108 GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
cutting the plane into sectors of 45° each. The accompanying chart (PI. X) shows 
the disposition of the oxides, the length of the radii representing the relative amount 
of the respective constituents. The silica can not be feasibly shown in these dia¬ 
grams because of its large amount. It seems desirable to separate the ferrous and 
ferric iron whenever possible, particularly in rocks of this kind where the asgirine 
molecule enters. It is a satisfaction to feel that such a separation is in this case per¬ 
fectly warranted by the accuracy of the analyses made by the chemists of the 
Survey. Water is essential in these rocks and has been given a place in the diagram, 
while titanic oxide has been omitted, both because of the difficulty in representing 
it and because of its comparatively small importance in the composition of this 
group of rocks. 
The center of each diagram is located with respect to a pair of coordinates, of 
which molecular proportions of silica are the abscissas and molecular proportions of 
total alkalies are the ordinates. This was the most practical manner of representing 
the silica, and as it is admittedly the most important constituent of igneous rocks it 
is perhaps not illogical to distinguish it from the other oxides. In the case of this 
particular rock series the total alkalies are of next importance and have been empha¬ 
sized by being made the other determinant of the position of the diagram. No 
particular advantage could be seen in Iddings’s method of using the ratio of total 
alkalies to silica as ordinates—at least for these rocks, where it seemed only to com¬ 
plicate and obscure the relations which it was desired to show. 
To continue a simile which Iddings has used, likening these diagrams to ships 
moving away from the observer, it may be said that a large spread of sail indicates 
high alkalies and alumina; a large hull indic-aft's high iron, magnesia, and lime; a 
high mast indicates much alumina; large draft points to high magnesia; upper 
sails out to starboard stand for soda and to port for potash; ferric iron is situated at 
the starboard rail and combined water at the port; ferrous iron and lime make up 
respectively the starboard and port sides of the hull; the farther to the right the 
diagram is situated on the chart, the more siliceous is the rock; the higher it is the 
more alkalic. The converse is of course true. 
Because of the definiteness and uniformity of composition of the phonolite 
group, the four phonolite analyses have been averaged® and the resulting analysis 
represented by the diagram P. The other diagrams are numbered according to the 
table on page 104. To these thirteen diagrams is added, in its proper position, the 
diagram representing the average rock of the earth’s crust (marked E) according to 
Clarke’s latest computation. 6 
The position which the Cripple Creek rocks occupy with reference to this average 
rock shows at once that they are in general low-silica, high-alkali rocks; that, with 
the exception of the biotite trachyte (No. V), which in several respects diverges from 
the series, the most siliceous rock corresponds to the average rock in silica content, 
and the least alkalic corresponds to it in the proportion of alkalies. 
A comparison of the individual diagrams with that of the average rock indicates 
also that on the whole the Cripple Creek rocks are alumina rich. Iron, magnesia, 
and lime are below the average in all but the last five members, in which they are 
above. Ferric iron is on the whole high, while ferrous iron is low. Magnesia 
a See p. 66 
b Bull. U. S. Geol. Survey No. 228, 1904, p. 19. 
