PETROLOGY-ROCKS OF THE CRIPPLE CREEK VOLCANO. 107 
The above table shows a close relation between silica and alumina, which is 
better represented below: 
Ratios of alumina to silica in rocks of Cripple Creek volcano. 
P. 1 : 4.98 
V . 1 : 5.59 
VI .. 1 : 5. 18 
VII ... 1 : 4.82 
VIII .. 1 : 5.06 
IX . 1 : 4.82 
X . 1 : 5.56 
XI . 1 : 5.33 
XII . 1 : 4.94 
XIII . 1 : 4.78 
XIV . 1 :4.87 
XV . 1 : 4. 96 
XVI .a 1 : 5.83 
This agreement throughout the series is really very striking and constitutes one 
of the best proofs that genetic relations exist between all these rocks. It almost 
wholly excludes the possibility of there having been more than one original source, 
or, conversely, it furnishes very strong proof that these rocks were derived from a 
common magma. It shows, furthermore, that although considerable differentiation * 6 
has taken place, as indicated by the range of percentages on page 104, the silica and 
alumina have nevertheless clung together during the processes which caused this 
result—whatever lias affected the one has equally affected the other. 
This is the only instance known to the writer in which an almost absolute 
chemical consanguinity (the sole fundamental factor) has been shown to exist 
between the various rock types of a petrographic province. The nearest approach to 
proof of definite chemical relationship which has been found in the literature is the 
exposition of the serial relations of the various rocks in certain areas, as in the Little 
Belt Mountains in Montana 0 and in "the Magnet Cove laccolith in Arkansas.^ 
It is believed, moreover, that this practically constant ratio of alumina to silica, 
together with the intimate, gradually changing relationship of the other oxides 
throughout the series, constitutes the best evidence yet brought forward toward 
proving what has heretofore been only assumed e —that the common magma was 
originally homogeneous. 
From a mineralogical point of view, this alumina-silica ratio, which is approxi¬ 
mately 1:5, indicates a large proportion of molecules having the orthoclase-albite 
ratio, 1:6, combined with minerals like nepheline, analcite, the remaining plagio- 
clases, and the pyroxenes, which in general have a ratio of alumina to silica lower 
than 1:5. 
COMPOSITION OF THE ROCKS EXPRESSED BY DIAGRAMS. 
Certain of these relations between these rocks are very well shown when the 
proportions of the essential oxides are represented graphically. For this purpose 
the general method adopted by Iddings-f has been used, but a few modifications 
have been introduced. The individual diagrams, each representing one analysis, 
are constructed on four lines in the same plane, intersecting at a common point and 
a The exceptional divergence in this case may be partially due to the decomposition which the rock has undergone. 
6 The term differentiation is here used in the same limited sense as employed by Pirsson. See Eighteenth Ann. Rept. U. S. 
Geol. Survey, pt. 3,1898, p. 573; Twentieth Ann. Rept. U. S. Geol. Survey, pt. 3,1900, pp. 566-567; and Bull. U. S. Geol. Survey, 
No. 237, 1905, p. 183. 
e Pirsson, L. V., Twentieth Ann. Rept. U. S. Geol. Survey, pt. 3, 1900, p. 571. 
d Washington, H. S., Igneous complex of Magnet Cove, Arkansas: Bull. Geol. Soc. America, vol. 11,1900, p. 403. 
« Cf. Bull. U. S. Geol. Survey No. 139, 1896, p. 141. 
/ Prof. Paper U, S. Geol. Survey No. 18, 1903. • 
I 
