GRANITIC ROCKS OF PIKES PEAK QUADRANGLE 217 
of the granites is due to earth movements which preceded the 
Cambrian. 
The following pages treat almost exclusively of the granitic 
rocks of the area. 
ROCK TYPES 
The greater portion of the area studied, as shown by the 
accompanying sketch and the more complete map in the folio of 
the Geologic Atlas, is occupied by granites, gneisses, and asso- 
ciated schistose rocks which form an undulating platform under- 
lying the later formations. The prevailing composition of this 
complex is that of a typical granite with the addition of a small 
amount of fluorine, while the characteristic mineral constituents 
remain the same over an area of more than a thousand square 
miles, notwithstanding the fact that the exposures are represent- 
ative of bodies intruded at different periods, and crystallized 
under somewhat different conditions. The granites are light col- 
ored, usually pinkish, holocrystalline aggregates of feldspar, 
quartz and biotite with occasional hornblende and flucrite. The 
individual components vary in their size and relative abundance 
and in the perfection of their crystal form ; but in almost every 
instance the feldspar is larger, more abundant and somewhat bet- 
ter formed than either the quartz or biotite. These variations 
in the manner of aggregation and in the size of the constituent 
minerals give rise to well-defined types of granite which were 
distinguished and plotted in the field. 
Although some sixteen varieties of granite were distinguished 
during the mapping, later study has shown that all masses of 
prominence may be referred to one of four clearly defined types 
which have been named,’ the Pikes Peak, the Summit, the Crip- 
ple Creek, and the Fine-grained types respectively. 
PIKES PEAK TYPE 
A large part of the area of the accompanying map is occupied 
by a single type of granite, called the Pikes Peak type, from its 
* Geological sheet. Pikes Peak folio, No. 7, Washington, 1895. 
2 Bull. Geol. Soc. Am., VI, 1894, pp. 471-473. 
