1874.| Copper Mines of Lake Superior. 167 
It was opened in 1849, and has been worked uninter- 
ruptedly ever since, stemming the tide of low prices when 
almost every other mine was carried down the current. 
The lowest level is at 1330 feet along the dip of the bed, 
and therefore on the incline of the shaft from surface, and 
the longest level is 1600 feet. The shafts and all the 
workings are opened in productive ground, where that can 
be followed ; but as the walls of the copper-bearing bed are 
never well defined, and as tracts of rich ground abruptly 
alternate with stretches of barren rock, there is found 
considerable difficulty in keeping to the lode, as it is called. 
Moreover, from being pinched and poor, or even barren, it 
will suddenly bulge to 20 or 30 feet of rich rock. The 
hanging wall is composed of a fine-grained, compact, bluish 
trap, but the characteristic trap beneath is coarse-grained 
and amygdaloidal, and approaches in appearance to the 
copper-bearing rock. 
The copper bed, however, while likewise generally 
permeated with small amygdules, is of a deeper red and 
breaks with a more uneven fracture. The minerals which 
fill the amygdules in the barren bed, viz. quartz, calcspar, 
lawmonite, prehnite, not only fill the amygdules here, but 
likewise form irregular veinlets rich in copper; and the 
chlorite constituents of the rock prevail so largely in parts 
as to give it a deep green shade. Pellicles of native copper 
enveloped in chlorite often occupy the centre of the 
amygdules. We see here the tendency of the copper to 
aggregate with the quartz, and the same zeolitic minerals as 
-compose the ftssure veins of the Eagle River, and the 
bedded veins of the Ontonagon districts; and, therefore, if 
we attribute the formation of the one to aqueous agencies, 
are led to ask whether the same agencies have not had more 
to do with the formation of the beds and their mineral 
contents than has generally been attributed to them. 
Sheets of native copper occur between the joints of the 
trap in the copper bed, and formed evidently through 
infiltration, are found also between the trap blocks beyond 
the walls of the bed. An indication of subsequent aqueous 
action is seen in the streaks of clay which smear to a great 
depth the faces of the trap blocks. A single cross course, 
carrying quartz, but no copper, is said to have been met with. 
The width of the bed of copper-bearing ground is supposed 
to be about 70 feet; not that in any place 70 feet of 
productive rock has been found, but when copper has been 
lost on one wall, as much as 70 feet have been driven 
through what is supposed to be the same bed, and copper 
