GEOLOGICAL STRUCTURE OF THE DISTRICT. 
29 
breccia near the contact is in some cases, as on the 1,400-foot level, so full of granite 
fragments, some of them of large size, that it is difficult to determine in a small 
exposure whether the rock seen is breccia or massive granite. 
While in the Portland mine the contact between the granite and the breccia 
is generally convex toward the northeast, in Stratton’s Independence mine it is 
concave. In the Portland the granite projects into the breccia as a steep promon¬ 
tory; in the Independence the breccia lies in the curved embayment on the east 
side of this jutting point of granite. The benched form of the contact surface 
described in the Portland is also well exhibited in the northern part of the Inde¬ 
pendence embayment. The general dip of the contact is at least 60° N. or NW. 
Further details of this portion of the contact will be found in the description of the 
Independence mine on pages 449 to 465. 
The Golden Cycle shaft, 1,000 feet deep, is wholly in breccia, though the granite 
at the surface is only 1,400 feet distant to the southeast. This shows that the slope 
of the contact must be greater than 35°. 
On Big Bull Mountain the Safety mine, another shaft higher on the northern 
slope of the mountain, and a tunnel and several pits on the northeast side all show 
a nearly vertical contact. Near Grassy Creek there are no deep workings near the 
contact. Its course, as shown on the map, is suggestive of a low angle of dip, and 
a shallow pit on the slope southeast of Cameron indicates that this part of the 
breccia may be merely resting on an old surface of erosion. 
The workings of the New Century, Damon, Jerry Johnson, Sunshine, and 
Iloosier mines, though neither deep nor exclusive, all point to a very steep contact 
along the northeastern border of the breccia area. From Tenderfoot Hill to Car- 
bonate Hill the contact is not explored by deep workings, but a zone of greatly 
shattered granite and schist bordering the fragmental volcanic material is indica¬ 
tive of violent eruptive forces and of a steep contact. At the C. O. D. mine the 
schist-breccia contact is fairly steep. 
The island-like mass of shattered granite which forms parts of Bull and Iron¬ 
clad hills lies almost in the center of the main volcanic area and derives exceptional 
interest from its rather remarkable position. The workings of the Bogart, Sheriff, 
War Eagle, and Ramona No. 2 mines show that to the south the contact of this 
granite with the breccia is steep. This is confirmed by the absence of granite in 
the American Eagle mine, though the bottom level, 1,500 feet deep, extends to 
within 400 feet of a point vertically under the contact at the surface. The char¬ 
acter of the contact on the west, north, and east sides of the mass is unknown, 
though the outline of the granite as shown on the map (PI. II, in pocket) indicates 
a high angle of dip. 
Granite, unfractured and with a steep contact surface, appears on level 11 of 
the Isabella mine, about 800 feet vertically below the Emma No. 2 shaft house. 
This granite is of the Pikes Peak type, however, while that of the Bull Hill area 
and of the tongue which extends west of Cameron is of the Cripple Creek type. 
Whether this body of granite occurring at so great a depth represents the crater 
wall, of which the average slope from the surface down is in such case not much 
over 45°, or whether it is a huge fragment which parted from the side of the chasm 
above and sank into the pit, it is impossible to say. 
13001—No. 54—06-4 
