PETROLOGY-ROCKS OF THE CRIPPLE CREEK VOLCANO. 
113 
ORDER OF SUCCESSION OF THE ROCKS. 
It is impossible to believe that near the beginning of eruptive activity effusive 
types were not more abundant than the few occurrences mentioned would indicate. 41 
It is true, however, that by far the greater part of the rocks which now remain are 
intrusive. Latite-phonolite, together with syenite, was probably the first of these 
to be erupted. It was followed by phonolite, Bull Cliff trachydolerite, and the 
basic dike rocks in the order given. This sequence is a common one, beginning 
with a rock of intermediate composition, passing to one extreme, the phonolites, 
and thence finally to the other extreme, the basic dike rocks. These last do not 
appear to conform exactly to the general sequence, since the Isabella trachydolerite 
was probably erupted after the vogesite and monchiquite, but they are so similar 
in composition that they may best be considered as one group. In connection 
with the sequence, the radial grouping of the dikes is of interest. 
SUMMARY. 
The Tertiary volcanic rocks of the Cripple Creek district, except the rhyolite, 
afford an excellent illustration of a petrographic province, i. e., they have clearly 
been derived by differentiation from a single originally homogeneous magma. 
The average rock of the Cripple Creek volcano is characterized by very high soda, 
high potash, high alumina, noticeable sulphuric anhydride, chlorine, and zirconia, 
and low ferrous iron, magnesia, and lime. • All the rocks have approximately the 
same ratio of alumina to silica. 
a Cl. p. 20. 
