GRANITIC ROCKS OF PIKES PEAK QUADRANGLE 220 
this crust is due to chemical action. The great diurnal changes 
in temperature, the dryness of the air, and the direct action of 
the sun tend to promote rapid changes in the amount of moisture 
present, and this in turn would cause alternations of solution 
and precipitation. Throughout the nights and the winter sea- 
sons the rocks receive by capillary action a considerable supply 
of moisture which during the day and the summer would take 
some of the material from the interior and carry it to the surface, 
where there would be rapid evaporation and precipitation. Such 
action must be slow, as the material carried out is but slightly 
soluble even under favorable conditions; and yet this very 
insolubility helps in the final result by rendering at least a por- 
tion of the deposited material independent of the rains. The 
increased amount of silica in the crust seems to corroborate this 
hypothesis of chemical action.t. The formation of a crust on the 
rhomboidal joint blocks, together with the closeness of grain of 
the rock accounts in great measure for the angularity of the 
blocks strewn over the summit, and may in part account for the 
present topographic preéminence of this portion of the massif. 
CRIPPIEE (GRE EK Wy PE 
The granites grouped under this title, compared with those 
of the preceding types, appear finer than those of the Pikes 
Peak type and more evenly grained than those of the Summit 
type. They are finely coherent saccharoidal aggregates of 
microcline, vitreous quartz, and glistening biotite with occa- 
sional microscopic individuals of zircon, hematite, magnetite, 
and apatite. When phenocrysts are present they are usually 
microcline, although in an exposure at the Placer Mill northwest 
of Cripple Creek, broad glistening flakes of biotite are porphy- 
ritically developed. 
The most prominent constituents are small, rectangular crys- 
tals of fresh pink microcline which occasionally reach the length 
of half an inch (Fig. 8). The twinning network is medium coarse 
t CrosBy (Merrill, Rock Weathering, p. 255) suggests also the deposition of iron 
oxide. 
