238 EDWARD B. MATHEWS 
called “Sentinel “Poimt)(12\400) feeh)py wkieldsparn us athe most 
important constituent, with quartz very abundant in somewhat 
smaller grains. The mica occurs in both single individuals and 
in aggregates of minute flakes. A thin section of this rock is 
composed, almost entirely, of quartz and microcline, the latter 
showing a few lamellae of perthitic plagioclase. 
II. (2531.) A porphyritic granite of the Summit type col- 
lected from the divide tunnelled by the Colorado Springs Water- 
works (elevation about 12,000 feet). This is composed of 
feidspars and large grains of quartz in a fine grained, reddish 
to purplish groundmass. 
III. (2430.) A fine grained variant of the Summit type col- 
lected on the head waters of the Middle Beaver, nearly opposite 
the Bear Creek road to the Colorado Springs Water-works. . The 
prominence of the biotite against a fine grained groundmass of 
feldspar, and the peculiar purplish hue due to the disseminated 
fluorite, are the chief characteristics. 
IV. (2369.) A fine grained granite of the fourth type taken 
from Smith’s Gulch not far from Current Creek P. O. This is 
composed of quartz and microcline with small amounts of mica. 
V. An average of the preceding. 
The following conclusions based on a comparative study of 
the analyses seem to be warranted by the figures. When the 
individual analyses and their average are reduced to molecular 
proportions and compared with an average of twelve type analy- 
ses given by Zirkel* and several analyses given by Rosenbuch? 
similarly reckoned, it is seen that they all are richer in silica than 
the averages given in the text-books, though not richer than 
individual specimens from many areas. The sum of the alkalis 
seems to conform to that of the averages but the granites of 
the Pikes Peak area are relatively richer in potassium. This 
relation between the alkalis becomes of additional interest when 
the occurrence of nepheline-bearing rocks near Cripple Creek is 
considered. 
t Lehrbuch der Petrographie, 2te. Aufl. II, p. 29. 
2Elemente der Gesteinslehre, p. 186. 
