ANALYTICAL ABSTRACTS. 539 



Comments. — The suggestion of the two additional unconformities in the 

 Huronian of the Marquette district is so tentative that no criticism of it is 

 necessary. The suggestion implies that Dr. Wadsworth thinks this outcome 

 the most probable one. It appears to the writer, however, that it is far more 

 probable that the true explanation is that there are only three unconformable 

 pre- Kev/eenawan series. The additional unconformities are probably sug- 

 gested by the considerable local variation in the character of both the Lower 

 Huronian and Upper Huronian series, so that in different parts of the district 

 the same series has very different aspects. 



Lane 1 holds that certain of the ore bodies of the Marquette district are 

 produced by abstracting iron oxide from amphibolites and depositing this 

 material at other places. The water is regarded as upward moving, hence 

 the ore bodies rest upon the diorites as foot walls. It is not denied that in 

 other places the iron is derived from a carbonate, or that silicia is replaced by 

 the iron oxide. At the Volunteer mine the ore seems in part to have replaced 

 the sandstone. 



Bell reports on the Sudbury mining district f The rocks are divided into 

 three groups, in ascending order: (i) A gneiss and hornblende - granite 

 series — Laurentian. (2) A series comprising quartzites, massive graywackes, 

 often holding rounded and angular fragments; slaty graywackes, with and 

 without included fragments ; drab and dark - gray argillites and clay - slates ; 

 dioritic, hornblendic, sericitic, felsitic, micaceous and other schists ; and 

 occasionally dolomites, together with large included masses or areas of pyri- 

 tiferous greenstones. This group constitutes the ordinary Huronian of the 

 district. (3) A division consisting of a thick band of dark -colored silicious 

 volcanic breccia and black slate (generally coarse), overlaid by drab and dark- 

 gray argillaceous and nearly black, gritty sandstones and shaly bands. The 

 breccia is underlaid in places by quartzite conglomerate. (4) In addition to 

 these, dikes of diabase and gabbro cut through all the foregoing, and are, 

 therefore, newer than any of them, although they may not belong to a later 

 geological period. 



Flanking the Huronian rocks on the southeast is gneiss, and on the north- 

 west a mixture of gneiss and hornblende - granite. The first of these rocks is 

 of the characteristic Laurentian type, but the hornblende - granite and quartz - 

 syenite on the northwest are not always characteristic of the Laurentian. 

 These rocks, however, pass into the gneiss in such a way, and are mingled with 



1 Microscopic characters of Rocks and Minerals." A. C. Lane. Rep. State Board 

 Geol. Sur., Mich., for 1891-2, Lansing, 1892, pp. 176-183. 



2 Report on the Sudbury Mining District, by Robert Bell. Annual Rep. Geol. & 

 Nat. Hist. Sur. of Canada for 1889-90. Vol. v, Part F, p. 95, with a geological map. 



