106 GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
As in the case of some similar computations/ the middle members of this rock 
series show an approach to a constant ratio of potash to soda, the ratio in this case 
being about 1 : 2 , while the end members show greater variations, the ratio ranging, 
in round numbers, from 1 : 2.9 to 1 :1.4. This is a rather smaller range than has been 
found in many petrographic provinces . * * 6 These ratios also show a general increase 
in the prominence of potash from the phonolite to the monchiquite end of the series, 
though there are several exceptions to this rule. 
Without giving the figures for each rock, it may be stated that computation 
shows that the ratio of molecular proportions of total alkalies to the sum of Fe 2 0 3 , FeO 
MgO, and CaO ranges from 1:0.14 in the phonolites to 1 :5.54 in the monchi quite, 
with fairly even gradation between these limits. 
The ratio of molecular proportions of the sum of Fe,0 3 , FeO, MgO, and CaO to 
silica has a range of 1:33.8 in the phonolites to 1 : 1.6 in the monchiquite, illustrating 
the well-known tendency for these basic oxides to increase as silica falls. 
The following table brings out the fact that, beginning with the phonolites, the 
alumina and silica decrease less rapidly than the total alkalies, and that the change 9 
is fairly uniform. 
Ratios of total alkalies to alumina and silica in rocks of Cripple Creek volcano. 
K2O-F N&20. 
AI2O3. 
SiC>2. 
K2O-F Na20. 
A1203. 
SiOj. 
p. 
1 
0.98 
4.86 
XI. 
1 
1. 41 
7. 52 
V. 
1 
1.17 
6.54 
XII. 
1 
1.43 
7. 09 
VI. 
1 
1.02 
5.27 
XIII. 
1 
1.50 
7.16 
VII. 
1 
1.13 
5.50 
XIV. 
1 
1.59 
VIII. 
1 
1.14 
5. 77 
XV. 
1 
1.56 
7.72 
IX. 
1 
1.20 
5.77 
XVI. 
1 
1.51 
8.85 
X. 
1 
1.26 
7.01 
In the more alkalic rocks the close approach to unity of the ratio of alkalies 
to alumina is of interest when compared with the amount of minerals in those rocks 
which have this ratio—orthoclase, albite, nepheline, and analcite—together with 
the slight amount of alumina which enters into the pyroxenes in those rocks. In 
the phonolites soda orthoclase, nepheline, and analcite make up about 85 per cent 
of the rock. The excess of alkalies over alumina in this case is due to the soda in the 
segirine and to the excess of soda over alumina in sodalite and nosean. In the more 
basic end of the series the predominance of alumina over alkalies, when taken in 
connection with the comparatively small importance of the anorthite molecule, is of 
interest in showing that the alumina must enter largely into the composition of the 
pyroxene, forming augite instead of diallage or diopside, the minerals which would 
naturally be expected in such rocks. 
a Weed and Pirsson, Geology of the Castle Mountain mining district, Montana: Bull. U. S. Geol. Survey No. 139, 1896, 
pp. 137, 138. 
6 Weed and Pirsson, op. cit., p. 137. Washington, H. S., op. cit., p. 402. Iddings, J. P., The igneous rocks of Electric Peak 
and Sepulchre Mountain: Mon. U. S. Geol. Survey, vol. 32, 1899, p. 136. Adams, F. D., The Monteregian hills: Jour. Geol., 
vol. 11, 1903, pp. 265, 271. Pirsson, L. V., Igneous rocks of the Highwood Mountains, Montana: Bull. U. S. Geol. Survey No. 
237, 1905, pp. 172, 173. 
