72 Lieut. Ruggles on the Copper Mines of Lake Superior. 
distances from their axes, subjecting them but partially to the in- 
fluence of the great disturbing cause. It is also to be observed 
that in the immense mining district embracing portions of Illi- 
nois, Wisconsin and Iowa, galena is generally found in the older 
secondary limerock, in fissures ranging north and south, east and 
west. The former is usually crystallized in cubes, and the latter 
frequently laminated. At Dubuque these fissures lead through 
extensive caverns, in which immense masses of galena are found. 
The geological position of this ore is precisely that which we 
might reasonably anticipate, as resulting from the projection of 
metallic lead, enveloped in a dense atmosphere of sulphur from 
the fountain of igneous action, through fissures in the rock 
strata, resulting from concurrent disturbing causes, under the 
pressure of an immense mass of water—otherwise the sulphur 
would have escaped by sublimation, and the lead sunk or entered 
partially into other combinations. This opinion is in a measure 
confirmed by the fact, that blende, sulphuret of zinc, is very gen- 
erally associated with, and indeed usually overlying, galena in 
the mining region; which accords with reason, as indicated by 
their relative specific gravity—being 4 to 7.5—taken in con- 
nection with the sublimation of zinc unconfined after fusion. 
‘Thus, experience seems to prove that atmospheric pressure is en- 
tirely insufficient to produce condensation, and hold this metal in 
combination. Galena is also found in beds of alluvium, where: 
it may have been projected in its nascent state, or subsided under 
the abrasion of the subsequently receding waters, which destroyed 
extensive portions of the metalliferous limestone stratum. Ex- 
perience shows, moreover, that galena is very generally imbedded 
in, or associated with, a species of plastic mineral clay, with indi- 
cations of oxidation. The combinations of copper at Mineral 
Point and Prairie du Chien, are probably accidental projections 
from the seat of igneous action, and consequently of rare occur- 
rence in that district, where the limerock is supposed to average 
one thousand feet in thickness. This corresponds, I conceive, 
with observations made in the copper-mining region, connected 
with the disappearance of mineral veins in very thick, overlying 
conglomerate. 
In most cases, boring should precede mining, as this simple 
and comparatively cheap operation will decide the question of the 
existence of ores in profitable quantity. 
Fort Wilkins, February 26, 1845. 
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