TERTIARY VOLCANIC ROCKS-BRECCIA. 
97 
SUMMARY. 
Nothing of very general application can be stated regarding the relative age of 
these three classes of dikes. In the Pinto mine the intersection of the Isabella 
and Pinto dikes is shown, and though, owing to the nature of the workings, a final 
opinion can not he reached, apparently the larger (Isabella) dike cuts and is there¬ 
fore younger than the northward-trending Pinto dike. In the Block 8 mine a 
large dike which corresponds in every way to the Isabella dike cuts and faults the 
smaller monchiquite dike. Whether or not other dikes of the same respective 
classes are contemporaneous with these was not determined. On the assumption 
that they are, it may be said that the Isabella trachydolerite is younger than either 
the vogesite or the monchiquite, but no evidence has been found as to the relative 
age of the last two. 
The foregoing descriptions make it plain that there are marked mineralogical 
and textural differences in these three groups of rocks. The analyses, on the other 
hand, show that the rocks are very closely related chemically and that they differ 
scarcely more from one another than they do from other rocks in the classificatory 
division in which they have been placed. For this reason the names assigned to 
them are not wholly satisfactory, but nevertheless seem to be the best now in use. 
It is possible that analyses of other specimens would show what is suggested by 
some of the thin sections—a still closer chemical relationship between the three 
groups and perhaps a complete gradation from one extreme to the other. 
Finally, these basic dike rocks as a whole show close relationship to the type 
just preceding, the Bull Cliff trachydolerite, and through it are connected with all 
the foregoing Tertiary rocks. They complete, in the order recognized as normal, 
the series of eruptions from the Cripple Creek volcano. 
BRECCIA. 
The pyroclastic or fragmental volcanic rocks of the district have an extensive 
development. The largest area is a rude ellipse with the longer axis extending 
about 4 miles in a northwest direction from just north of the summit of Big Bull 
Mountain to the pass between Carbonate and Tenderfoot hills. The greatest 
width is about 2\ miles, reaching from the west side of Guyot Hill to Grassy Creek 
at Cameron. Masses of considerable size occur on Mineral Hill and Rhyolite 
Mountain, and smaller bodies are found on Copper Mountain, east of Galena Hill, 
on the summit of Big Bull Mountain, and on the upper part of Mount Pisgah just 
west of the area covered by the map. 
The fragmental character of these rocks is generally discernible with readiness. 
They are, moreover, wholly fragmental, and do not consist, as is often the case, 
of numerous fragments held in a matrix which solidified about them from the 
molten state. They are made up of angular pieces which are, on the average, equi- 
dimensional and which vary in size from very minute grains to blocks over a meter 
in diameter. The name “ breccia ” is here applied to all these rocks, though “ tuff ” or 
“agglomerate” are in some cases more appropriate terms. There is usually no 
assortment of the material, and coarse and fine pieces occur together. The variety 
