Inftucncc of Country Rock. — IVccd. 183 
at Castle, J\lont., and a score of other places where the ores 
occur in limestone, it is evident that different action has taken 
place. Here, as is so very commonly the case, the ore-bodies oc- 
cur in limestone near the. contact of the igneous intrusions. The 
heat of the latter and the vapors of the cooling magma have al- 
tered these limestones to more or less coarse-grained marbles, 
if the limestones were pure, or to mixtures of garnet, epidote 
and other silicates, if the limestones were impure. The latter 
is the case at the Trout mine and other contact ore-bodies near 
Phillipsburg, Montana, where the Granite Mountain vein (of 
c[uite different and drier ore, without lead) occurs in the gran- 
ite. In this case the gangue is largely silica ; and it is probable 
that similar solutions circulating as a result of the heat sup- 
plied by the igneous intrusion, and, in part, at least, in fissures 
formed in rocks by the intrusion, formed one set of ores in the 
limestone and another in the granite. 
Another instance of the difference of mineral contents in 
the same ore-deposit in different rocks is shown by the Elkhorn 
mine at the town of that name, Jefferson county, Montana. 
This deposit is peculiar in its structural relations, being sim- 
ilar to tlie well-know^n saddle-reefs of Australia in that it 
occurs in the 'Saddleshaped mass along the axis of a steeply 
pitchmg anticlinal fold. The rocks are sedimentary, of prob- 
able Cambrian age, and dip at steep angles toward the con- 
tact of a great mass of granite and other intrusive igneous 
rocks. The ore occurs at the contact between an altered shale 
and a massive crystalline limestone, and both rocks have been 
changed by contact metamphorism. The bedding-plane is 
ore-bearing only where the general dip of the rocks is disturbed 
bv flexures. In addition to the ore found along the contact, a 
number of very large ore-bodies have been found at some little 
distance in the dolomyte, though always in the same structural 
position. The ore found along the altered shale-dolomyte con- 
tact is essentially a "dry" quartzose milling-ore; that of the 
dolomyte, mainly galena, with accessory sphalerite and pyrite, 
Both ores are connected by "pipes" and stringers, and both 
the field and the microscopic evidence shows that they were 
formed by the same solutions and at the same time. There is 
■ no escape from the conclusion that the dift'erence in the miner- 
alogical character of the ores is the result of the dift'crcnt na- 
ture of the enclosing rock. 
