26 GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
the central volcanic abyss. Its floor pitches steeply to the south and on an east- 
west line drawn through the summit of Gold Hill may attain a depth of from 1,500 
to 3,000 feet below the present surface. This and other similar embayments in 
the rocks rimming the volcanic neck are doubtless due to the tendency of the vol¬ 
canic forces to find relief in a lateral as well as in an upward direction as the} T neared 
the surface. They may be compared to the flaring enlargements noticeable on the 
side of issue of a bullet hole through plate glass. 
At the Pointer and Mint mines, on the southwest side of Gold Hill, the granite 
and breccia are locally separated by an intrusive mass of syenite. The general dip 
of the pit wall in this vicinity, however, as shown by comparison of its outcrop 
with its exposure in the Ophelia tunnel, is between 70° and 80°. In the vicinity 
of Anaconda and in the Mary McKinney mine the contact between the granite and 
the breccia is nearly vertical, and continues so along its irregular course toward 
Elkton. 
In the Elkton mine the contact between the granite and breccia is well shown. 
North of the Thompson shaft of the Elkton property the contact runs nearly east 
and west, while east of the shaft it runs nearly north and south. This shaft is thus 
situated within a local granite promontory which juts northeastward into the 
breccia of the volcanic neck. The contact as seen in the Elkton mine is irregular 
in detail, but it is clear from its position on successive levels that the promontory 
steeply overhangs the breccia. The granite as a rule shows considerable shattering 
near the contact, which, however, is usually fairly sharp and is too irregular to be 
a fault contact. Fragments of granite are abundant in the breccia, often for dis¬ 
tances of more than 300 feet from the contact. 
Although the original contact between the granite and breccia in the Elkton 
mine is not a fault plane, there has been faulting in the vicinity. The Thompson 
fault or so-called “Dead vein” lies usually just north of the contact. It strikes 
N. 72° W. and dips at 83° S. It is sometimes a simple fissure containing a foot 
or more of soft slickensided gouge, sometimes two or more narrower parallel fissures 
filled with similar material. The throw is apparently normal and the displacement 
probably not more than 100 feet. At a few points, as on level 4, the fault locally 
marks the contact between granite and breccia. The fault is an example, similar 
to others which will be described later, of local movement near the original contact. 
Such fault fissures in most cases more nearly approach a plane surface than does 
the adjacent granite-breccia contact. The breccia had filled the volcanic conduit 
and become somewhat indurated before the faulting took place. 
Between Elkton and Victor the contact passes between Squaw and Battle 
mountains and is approximately vertical. It is well exposed on nearly all the levels 
of the Ajax mine and has in many places been drifted on for considerable distances. 
It is prevailingly steep, dipping north or northeast at angles ranging usually from 
45° to 85°. The average dip is probably between 70° and 75°. Near the shaft, 
particularly on the upper levels, the contact has an east-west course. But, as is 
shown on Pis. II and V, the Ajax shaft is situated in an embayment in the granite 
between the Portland promontor}^ on the east, presently to be described, and 
the northward turn of the contact, which carries it through the saddle between 
Squaw and Battle mountains on the west. This northerly turn is noticeable on all 
