426 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
escence in Pumpelly’s view. On the other hand, the trappean rocks them- 
selves are for the most part devoid of copper, except such as is plainly 
secondary. Copper in a sulphureted form I have, however, observed in 
the coarse gabbros of Duluth, in the green uralitic gabbro of Mount 
Bohemia, and in similar coarse rocks in one or two places on the north 
shore of Lake Superior. It is commonly said that copper occurs in the 
conglomerates and sandstones only where it could have leached directly 
downwards from an overlying trappean mass; and with one exception the 
statement is undoubtedly correct. The exception is that of the Nonesuch 
cupriferous sandstone, which is, however, a very important exception, since 
this rock not only has no overlying diabase, but is separated from the near- 
est trappean flow beneath it by many hundred feet of detrital material. 
As previously shown, this sandstone is unusual for its large proportion of 
basie detritus. Its copper can only be connected with a trappean source 
by supposing it to have formed part of this detritus in the sulphuretted con- 
dition, and afterwards to have been dissolved and redeposited in a native 
state. This is a supposition which would seem on the whole, however, to 
be rather more violent than to regard the copper as having come from the 
overlying sandstones, and as having been arrested in its descent on meeting 
a layer so rich in basic detritus as to be able to furnish the requisite supply 
of precipitating agent. 
From the facts and theoretical considerations thus given, may be for- 
mulated a few simple rules to guide the explorer for copper in the regions 
traversed by the Keweenaw Series. Thus the explorer, should he be 
searching for transverse veins, should bear in mind that epidote, prehnite 
and chlorite are the favorite associates of the copper; that veins carrying a 
greatly predominating quantity of calcite are not likely to be cupriferous ; 
that laumontitic veins have hitherto not proved to be sufficiently rich for 
exploitation; that a vein which may be very rich and wide in the amygda- 
loidal or other soft and easily decomposed rocks will pinch to a mere seam 
and become barren within the massive and more compact layers; that, 
hence, the intersection of a vein with such amygdaloidal or other soft beds 
should always be searched for; that the copper occurs in these veins with 
extreme irregularity; and finally, that a vein found traversing decomposed 
