PETROLOGY-ROCKS OF THE CRIPPLE CREEK VOLCANO. 
103 
MINERALOGICAL CHARACTERS. 
As may have been inferred from a perusal of the description of these rocks, 
they have unquestionably been derived from some common source, or, as petrog- 
raphers say, from the same magma. The calculation of all the analyses into per¬ 
centages of constituent minerals would furnish a very valuable table for comparison 
of likenesses and differences throughout the series, but owing largely to the number 
of variable factors which would enter into such calculations it was found impracti¬ 
cable to do this. Since, with the exception of two varieties, the rocks are fine¬ 
grained porphyries it was likewise impossible to apply Rosiwal’s 0 metric method 
at all easily or accurately. But the more important minerals may be tabulated to 
show the mineralogical similarities and differences throughout this rock group. 
The characteristic and distinctive mineral of the phonolites is nehpeline; olivine 
may be said to occupy an analogous position in the basic rocks. The tegirine of 
the former is balanced by the augite of the latter. The abundant alkali feldspar 
of the one class has a prominence which is occupied by calcic plagioclase and by 
hornblende and biotite in the other. Xosean and sodalite are noticeable con¬ 
stituents of the phonolites; the basic rocks contain considerable quantities of 
apatite. Intervening members of the series are in general characterized by none 
or by smaller amounts of these distinctive minerals, or by a combination of minerals 
from both extremes. They also contain minerals of an intermediate composition, 
like the sodic plagioclases and aegirine-augite. So much for the mineralogical 
differences. The points of similarity are not so numerous, but are perhaps more 
striking. The most notable is the fact that orthoclase and analcite occur in 
practically all members of the series. Another point of likeness is that the extreme 
members are linked together through the intermediate character of the middle 
members just outlined. These features can perhaps be better presented in a table. 
Characteristic minerals in roclcs of Cripple Creek volcano. 
Phonolites. 
Intermediate rocks. 
Basic-dike rocks. 
Nepheline. 
Soda orthoclase. 
Albite. 
Algirine. 
JEgirine-augite. 
Sodalitel 
(Sodalite. 
Nosean J 
|Nosean. 
Orthoclase. 
Orthoclase. 
Analcite. 
Analcite. 
Apatite. 
Apatite. 
Augite. 
Augite. 
Oligoclase.. j 
(Labradorite. 
Biotite.i. 
| Biotite. 
HornblendeJ 
[Hornblende. 
Olivine. 
The conditions of cooling, as revealed by the texture of these rocks, apparently 
had but slight effect on the mineral composition. To be sure, the plutonic type, 
the syenite, often contains no analcite, but in other respects the mineral develop¬ 
ment is practically independent of texture. 
a Verh. Wien. geol. Reichsanst., 1898, vol. 32, pp. 143 fi. 
