486 



THE GEOLOGY OF MINNESOTA. 



[Copper. 



being small calcite grains still existing in some of the depressions, it appears that 

 this metallic sheet was deposited on a calcite coating, or between two calcite surfaces, 

 since both surfaces are roughened in the same manner, as if by fine calcite teeth, or 

 rhombs. 



Remark. The singular fact that in the lake Superior region copper and silver 

 are thus associated without being alloyed, was first noted by Dr. C= T. Jackson,* and 

 has been confirmed by several other geologists since. When they are alloyed, as 

 sometimes occurs in pieces extracted by the ancients, it is probably due to fusion at 

 the time of extraction, since it is a matter of history that the Indians obtained copper 

 in that region by applying heat to the rocks, f N. H. w. 



No. 583. COPPER (with attached wystals of calcite and adult* ri<i). 



Minong mine, Isle Eoyale. 



Ref. Annual Report, x, page 54; American Geologist, vol. xxiii, page 317. 



Metallic copper is crystalline, in slender rods that are flanged and pointed like 

 a spear-point, the alternating twins furnishing dull corners, slightly barb-like, which 

 increases the resemblance to spear-heads. The calcite is in large masses, conspic- 

 uously cleaved and involved with the copper. It is not apparent which is the older. 

 Upon these is an abundant deposition, as a coating, of fine crystals, which are flesh-red, 

 which are so numerous that they constitute sometimes a massive mineral, whose 

 free surfaces, as in the small cavities, are the only parts in which the crystalline 

 facets appear. This mineral is insoluble and infusible, or difficultly fusible. In tube 



it gives no water. 



\ 



N 



I/O 



\ 



\l 



View 100. 



View 010. 

 FIG. 27. CRYSTAL FACES OF ADULARIA IN NO. 385. 



Mic. The mineral has a glassy transparency between crossed nicols, but contains 

 numerous particles of non-translucent impurities. Its fracture is irregularly con- 

 choidal, but is governed partially by the cleavage. The crystals, broken at random, 

 give more numerous cleavages in which the axis n m is vertical, indicating the basal 

 cleavage, while fragments that show the axis n f vertical are rare. The crystals are 

 confusedly mingled, and sometimes twinned, or compound by parallel growths. 



In the geodic cavities are innumerable compound faces, bounded by prismatic 



* Report on the geological andmincralogical survey of the mineral lands of the United States in the state of Michigan, 1849, 

 pp. 386, 481. Compare, also, Fourteenth Report a/tlir Minnrsotd Hurn-y, for 1885, p. 319. 

 t Voyage du fiieur de Ctutmplain, Paris, 1613, p. 246. 



