48 
COKUNDUM IN THE UNITED STATES. 
[BUIjIj. 1H() 
strike as it cuts through the slate is N. 10° E., the dip being about 
45° W. 
This rock is undoubtedly of the same character as that described 
by Kunz as occurring at Ruby Bar. From the occurrence of these 
two corundum-bearing dikes of andesite it would seem that the source 
of the sapphires found in the various bars along the Missouri Rivei 
is a series of small parallel dikes with a slight northeast-southwesl 
trend, like those described. As the sapphires are scattered so spar- 
ingly through these dikes, the amount of decomposition and erosior 
that was required to liberate those that are now found in the graveh 
must have been simply enormous. 
The beds of gravel in which the sapphires occur are from 10 to 5( 
Fig. 8.— Spokane Bar sapphire deposits, Lewis and Clarke County, Mont. 
feet thick and rest for the most part upon slate, in bluffs that rii 
nearly 50 feet above the river. At Emerald Bar the gravel beds ai 
nearly 130 feel above the river and rest upon granite rock. 
Most of the mining has been done at Spokane and Eldorado ban 
The former locality is near the center of the sapphire deposits and o 
the west side of the river about 16 miles due east of Helena. In 181 
these beds of gravel, which are 8 to 18 feet thick, were extensive! 
mined by an English company known as the Montana Sapphire an 
Ruby Company. The gravels were washed by hydraulic methods an 
a great many gems were obtained, most of which were sent to Eni 
land. This company also controlled or owned French Bar and Dan i 
Bar on the west side of the river and Eldorado Bar on the east sid 
