1866.] BAUERMAN COPPER-MINES OF MICHIGAN. 455 



Boston lode is also very epidotic, and in the hollows carries large 

 quartz crystals and masses of prehnite. 



The Albany and Boston conglomerate is a deposit of considerable 

 interest, as it forms a well-marked horizon in the rocks of the 

 Portage district, and carries in places a very considerable amount of 

 copper. It is about 30 or 32 feet in thickness, and is included 

 between a soft argillaceous sandstone floor, 4 feet thick, and a clay 

 roof of 9 inches in thickness. The latter contains at times parallel 

 sheets of copper, and is locally known as the "fluean lode." The 

 pebbles are chiefly of red jaspideous porphyry, and are for the most 

 part well-rounded, varying in size from about half an inch up to six 

 or eight inches in greatest diameter. The cementing material varies 

 considerably, being mainly calcareous at the Pewabic, and a compact 

 epidote at Rhode Island mine. The more usual character^ however, 

 is a granular mixture of epidote, quartz, and finely divided rock- 

 matter with small specks of copper. 



More remarkable conditions, however, have been observed in some 

 portions of this rock at the Albany and Boston mine, where the 

 cement is in places entirely metalhc, the copper forming closely- 

 fitting shells over the pebbles, and at times permeating them to such 

 an extent as to form, with the siliceous mass, a kind of copper-con- 

 crete, which of course is extraordinarily tough, such pebbles being 

 capable of passing almost unaltered in form through the jaws of the 

 powerful rock-breakers employed in the dressing-floors. In places, 

 however, the copper in the cement of the coarse conglomerate has 

 been changed to chrysocolla and red copper ore, both of the octahedral 

 and fibrous forms, associated with which are calcspar, prehnite, and, 

 probably, cupreous allophane. Out of the total thickness of 32 feet^ 

 only the lower portion of the bed, from 10 to 15 feet in thickness, is 

 cupriferous ; so that in this respect the conglomerate resembles the 

 amygdaloids. 



The lower metalliferous, or Isle Royale series, is a belt of 

 amygdaloids similar in general particulars to those of the Pewabic 

 group. It includes two great lodes: — the grand Portage, worked in the 

 mine of the same name on the south shore of Portage Lake ;. and the 

 Isle Royale, which is opened on the Huron mine, also on the south 

 side, but has also been traced for several miles on the north shore. 

 It is a pale-green amygdaloid containing quartz, steatite, chlorite, 

 epidote, and copper in smaU quantity, from 24 to 33 feet in thick- 

 ness, yielding about 1 per cent, of copper when dressed. Below the 

 Isle Royale is another lode called Mabbs lode, which has recently 

 been discovered ; and it is not quite certain whether it be a parallel 

 belt or an actual fissure- vein, as its dip is much steeper, being 75° 

 instead of from 52° to 60°, which is the amount of inclination of the 

 higher belts. 



6. Mines of the Northern District. — In the district of Keweenau 

 Point the cupriferous amygdaloids occupy but a subordinate position, 

 the whole of the produce being derived, with a very few exceptions, 

 from true lodes, or, as they are called in the district, fissure-veins, 

 which, as a rule, are nearly vertical, and cross the trappean formation 



VOL. IXII. PART I. 2 I 



