110 GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
bodies of latite-phonolite. The former certainly appears to decrease in amount 
with depth; but, as has been shown, its amount is so small relative to the total mass 
of eruptive rock that this error is practically insignificant. With the latite-phonolite 
the case is different. Large bodies are known to occur which are flat and ultimately 
give out in depth or diminish to comparatively narrow dikes; but, because of this 
flat shape, the actual horizontal extent of these bodies is in several known cases 
considerably greater than their area exposed on the surface. Moreover, flat bodies 
which do not appear at the surface have been encountered in some of the mine 
workings and this makes it fair to assume that still other buried masses of latite- 
phonolite exist at various depths. So, while the assumption that the present 
exposed area of this rock represents its relative volume is unfounded, still it does 
not seem at all improbable. But even if this assumption be wholly untrue the 
error which its use introduces is not so great as might at first appear. In all 
cases where the latite-phonolite has been found to give out in depth it has been 
underlain by breccia. But as has been shown, the breccia is composed of about 
half latite-phonolite; the other half, phonolite, is not very different in composition 
from latite-phonolite. Any error which may result from this assumption will 
therefore tend only to throw the computed composition of the average rock a little 
farther from phonolite than is actually the case. 
On the whole, therefore, the first of the following assumptions seems warranted, 
as well as the remaining ones: 
1. The volumetric proportion of the various rock types corresponds to their 
areal proportion on the present surface. 
2. The same specific gravity (about 2.55) may be assigned to all the rocks 
except the breccia, which is given 0.8 of that value. 
3. The breccia is composed of 5 per cent granite, gneiss, and schist; 50 per cent 
phonolite; and 45 per cent latite-phonolite. 
4. The narrow- phonolite dikes make up 1 per cent ot the total massive phonolite. 
5. The basic dikes have an average width of 4 feet, but because of their ready 
decomposition and consequent frequent escape of detection this width is increased 
to 6 feet. 
6. Of the basic dikes, the Isabella trachydolerite comprises 50 per cent, vogesite 
10 per cent, and monchiquite 40 per cent. 
The areas of the rocks were accordingly measured on the map by means of 
planimeter and scale, and are shovm in the table below: 
Areas of the Cripple Creek volcano rocks. 
Rock. 
Actual 
area.a 
Area cor¬ 
rected for 
breccia. 
Rock. 
Actual 
area.a 
Area cor¬ 
rected for 
breccia. 
Phonolite. 
Sq. miles. 
6.542 
Sq. miles. 
9.344 
Isabella trachydolerite 
Sq. miles. 
0.005 
.001 
.004 
6.636 
Sq. miles. 
0.005 
.001 
.004 
Biotite trachyte. 
. 130 
.130 
Vogesite. 
Latite-phonolite. 
.678 
3.039 
Monchiquite. 
Syenite. 
.143 
.143 
Breccia. 
Bull Cliff trachydolerite. 
.046 
.046 
12. 712 
a Of this, 4.230 square miles of phonolite and 0.169 square mile of breccia are not included by the Cripple Creek map. See 
Pikes Peak folio: Geologic Atlas United States, folio 7, U. S. Geol. Survey, 1894. 
