GRANTTIG ROCKS OR PIIGES PEAK QUADRANGLE, 227 
quartz occurs in small oval, or irregular, disks which have the same 
orientation over considerable areas of the feldspar. Although these 
disks may lengthen out, they do not have the branching-radial 
arrangement characteristic of some of the other occurrences. 
The biotite occurs in flakes without good crystal outline, and 
locally shows quite an advanced stage in the alteration towards 
chlorite and lenses of quartz formed between the foliae. The 
same slide may show perfectly fresh pieces of biotite associated 
with that which has become thoroughly chloritized. Unlike the 
mica of the Pikes Peak granite, the biotite of the Summit type 
is of the first order with the plane of the optic axes parallel to 
the principal ray of the percussion figure. 
Hornblende, titanite, and magnetite are practically wanting 
in this type, although a few fresh irregular grains of the latter 
were noticed in a single slide. 
The most characteristic mineral in the Summit type is fluorite. 
This is present in every section but one made from the Summit 
granites. It is commonly in small irregular areas and rarely in 
well-defined crystals. When the crystal contours are evident 
the little squares suggest either cubes or octahedrons. The min- 
eral is especially characterized by a highly perfect octahedral 
cleavage which is well developed in the larger areas, but is 
lacking in the minute crystals. The anhedral areas are clear 
and either colorless, purple, faintly pink, or green. The pigment 
is unevenly disseminated through the grains, and seems to be 
more intense about inclusions than in the clearer parts of the 
mineral. Between crossed nicols the areas remain perfectly 
isotropic, and in ordinary light the mineral shows a shagreened 
surface corresponding to its very low index of refraction. All of 
the properties enumerated are characteristic of fluorite. The 
view that this is fluorite is corroborated by the high percent- 
age of fluorine in the bulk analyses and the presence of fluo- 
rides in the veins of adjacent areas.t Microchemical tests were 
made, but failed to give conclusive results. 
*E.g., St. Peter’s Dome (Bull. U. S. Geol. Sury., No. 20), and Cripple Creek 
(Sixteenth Ann. Rept. U. S. Geol. Surv., II, 1895). 
