GRANITIC ROCKS OF PIKES PEAK QUADRANGLE 225 
more resistant to the weathering agencies, while the friable rock 
beneath has fallen away leaving the crust as a projecting edge. 
Such a crusting over friable material often leads to fantastic 
shapes, as represented in Fig. 6. The final result of the weather- 
ing is the formation of a thick coating of talus and granite gravel, 
composed of relatively fresh fragments of the rock and _ its 
mineral constituents. . 
Fic. 6.—Fantastic forms due to weathering and surface hardening. 
SUMMIT TYPE 
The rocks of the Summit type show a very constant texture 
closely allied to that of granite-porphyry (Fig. 7). They are 
composed essentially of small gray feldspar phenocrysts embed- 
ded in a finely granular aggregate of hypidiomorphic, quartz, 
smaller feldspars, biotites, and minute grains of fluorite. 
Microscopic zircon, magnetite, hematite, and micropegmatitic 
intergrowths of quartz and feldspar are also present. 
When fresh the color of the rock is purple, ranging from pur- 
ple-violet to carmine-purple.t As the rock becomes weathered 
the color becomes’ less pronounced and fades to light neutral 
gray and brown. | 
The minerals composing the Summit type differ very slightly 
from those described under the preceding type. Quartz is more 
*Nos. 23, and 26; of Radde’s International Farben scala. 
