MINERALOGY: LARSEN AND WELLS 
361 
One of these specimens proved to be the unusual mineral gearksutite, 
and the other was made up of a peculiar kaolinite and a new fluoride- 
sulphate, creedite. It is the purpose of the present paper^ to describe 
these minerals. 
The vein is entirely in Tertiary volcanic rocks made up of lava flows 
and tuff beds of rhyolite and quartz latite, with subordinate andesite. 
It can be followed for perhaps half a mile and is variable in width but 
is not uncommonly wide enough to make the mining of the fluorite 
profitable. Much of the vein filling is nearly pure fluorite but locally 
barite is abundant. The wall rock shows considerable alteration and 
commonly carries much disseminated pyrite. 
Gearksutite. — The lower tunnel of the mine, whose portal is about 
100 feet east of one of the hot springs, follows the vein and some distance 
from the portal it passes through a large body of soft, highly altered rock, 
giving rise to considerable caving. This soft material is a highly altered 
rhyolite or quartz latite, probably a tuff, which carries very abundant 
balls, up to several inches across, of snow white, powdery gearksutite. 
These balls are easily separated from the altered rock and resemble a 
very fragile chalk or kaolinite. When wet they become plastic and 
resemble ordinary dough and are without grit. 
A microscopic examination shows that the material is homogeneous 
and is made up of an aggregate of shreds so minute as to be recognized 
only with a high magnification. It has a mean index of refraction 
of 1.454 =i= 0.003 and a moderate birefringence. The specific gravity 
measured by the pycnometer method is 2.768. 
This agrees with the properties of gearksutite from Greenland,^ which 
"occurs in white, chalky aggregates of minute fibers or prisms which are 
optically negative and have a moderate axial angle. X is normal to the 
fibers and Y makes a large angle with them. 
The mineral is probably monoclinic with X = b. 
a = 1.448 =b 0.003, /3 = 1.454 ^ 0.003, y = 1.456 ± 0.003." 
A chemical analysis of the material from Wagon Wheel Gap shows 
its identity with gearksutite. In the following table Analysis 1, by 
R. C. Wefls, is of gearksutite from Wagon Wheel Gap, Colorado. 
Analysis 2, by G. Lindstrom, is of gearksutite from Ivigtut, Greenland, 
and Analysis 3, by Hillebrand, is of that mineral from St. Peters Dome, 
Colorado. Column 4 gives the theoretical composition,^ on the basis 
of the formula CaF2.Al(F,OH)3.H20. with F:OH = 2:1. 
