GEOLOGICAL STRUCTURE OF THE DISTRICT. 
25 
different rock fragments as to indicate that the great chasm in the granitic rocks, 
after its initial filling with breccia and intrusive rock, was more than once partly 
or wholly cleared by renewed explosions and refilled with their products. 
These eruptions undoubtedly built up a volcanic cone upon the plateau, but 
the materials of this cone have been removed by erosion very nearly to the original 
plateau surface. Consequently the main central area of breccia shown on the 
geological map more nearly represents a plan of the volcanic throat than it does 
the extent of the former cone. It is important to determine as closely as possible 
how much of this area represents the outline of the deep volcanic chasm or throat 
and how much is merely a residual portion of the practically vanished cone, and 
thus rests upon the old plateau surface. It is also important to ascertain, if pos¬ 
sible the inclination and general character of the contact between the breccia which 
fills the volcanic throat and the granitic rocks through which the eruptive materials 
forced a passage. A convenient plan is to begin in Poverty Gulch, just east of 
Cripple Creek, and thence to trace the outline of the volcanic neck in a circuit past 
Anaconda, Guyot Kill, Elkton, Squaw Mountain, Victor, Bull Cliff, Ironclad Hill, 
and Globe Hill. 
The contact is exposed at several places in the Abe Lincoln mine, in Poverty 
Gulch. For the first 360 feet in depth it seems to have a dip of 80° or 85° S. For 
the next 140 feet the general dip is probably not over 30°. A short distance east 
of the Abe Lincoln mine the Chicago tunnel enters Globe Hill from Poverty Gulch 
and extends eastward to the Plymouth Rock shaft, between Globe and Ironclad 
hills. The tunnel is chiefly in breccia, which near Poverty Gulch is composed 
largely of schist fragments. Lateral branches, however, penetrate solid schist to 
the north and south of the line of the main tunnel. The schist on the north is 
continuous with a mass exposed at the surface in Poverty Gluch. That on the 
south is continuous with the schist exposed at Fairview. The two contacts dip 
toward each other at from 35° to 45°. If these dips are maintained, the Fairview 
and Poverty Gulch schist masses should unite at a depth of from 200 to 300 feet 
below the tunnel. 
In the Midget and Conundrum mines, on the west slope of Gold,Hill, the con 
tact between the ancient crystalline rocks and the breccia is irregular, is at many 
points poorly defined, and has not been exposed in a sufficient number of places to 
fully establish its form and character. All indications, however, point to an increas¬ 
ing steepness of the contact toward the south, and in the Good Will tunnel, which 
passes from granite into breccia somewhat less than a thousand feet south of the 
Midget shaft, the contact is nearly vertical. In the Abe Lincoln and particularly 
in the Conundrum and Midget mines the gneiss is more or less shattered for some 
distance from the contact, and the breccia is largely composed of gneiss and schist 
fragments. 
While the contact as exposed in the Chicago tunnel and in the Abe Lincoln, 
Midget, and Conundrum mines is not so steep as it becomes farther south, the breccia 
of the northwest slope of Gold Hill clearly occupies a pit produced by explosive 
volcanic activity and does not rest in an eroded hollow in the prevolcanic plateau. 
Lying between the main rim of schist and gneiss on the west and the Fairview 
promontory of schist on the east, this pit is apparently a marginal embayment of 
