98 GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
most commonly met with consists of fragments whose maximum size is 2 to 3 
cm., in a more finely granular matrix. In some cases all the grains are of small 
size, and then the true clastic nature of the rock is not always easily detected. 
Occasionally, however, a distinct banding is apparent, the material being well 
assorted into even layers a millimeter or less in thickness of extremely fine mate¬ 
rial, alternating with layers sometimes several centimeters thick of coarser yet 
still very small particles. Bedded tuff of this kind is seen on the slope north of 
Altman, at the southwestern base of Bull Cliff, and in the Lucky Guss No. 2 and 
Portland mines. In some cases the bedding is nearly or quite horizontal; in others 
it is much inclined. The appearance of some of this bedded tuff suggests that it 
is a deposit in quiet water. 
All these rocks are considerably altered, even at the greatest depths attained 
in the mines. The freshest are soft, not very compact, of reddish, purplish, or 
bluish-gray color. Slight weathering changes them to brownish, probably by 
conversion of the iron into limonite. Most of the rock appearing at the surface 
has a light-yellowish color and is compact, owing to decomposition by atmospheric 
agencies. Hydrometamorphism, however, has been only one factor in the altera¬ 
tion of these rocks. Impregnation by dolomite and pyrite is a very common 
feature and may be attributed to the action of gases and solutions emanating 
from the seat of the Cripple Creek volcano. Small crystalline grains of pyrite or 
characteristic little cavities which lesult from its oxidation frequently serve to 
distinguish fine-grained, decomposed breccia from massive latite-phonolite. When 
rock thus heavily pyritized is exposed on a dump to the action of the atmosphere 
it frequently swel s, cracks, and crumbles to a greenish-yellow powder. 
Near the contact with granite and with schist the breccia generally contains 
a greater or less proportion of those rocks or of their constituent minerals. But 
the presence of these materials is uncommon in the great mass of the breccia. It 
appears, therefore, that the ancient crystalline rocks which occupied what is now 
the throat of the volcano were shattered and thrown out of the opening by violent 
explosions. 
Cross, judging from specimens generally much decomposed, was of the opinion 
that the breccia is made up largely of andesitic rock. The extensive mine work¬ 
ings now give much additional information as to the character of these fragmental 
materials. In spite of the alteration which it has suffered, an examination of the 
breccia shows that it is made up chiefly of phonolite and latite-phonolite. The 
abundance of phonolite fragments in the breccia and the sharp contact of the 
massive phonolite with it prove that there took place at least two phonolite intru¬ 
sions of considerable magnitude. But with the latite-phonolite the case appears 
to be different. Sharp contacts with the breccia do appear, it is true, as in the 
Trail tunnel, but in general the massive rock passes, by a gradual increase of shat¬ 
tering, into true breccia composed of various materials. In several places, as on 
the northeast knoll of Battle Mountain, the latite-phonolite is simply crushed and 
the fragment^ are only slightly dislocated. At increasing distance from the massive 
rock the disturbance becomes greater and the derangement of the grains finally 
admits the mingling of foreign fragments. In many places, particularly on the 
western slope of Bull Hill, an intermediate stage of this shattering has resulted 
