MINERALS FROM LAKE SUPERIOR. 409 



the " Kammkies " of German authors. In this variety, the crystals 

 are not united regular^ by a plane of the prism, or by one of the 

 macrodome planes, as in the true twins of Marcasite, but are simply 

 formed at the free end of the radiating lamellae, the broad surface of 

 the latter representing the basal plane. A point of much interest, in 

 connexion with these specimens, is the occurrence of common or cubi- 

 cal pyrites in the same vein. The latter species occurs in different 

 parts of this vein, in small but distinct crystals — combinations 

 of the cube and octahedron, with the cube faces predominating. 

 "Where representatives of the separate conditions of a dimorphous sub- 

 stance thus occur together, the cause by which the dimorphism was 

 produced is not readily explained. In the present instance there were 

 no data to shew that one condition had originated at an earlier or 

 later period than the other, and yet such must in all probability have 

 been the case. 



Some of the marcasite specimens from this spot had already entered 

 into decomposition when first obtained — the products being an efflo- 

 rescence of sulphur in one instance, and, in others^ the formation of 

 sulphate. The latter was also in itself altered, by the partial conver- 

 sion of the FeO into Fe^O^, its solution yielding an abundant blue 

 precipitate with ferrocyanide of potassium (" yellow prussiate.") 



4. Molybdenite, MoS^. Several veins of quartz, in which this 

 mineral is abundantly distributed, occur on the shore of Sea-beach 

 Bay, near Black River (Lat. 48^^ 46^ N.; Long. 87^ 17' W.). Some 

 specimens from one of these veins discovered by Mr. Salter, the sur- 

 veyor, gave me (by mechanical analysis) very nearly 4^ per cent, of 

 Molybdenite, an amount equivalent to about 100 lbs, pet' ton. Copper 

 pyrites is also present in the quartz. 



5. Barytine, BaO, SO'.r— It has long been known that many veins 

 of Heavy Spar or Barytine occur on the north shore of Lake Superior, 

 several of these veins being almost free from colouring matter, and 

 hence of good quality as a paint material ; but I am not aware that 

 any crystals from this region have hitherto been described. From 

 the vein in Neebing township (about ten or twelve miles from Fort 

 William,) in which the cockscomb variety of marcasite (described in 

 Note 3) was obtained, I procured a great number of small crystals of 

 this mineral, of a pale yellowish or reddish colour. The same forma 

 were present in all, producing a combination of: 1, the base, oooo; 

 2, a front-polar or macrodome, i to ; 3, a second or lower front-polar 



