COPPER RANGE OF DOUGLAS COUNTY, WISCONSIN. Pay | 
regular bedding. On Middle river the original lines of depositon have been entirely 
obliterated, and the very argillaceous sandstone transformed into a transverse cleav- 
ing slate, somewhat micaceous. 
* * * * * * * * * 
That the Keweenawan eruptive rocks are bedded, in a certain sense, there can be no 
doubt. Thedipand strike * * * * can be determined at almost any locality, where 
the rocks are exposed, with but little difficulty. * * * * Owing to a great inequal- 
ity in the hardness of the different beds, the softest have been eroded, and the hardest 
and firmest have remained; and now outcrop in the form of bare ridges of rock. These 
ridge-like exposures are very prominent and characteristic between Black river falls 
and the Aminicon river. They appear to arise directly from the soil, like a great 
stone wall, and, at least in one or two instances, pass across the country for a mile or 
more in an almost straight line, with a height of 30 or 40 feet, and a thickness of 50 to 
100 feet. Other ridges vary in length from a few feet to a quarter of a mile or more. 
On the north, the face of each ridge is usually precipitous and somewhat jagged, 
owing to the exposure of the edges of thelayers. The south side descends to the soil 
with the inclination of the bedding. Ordinarily the dip is readily obtained, and the 
trend of the ridges is usually the strike of the formation. East of the Aminicon, the 
wall-like exposures are less prominent. 
* * * * * * * * * 
By far the most common rock is a greenish to dark-gray diabase. Near the sand- 
stones on the north, this rock often has an amygdaloidal structure. There are also 
beds or layers of amygdaloidal diabase 1,000 or 2,000 feet south of the sandstones. 
The amygdules are usually either epidote, prehnite or chlorite. A less common rock, 
but one forming massive beds, is a fine-grained, nearly black kind, having a marked 
conchoidal fracture, and differing much from the ordinary diabase (Pumpelly’s “ash- 
bed type”). Coarse-grained, red-and-black-mottled basic rocks (gabbro) also occur. 
Felsitic porphyry has been seen only on Black River.’ 
Native copper occurs throughout this district in three ways, accord- 
ing to Mr. Sweet :—?’ 
1. Indiscriminately scattered through belts of epidotic and calcareous rock of 
various thicknesses, and lying usually with the bedding of the formation, as at the Per- 
cival mine. 2. Irregularly disseminated fine particles of native copper in the layers 
or beds of diabase, as at the Fond du Lac mine. 3. In true fissure veins, as at the 
Wisconsin mine. 
Mining has been attempted at a number of points, but unsuccessfully. 
The following are further quotations from Mr. Sweet’s descriptions: 
The most western exposure of the eruptive rocks of the Western Lake Superior 
district that I have seen, is a short distance west of the lower falls of Black river.* 
* * * * * * * * * * 
1 Having examined in detail a complete suite of rocks from this region, I find them identical in 
all respects with those of the Keweenaw Series farther east. See also R. Pumpelly, Geology of Wis- 
consin, pp. 42 and 48. R. D. I. 
2Geology of Wisconsin, p. 357. 
3Do., p. 340. 
