PETROLOGY OF THE DISTRICT-GRANITE GROUP. 
101 
rhyolite and grits is at nearly every place a sharp and close one. The rhyolite at 
the contact appears to be no more weathered than elsewhere, but it does show a 
narrow zone of finer, denser texture in immediate contact with the grits. The flow 
structure is parallel to this contact. In many places the upper part of the rhyolite 
is broken and cracked, and the fragments are angular. While the grits appear to 
have been in most places unconsolidated at the time of the intrusion and have fallen 
down into cracks and between fragments of the rhyolite, in a few cases the rhyolite 
seems to have broken across some of the mineral grains of the grits. There are 
indications just south ot the boundary of the area shown in the map and also on the 
top of Grouse Hill that the grits partly underlie the rhyolite. Therefore, while final 
proof is perhaps lacking, it seems almost certain that the rhyolite is younger than 
the grits. 
PETROLOGY OF THE DISTRICT. 
In the foregoing pages of this chapter an endeavor has been made to present 
the distribution and the petrographic and chemical character of the various igneous 
and metamorphic rocks which occur within the Cripple Creek district. Short 
summaries have been given of the prominent features of the best-defined types, 
and in some cases a few words have been devoted to showing relations between 
certain of these types. It is the intention in the present section to view these 
rocks more broadly by considering them as products of crystallization from magmas. 
It will be the aim to hold closely to the facts as shown by the field occurrences 
and by the microscopic and chemical investigations and to avoid such considerations 
of the origin of the rocks, magmatic differentiation, and the like, as are purely 
theoretical. 
For this purpose it will be advisable to disregard the gneisses and schists, in 
which metamorphism has been so intense that their origin is largely a matter of 
conjecture. The rhyolite, also, will not be considered, as it came from an extrane¬ 
ous source and is in no way essentially related to any of the other rocks of the 
district. The remaining rocks are igneous and may be divided on the grounds of 
magmatic relations into three groups, which, in order of age, are (1) the granites, 
(2) the olivine syenite with its related rocks, and (3) the rocks of the Cripple Creek 
volcano. 
GRANITE GROUP. 
Although there seems to be no very evident relationship between the several 
varieties of granite found in the district, Mathews has shown that when studied 
over a larger field, they present a marked-uniformity in mineralogical and chemical 
composition. An average analysis which he has published 0 illustrates the char¬ 
acter of this granitic type. 
a Jour. Geol., vol. 8, 1900, p. 237. 
