236 EDWARD B. MATHEWS 
of the constituents, and not in the specific character of the min- 
erals present. 
The second table shows a variation in the size of the quartz 
constituent from grains averaging 5™™" in diameter in the Pikes 
Peak type to those of 4%™™ in the fine grained granite. A simi- 
lar variation is noticeable in the mica, from flakes of 0.5—-1™™ in 
the fine grained type to those of 3-4™™ in the Pikes Peak type. 
The microclines also show a similar change in the same direction, 
whether they are phenocrysts or not; and in addition the fine- 
grained granites show no feldspars porphyriticaliy developed. 
This uniform change in the size of the constituents can only 
result in the production of a similar variation in the coarseness 
of grain, as shown in the tables. 
Table I, together with the chemical composition of the 
rocks, brings out the similarity or family likeness existing 
between the different granites; a likeness that signifies their 
origin from a common magma relatively rich in silica and fluo- 
rine. Table [I], with the field relations, substantiates this view 
and explains the many local differences shown in texture, or 
mode of aggregation, of the different constituents. The coarse- 
grained Pikes Peak and Cripple Creek granites formed large 
masses, while the Summit and fine grained rocks occur in physi- 
cal conditions sufficiently variable to account for the variations 
in texture which distinguish the rocks of these types. 
CHEMICAL COMPOSITION 
The marked uniformity in the mineralogical composition of 
the various granites from all portions of the area suggests a 
similar uniformity in the chemical composition. The abundance 
of quartz and perthitic microcline, with the small amounts of 
plagioclase, mica, and accessory minerals, indicate a relatively 
high percentage of silica and the alkalis, with a comparatively 
small amount of calcium, iron, and magnesium. The presence 
of fluorite, also, suggests the actually small, but relatively high, 
percentage of the unusual constituent fluorine. These inferences 
from the mineralogical composition are fully sustained by the 
