MUSEUM OF COMPARATIVE ZOOLOGY. 55 



and if it is part of the same formation that the adjacent granites are, 

 then the latter are younger in their present position tliun the iron ore 

 of Republic Mountain. 



The preceding rocks (128, 129, 130) naturally fall under the name of 

 greisen, but as they seem simply to be the modified edge of the " Lau- 

 rentiau " granite, we prefer to apply the name granite to them. The 

 practice of giving a different name to every little local modification in 

 rocks has been a constant source of confusion in lithology. This prac- 

 tice has perhaps never been carried to greater extent than it has been 

 in this district.* 



South of Ishperaing, on the line of the Chicago and Northwestern 

 Railroad, a gray gneiss (192, 193) was seen dipping W. 33°, cut by the 

 common reddish granite (194), which sends veins through it. Figure 26 

 was taken here from the side of a little clitf (195). The gneiss, at the 

 points in which it is cut by the granite, is less schistose, and becomes 

 more granitoid in its structure. A few rods west, on the north side of 

 the railroad ti'ack, this granite is seen in contact (197) with and cutting 

 a quartzite that resembles the ordinary " Huronian " quartzites. The 

 granite is here in large masses, but shows its intrusive character when 

 in contact with the gneiss, quartzite, or schists. About one eighth of a 

 mile nearer Ishpeming the granite (202) was seen in contact (200) with 

 and contorting schists (201, 203). This shows its intrusive character on 

 both sides of the schist, the contact being well marked in many places. 

 On the same elevation a line-grained granite (204) was seen to be intru- 

 sive in a dark green nodular schist, contaiuiug large irregular masses 

 of feldspar (205). These schists and granites are in the area mapped 

 as " Huronian " by Mr. Brooks. 



The granite breaking through the " diorite" at Picnic Point has been 

 referred to (page 39). This appears to be the same as the reddish 

 granite that occurs at the mouth of Dead River (62, 63). The lamina- 

 tion of this latter granite strikes S. 80° E. The reddish granite of the 

 entire region appears to be lithologically the same. That breaking 

 through the "diorite" at Picnic Point (56) is seen to be, under the 

 microscope, a crystalline aggregate of feldspar, quartz, and hornblende, 

 with magnetite. The feldspar is in part clear orthoclase, but mostly a 

 pinkish decomposed one without definite polarization. This, according 

 to the present imperfect method of microscopic analysis, is presumably 

 orthoclase, but we believe it may or may not be so. This feldspar is 

 now composed of a fibrous decomposition product, kaolin (?), with ox- 



* SeeGeol. of Mich., Vol. II. 



