ORIGIN OF THE COPPER DEPOSITS. 425 
Thus the differences in origin of the several classes of copper deposits 
—conglomerate beds, cupriferous amygdaloids, epidote veins parallel to the 
bedding, and “fissure” veins transverse to it—which at first sight seem to 
be great, on closer inspection for the most part disappear. They are all 
the result of the percolation of carbonated waters, which, in the lines of 
fissure, the open textured amygdaloids, and the nearly equally open con- 
glomerates, found the least resistance to their passage, and at the same time 
the greatest susceptibility to their altering power. This susceptibility de- 
pended partly upon the very openness of these different rocks, but also, in 
the case of the amygdaloids, in the presence of a large proportion of glass 
basis, the most readily alterable substance among rock constituents. 
The source and the cause of the arrest of the copper which was carried 
in with the altering waters are other and more difficult questions. Its 
home has commonly been regarded as being within the mass of the trap- 
pean flows themselves, with which it is supposed to have come to the sur- 
face. Another view is that it was originally deposited in a sulphuretted 
form along with the detrital members of the series, from which it was sub- 
sequently leached, partly in the shape of a sulphate, but principally as a 
carbonate and silicate. The latter is the view which Pumpelly has elabo- 
rated;! to whom also is due the credit of having advanced the only satis- 
factory view as to the cause of arrest of the copper in the places where it is 
now found. He has shown the existence of an intimate relation between 
the precipitation of the copper and the peroxidation of the ferrous oxide of 
the augitic constituent of the basic rocks; a relation so constant as to ren- 
der irresistible the conclusion that in this ferrous oxide is to be found the 
precipitating agent of the copper. To this I would add that the ferrous 
oxide of the magnetite, and of the unindividualized magma of the vesicular 
layers, has also been concerned in this reaction. 
While this explanation of the precipitation of the copper seems satis- 
factory, we have too little to go upon in deciding between the two views 
above referred to as to the source of the metal. Too few signs have been 
observed of the existence of copper in the upper sandstones of the series, 
such as would be expected were this its home, to allow of an easy acqui- 
1Geology of Michigan, Vol. I, Part ILI, p. 43. 
