COPPER. 141 



Atlantic coast (see Example 21), even going so far as to regard 

 the former as the western equivalent of the latter. 1 



There are three principal mining districts the Keweenaw 

 Point, on the end of the Point ; the Portage Lake, in the middle ; 

 and the Ontonagon, at the western base. Mines have also been 

 worked on Isle Royale, and copper is found in small amounts 

 on the north shore. The Portage Lake district is now the princi- 

 pal and almost the only producer. In the first-named district most 

 of the mines are on original fissures, which have later become 

 much enlarged by the alteration of the walls. They are usually 

 from 1 to 3 feet broad, but may reach 10, 20, and 30 feet, this last 

 in the looser textured rocks. These expansions are also richer in 

 copper. The veins stand nearly vertical and cross the beds at right 

 angles. They were the earliest discovered and the first to be ex- 

 tensively worked. The metallic masses, both large . and small, 

 occur distributed through the gangue. The best known mines of 

 the district are the Cliff, the Phoenix, and the Copper Falls. 



2.04.12. In the Portage Lake district the mines are either in 

 conglomerate (Calumet and Hecla, Tamarack, Peninsula, etc.) or 

 in amygdaloidal, strongly altered diabase, certain very scoriaceous 

 sheets of which are known as ash-beds (Quincy, Franklin, Atlantic, 

 etc.). In the conglomerates the copper has replaced the finer frag- 

 ments, so as to appear like a cement, and often the boulders them- 

 selves, or particular minerals in them, are permeated with copper. 

 The amygdaloids have copper in their small cavities, but in the 

 open or shattered rock it fills all manner of irregular spaces, often 

 in fragments of great size. It is associated with calcite, the 

 zeolites (often of great beauty), epidote, and chlorite (the last 

 containing Fe 2 O 3 ). 



2.04.13. In the Ontonagon district the copper follows planes 

 approximately parallel to the bedding of the sandstones and igneous 

 rocks, and in one case at least (the National mine) along the con- 

 tact between the two. The copper is quite irregular in its distri- 

 bution, but has the same associates that are mentioned above. 



2.04.14. In their practical bearings the mines are classed as 

 Mass Mines, Amygdaloid Mines, and Conglomerate Mines, accord- 

 ing to thje'size of the masses of copper or to the character of the 

 inclosing rock. 



2.04.15. On the Origin of the Copper. The original source of 



1 C. T. Jackson, Amer. Jour. Sci., i., XLIX., 1845, pp. 81-93. 



