422 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
calcite bodies, those irregular, solid, branching forms, that are locally known as horn- 
copper, often many hundred pounds in weight; in the epidote quartz, and prehnite 
bodies, it occurs as thread and flake-like impregnations; in the foliaceous lenticular 
chloritic bodies, it forms flakes between the cleavage-planes and oblique joints, or-in 
places—and this is more particularly true of the fissure veins, which we are not now 
considering—it replaces the chloritic, selvage-like substance till it forms literally 
pseudomorphs, sometimes several hundred tons in weight. 
The copper in these deposits is not restricted to that portion of the bed 
which was originally vesicular, but runs from it downward irregularly into 
the originally compact portions, following always a great alteration of the 
rock. The copper, however, tends always to be very irregular in distribu- 
tion, and, even in the longest worked and most reliable amygdaloids, has 
frequently to be searched for through many feet of barren rock. In this 
search the diamond drill is now extensively used, the miners being guided 
in its use by the occurrence of seams of calcite and epidote, and other 
alteration forms, which, when followed up with the drill, are often found to 
lead to pockets containing much copper. 
In one class of amygdaloids, those of the ashbed type,—which I agree 
with Wadsworth in regarding as merely very highly scoriaceous and open 
lava flows, into whose interstices the intermingled detrital material has sub- 
sequently been washed—the distribution of the copper is sometimes more 
uniform than in the ordinary cupriferous amygdaloids, so that the whole of 
the bed may be broken down and taken to the stamps, as is done for 
instance at the Atlantic mine. _ 
The copper deposits of the Ontonagon region have not had the study 
given to them that has of late years been devoted to those of the Kewee- 
naw Point and Portage Lake districts; so that it is not possible to be quite 
so positive in our statements in regard to them. The copper of this region 
never occurs in transverse fissures, but either lies in irregular accumula- 
tions—often solid masses many tons in weight—associated with much 
epidote and calcite, distributed along the course of diabase beds, or else 
occurs with more persistent and vein-like aggregations of epidote and cal- 
cite. The latter coincide always with the bearing of the formation, and 
commonly also with its dip, but in some cases, as for instance in the once 
famous Minnesota mine, dip at a higher angle than that of the formation, 
