PETROLOGY-ROCKS OF THE CRIPPLE CREEK VOLCANO. 
] 09 
exceeds the average only in two rocks; but lime is about equal to or greater than the 
average in most of the types. Water is on the whole a little high. 
The combination of high alkalies and alumina with moderate silica gives to this 
rock group the distinctive and rather uncommon feature of containing practically no 
quartz. 
AVERAGE ROCK OF TIIE CRIPPLE CREEK VOLCANO. 
Another and perhaps more valuable comparison might be made with the 
average rock of the earth’s crust if some quantitative relation could be instituted 
between the Cripple Creek rocks and the composition of the average rock of the 
volcano thus be obtained. 
By making certain assumptions, which are more or less supported by fact or 
probability, it is possible to obtain an approximate idea of the composition of the 
material extruded from the volcanic reservoir which must have underlain the 
Cripple Creek district. 
It has been shown in a preceding chapter (p. 21) that the main breccia area 
practically occupies the throat of the volcano and that the walls of this conduit 
are very steep. If all the masses of eruptive rock which appear at the surface 
continue with uniform size in depth, then their volumes would bear to each other 
the same proportion as their areas. But it is certain that the size of these masses 
does vary at different depths. Mine workings which extend as deep as 1,500 feet, 
however, do not indicate any noticeable change in the relative abundance of the 
various rock types, with the exception of two cases that will be considered presently. 
There is another fact also which tends to make a relation on the basis of area 
fairly representative. An uncertain but considerable amount of volcanic material 
has been removed by erosion. If, therefore, the contacts of a mass diverge down¬ 
ward, showing that the amount of material is greater below, they would in general 
have converged upward and have inclosed less of the rock than the present surface 
exposure indicates, and vice versa. Accordingly, while the surface area as now 
shown may not be an exact average cross section of the mass throughout its vertical 
extent, it nevertheless approaches such an average. 
Furthermore, only a glance at the map is needed to show that phonolite, latite- 
phonolite, and breccia comprise by far the greater proportion of the volcanic rocks. 
The breccia is made up almost wholly of the two other rocks, in approximately 
equal amounts. The composition of the latite-phonolite does not vary greatly 
from that of phonolite. Finally, the amount of phonolite occurring within the 
limits of the area mapped is only about half as great as the amount which occurs 
outside those limits, but in the same general locality, and which must of course be 
included in the computation. It is therefore possible to predict, without further 
estimates, that the average rock will have a composition between phonolite and 
latite-phonolite, and that it will be nearer to phonolite. It is obvious, therefore, 
that any inaccuracies which may enter into the calculation of the relative amounts 
of the different rocks, be they theoretical or mechanical, will be very much mini¬ 
mized by this fact. 
The two exceptions to the general rule that the igneous masses have on the 
average a nearly vertical contact are the Bull Cliff trachydolerite and some of the 
13001— No. 54—06-9 
