contact between the granite and 
the breccia. The fault is merely a more conspicuous example of such local faulting 
near the contact as was noted in the Portland mine (p. 435), the fault fissures 
approaching much more nearly to a plane surface than does the adjacent granite- 
breccia contact. 
The breccia occasionally exhibits a banded structure similar to that observed 
in the Portland mine. Such structure may be seen on level 7, about 450 feet 
north of the Elkton shaft. The banding is here about vertical and is produced by 
very distinct and sharp alternations in the relative coarseness of the breccia particles. 
Similar lamination of the breccia occurs on level 6, about 200 feet south of the 
shaft, the structure in this case dipping north at an angle of 40°. As in the Port¬ 
land, this banding seems to be a local feature and passes by indefinite gradations 
into the ordinary breccia . 
Among the recognizable fragments composing the breccia, phonolite undoubt¬ 
edly predominates, both in the Elkton mine and in the Anaconda-Raven tunnel, 
which furnishes a section through Raven Hill. Fragments up to 6 inches in diam¬ 
eter are common, while much larger masses are occasionally found. The alteration 
of the breccia near the ore bodies consists chiefly of the metasomatic development 
of valencianite, quartz, fluorite, and a little sericite, pyrite in sharp minute pyrito- 
liedrons, and apatite in small greenish-white prisms. Carbonates, particularly 
dolomite, which are common in the altered breccia of other mines, are not abundant 
in the Elkton, and in many places seem to be entirely absent. 
The volcanic breccia of the Elkton workings is cut by several irregular intru¬ 
sive masses of phonolite. One of these is the dike of “purplish phonolite” which 
was noted by Cross® in the upper Raven tunnel and which is exposed on the surface 
passing just above the mouth of the tunnel and the Tornado shaft. The same 
a Geology and mining industries of the Cripple Creek district, Colorado: Sixteenth Ann. Rept. U. S. Geol. Survey 
pt. 2, 1895, p. 89. 
