452 



PROCEEDINGS OP THE GEOLOGICAL SOCIETY. [Apr. 11, 



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GQ 



Ph 



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dotic portions of the latter, and in two classes 

 of true lodes, — one of which crosses the strata at 

 right angles to their strike, forming the so-called 

 fissure-veins of the Keweenau Point and Eagle 

 Eiver districts ; while the other includes such 

 veins as run with the formation, and are found 

 in the Ontonagon district. These are very irre- 

 gular in character ; and the fact of their being 

 veins can only be made out by their relations to 

 the containing rocks as developed in deep work- 

 ing, the surface appearance being that of irre- 

 gular masses of epidote -breccia, continually 

 varying in thickness, although capable of being 

 traced for considerable distances. In the fissure- 

 veins the metal occurs in comparatively smooth 

 platy masses, of all sizes, and comparatively free 

 from intermixed rock ; while in the Ontonagon 

 lodes the masses are rough and on a very large 

 scale, arborescent in form, and usually contain 

 large adherent and included fragments of the 

 vein-stone — a breccia of epidotic rock and 

 quartz ; but in either case they may attain a 

 great size, and at times a weight of several 

 hundred tons. In the cupriferous amjgdaloids 

 of Portage Lake, masses of great size are not 

 found, the largest being that discovered about 

 two years since in the Mesnard location, which 

 weighed about 18 tons, and lay loose on the 

 drift covering the rock. In addition to the 

 masses, thin sheets of metal are common wherever 

 narrow transverse cracks occur in the rock, 

 either forming masses of tangled crystals with 

 a serrated edge, or, when of large size, being 

 more compact and not unlike worn-out copper 

 sheathing. The metallic kernels of the amyg- 

 daloids are rarely solid, but form thin shells 

 moulded to the wall of the cavity, or to the 

 crystals of some mineral previously deposited, 

 usually carbonate of lime. In the conglome- 

 rates, in like manner, the metal usually incrusts 

 the pebbles, sometimes completely investing, 

 but never entirely replacing them. When cry- 

 stallized in its oAvn form, the copper has also a 

 great tendency to appear in hollow crystals, the 

 simplest solid forms being formed in the hol- 

 lows of the larger masses. A fine example of 

 this has been recently furnished by the great 

 mass of the central mine, which on one of the 

 cuts disclosed a druse full of nearly perfect 

 crystals of tetrahedral form, without any se- 



