420 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
The conglomerate and sandstone deposits are simply portions of the 
beds of these rocks, in all respects of the ordinary character, save that they 
are impregnated with the native copper. Cupriferous deposits of this char- 
acter are for the most part confined to the thin conglomerate beds which are 
interstratified with the ordinary diabase flows; but one cupriferous bed of 
sandstone is known within the upper or purely detrital division of the Ke- 
weenaw Series, and separated from the nearest trappean flow beneath it by a 
thickness of many hundred feet of sandstone layers. This is the belt of 
dark colored sandstone and shale in which occurs the Nonesuch copper bed 
of the Porcupine Mountains. This belt has been traced from Keweenaw 
Point to Bad River, a distance of about 150 miles; and has been found to 
contain copper at a number of points in the vicinity of the Porcupine 
Mountains, and again on the Montreal River, the boundary line between 
Michigan and Wisconsin. 
In the cupriferous conglomerates and sandstones the copper occurs as 
a cementing material, and as a replacer of the constituent grains, being in all 
cases plainly of secondary origin, and a result of deposition from an aque- 
ous solution. Moreover, the cementing copper itself, 7. e., that which is 
to be seen in the thin section between the constituent grains molding 
itself sharply around their contours, is often also plainly a replacer of still 
smaller constituent particles. In the case of the Nonesuch sandstone of the 
Poreupine Mountain region a large proportion of the particles of cementing 
copper have within them a core of magnetite. It is indeed not improbable 
that in all cases the cementing copper is not a deposit in the original inter- 
spaces of the fragmental particles, but is always a replacer. 
In the thin sections of these cupriferous conglomerates the larger par- 
ticles of porphyry matrix, and fragments of the feldspars, are found to be 
replaced by copper in varying degrees, the metal in the case of the feldspar 
fragments tending to follow the cleavage directions. In the famous con- 
glomerate of the Calumet and Hecla mine in the Portage Lake region the 
copper has not only saturated the matrix, but has also entered into and 
more or less completely replaced large-sized pebbles and even bowlders 
several inches to a foot or more in diameter. Hundreds of such bowlders 
are picked each day from the heaps of rock, before it is taken to the stamps 
