52 CORUNDUM IN THE UNITED STATES. [bull. 180. 
this creek, so that the extent of the sapphire-bearing gravels is not 
known. The sapphires are similar in character to those of Rock 
Creek, but they are apt to be of lighter color and not of such a 
variety of colors. What little work was done on the creek during the 
past summer was by Franz Cobalt, of Helena, and according to him 
there were but few sapphires taken out of the gravels. Future devel- 
opment at this locality may show these deposits to be of considerable 
importance and extent. 
YOGO GULCH SAPPHIRES, 
The sapphires that arc the most widely known and that have 
attracted the most attention have been obtained in Fergus County 
near the entrance of Yogo Gulch, on the Yogo Fork of Judith River. 
This locality is on the eastern slope of Prospect Ridge of the Little 
Belt Mountains, about 75 miles northeast of Helena and 15 miles a 
little south of west of Utica, which is the nearest town, and which is 
on the Judith stage line. The sapphires were first found in the grav- 
els of Yogo Fork, and in following these up the creek their original 
source was located in dikes that extend across the county for a mile 
and a half. 1 
There are two parallel dikes about 800 feet apart, with a general east- 
west trend, which vary in width from 15 to 75 feet. The miner- 
alogical composition of the rock show r s that it has a close affinity with 
minette and shonkinite, as described on page 28. 
The alluvial deposits below these dikes have been pretty thoroughly 
worked for the sapphires, and mining operalions are now confined 
almost entirely to the dikes themselves. These dikes, the upper por- 
tion of which is thoroughly decomposed, have been worked by means 
of open cuts, the limestone making fairly firm walls. By hydraulic 
processes the decomposed rock was readily broken up and washed 
into sluice boxes. As the mining extended deeper the rock was much 
less altered and it was necessary to leave a great deal of it exposed to 
the atmosphere from one season to the next, before it could be broken 
up and run through the sluice boxes. At a number of points the 
almost perfectly fresh rock has been encountered, and from this it 
will be a difficult problem to separate the sapphires. The percentage 
of sapphires in the rock is small and if it were the unaltered rock that 
had to be worked for them the deposit would not be of economic 
importance. 
The sapphires occur embedded in this rock in distinct crystals from 
less than a millimeter in diameter to some that were over 15 mm. 
Their color, as far as I have observed them, is always a blue, varying 
from light blue to a very few that showed the dark blue of the Ceylon 
stone. The prevailing color is a bright blue. 2 
1 The geology of the district and a full description of the mines and workings, l>y W. H. Weed, 
will be found in the Twentieth Ann. Rept. U. S. Geol. Survey, Part III, 1899, pp. 454-460. 
2 Kunz: Am. Jour. Sci., 4th series, Vol. IV, 1897, p. 4:iU. 
