112 GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
By referring to the table on page 104, it is seen that, as was to be expected, this 
rock comes between the phonolites and latite-phonolites and agrees very closely 
with the most basic phonolite. 
A few words may be added concerning the meaning and value of this analysis. 
In arriving at this result the assumptions made have been perhaps wholly incapable 
of conclusive demonstration: they have been more or less arbitrary, and largely 
dependent on the personal factor. It is believed, however, that the result given is an 
approximation to the truth and hence of value. This average rock is not proposed 
as representing the “originally homogeneous magma” from which the various 
types of rock have been derived by segregation, differentiation, liquation, fractional 
crystallization, or other means. Some petrologists are led by principles of physical 
chemistry to believe that the material which is extruded from the main reservoir is 
often not representative of the total material in the reservoir. But it is believed 
that this analysis gives an approximate idea of the composition of the material 
which has issued from this volcanic reservoir; that is, the average rock of the 
Cripple Creek volcano. 
On comparing this analysis, which is represented on the chart (PI. X) as C, 
with that of the average rock of the earth’s crust, it is apparent that in silica con¬ 
tent the difference is not great, but it is very much richer in alkalies and contains 
noticeably more alumina. Ferrous iron, magnesia, and lime are very low in the 
Cripple Creek rock, but ferric iron and water are about the same as in the average 
rock of Clarke. 
The ratio of potash to soda in the average rock of the earth’s crust is 1: 1.75. 
The ratio of potash to soda in this average Cripple Creek rock is 1: 2.39, a figure 
considerably higher than the apparent mean value of 1: 2 in the table on page 105. 
It is believed that this illustrates the danger of placing too great confidence on 
deductions made from analyses without having regard to the quantitative relations 
of the rocks which the analyses represent. 
The ratio of total alkalies to the sum of Fe,0 3 , FeO, MgO. and CaO is 1:0.36. 
The ratio of this sum to silica is 1: 14.13. The predominance of ferric over ferrous 
iron accounts for the jegirine molecule in the phonolites. * 
The ratio of total alkalies to alumina and silica is 1:1.01 : 5.13, and of alumina 
to silica 1: 5.05. These last two ratios emphasize the nearly unit ratio of alkalies 
to alumina and confirm the statements made on page 107 as to the constant relation 
of alumina to silica in these rocks. 
The ratio of alkalies to alumina and silica is very nearly stoichiometric, but 
probably only by coincidence. 
The appearance of orthoclase and analcite as final separations from the molten 
material and the constant relation of alumina and silica throughout the series may 
be some indication of an approach to the eutectic composition for the original 
magma. 
Besides the essential oxides, high zirconia and chlorine and low phosphoric 
acid are characteristic of the Cripple Creek average when compared with the average 
rock of Clarke. 
If lavenite is present in the phonolites, the rock contains original fluorine. 
