Pa. eee pa te ee 
a a eve: -.. Geological Structure of Keweenaw Point. 71 
It is still a question among geologists and miners whether veins 
‘were filled by igneous injection or sublimation, or by aqueous 
_ and galvanic deposition. ‘This question is one of very great sci- 
entific and practical interest, and is exceedingly difficult to an- 
La -Swer so far as relates to the native copper and native silver of 
‘Lake Superior. 
The objections to the igneous origin of native copper are, Ist, 
- that the metal bears the imprint of crystals of prehnite, as seen 
_. in the specimen I lay before the section, and we cannot account 
for the fact that this zeolite was not rendered anhydrous by the 
_ Molten copper. 2dly, that if the copper was melted, since its 
- fusing poiut is much higher than that of silver, that the silver is 
not alloyed with the copper, but is separate from even a trace of 
it in chemical combination, though small particles and large lumps 
of silver are mixed and united with the metallic copper. 
These objections are equally strong against the theory of sub- 
limation of the copper, and since silver is not volatile at the 
highest temperature of our furnaces, we could not account for the 
presence of that metal by a simultaneous sublimation of the 
metals 
Against the theory of its aqueous deposition, or its origin from 
any solution of copper, it may be urged that if the metal was in 
chemical solution, no material capable of causing its decomposi- 
tion with the deposition of the copper in a metallic state exists in 
the vein, and no salt, if any supposed acid solvent, which would 
result from the decomposition of its combination, exists in the 
vein, Again, it would be impossible for the chasm to contain a 
Sufficiency of any copper solution, however concentrated, to pro- 
duce the solid metallic copper filling the fissure, for, as before 
observed, the masses of copper are from a foot to three feet in 
thickness, and occupy the whole space of the sundered rock. 
Galvanic segregation it has been supposed would explain the 
_ Origin of these copper veins. But we may ask, from what was 
the copper segregated? It is impossible for galvanism to create 
the metal from the ingredients of trap rocks, or sandstone ; and 
We can hardly imagine any arrangement of the rocks that would 
produce a galvanic battery with its poles so arranged as to effect 
the deposition of a vein of solid copper two or three feet in 
thickness. 
{t is well known that the trap rocks are magnetic, and that 
they possess polarity at the surfaces of disjunction. This has 
been fully substantiated by the researches of Dr. Locke and oth- 
ts on the Lake Superior mineral lands, but this magnetism is 
ce. Viously the effect of the earth’s inductive magnetism exerted 
on the very large proportion of magnetic iron ore entering into 
se the composition of the trap rocks, a quantity so large that I have 
Seen pig iron made directly from those rocks by fusion in a blast 
