MUSEUM OF COMPARATIVE ZOOLOGY. 45 



gray rock composed of hornblende, holding patches of quartz, wliich con- 

 tains crystals of tourmaline. The rock holds an abundance of magnetite, 

 in part at least toni from the ore through which it passed. The quartz 

 contains fluid and other inclusions. The scliist at this locality is an 

 argillaceous one (340) resembling that found at Republic Mountain, 

 Ishpeming, and Negaunee ; but it has been so affected in places by the 

 combined action of the magnetite and other dikes that it has become an 

 ottrelite schist (336). This rock has a dark gray groundmass, holding 

 crystals of ottrelite and minute ones of tourmaline. In the thin section 

 the groundmass is of a clear grayish-white color, and holds ottrelite, 

 tourmaline, and magnetite. This groundmass is composed of a clear talc- 

 like mineral in flakes and scales, apparently orthorhombic. In it occur 

 magnetite grains and irregular masses of this mineral, as well as greenish 

 micaceous scales. The ottrelite in the section is of a green color, shows 

 dichroism, varying from a light to dark green, and contains scales of the 

 talc-like mineral, magnetite, etc. In polarized light it shows a banding 

 generally of lighter and darker shades of the same color. Its edges are 

 broken, step-like, and iiTegular ; and it shows cleavage parallel to the 

 basal and lateral planes. In microscopic characters it and the masonite 

 of Warwick, R. I., are closely alike. The tourmaline is in elongated 

 crystals containing grains of magnetite. All the minerals in this rock 

 are of later origin than the magnetite. 



South of the Champion mine dikes of diabase (346) and "diorite" 

 (345) occur in the gi-anite and gneiss. A " diorite " that was undis- 

 tinguishable from that in the gneiss (345) was found at the east end 

 of the Champion mine, in the " Huronian." This " diorite " contains 

 patches of quartz, which hold crystals of tourmaline in radiated groups. 

 The " diorite " (345), running north and south in the " Laurentian," cuts 

 both the granite and the gneiss, and is composed of hornblende, biotite, 

 quartz, and magnetite. Traces of some of the larger and more porphy- 

 ritic feldspar crystals remain in a kaolinized mass, containing flakes of 

 biotite and grains of quartz. Were the origin of this rock unknown, it 

 would pass for a sedimentary hornblende schist, so far as the section in- 

 forms us of its nature. The diabase (346) is comparatively fresh, and 

 is undistinguishable from the diabases seen in the " Huronian." It is 

 composed of augite, plagioclase, a little orthoclase and quartz, mag- 

 netite, viridite, and some decomposed olivine. 



Another dike (343), cutting the gneiss at this locality, is seen to be 

 composed of hornblende, quartz, feldspar, and magnetite. 



On the northerly side of Republic Mountain a garnetiferous " diorite " 



