MUSEUM OF COMPARATIVE ZO(3lOGY. 



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bear, and cracks or breaks, tind faults or throws, are the result, though 

 these are not numerous in the Lake Superior region. Cracks so pro- 

 duced, and iilicd with material other than tliat constituting the adjacent 

 rocks, are called d^kes ; or, if the material bo crystalline and metalhf- 

 erous, vei7is. As iron ore in workable quantities docs not occur in this 

 form in this region, vein phenomena will not be considered here." Of 

 the Iluronian scries "the prevailing rock is a greenstone, or diorite, in 

 which, like the copper traps, the bedding Is usually obscure ; but the 

 intercalated schists and slates, which usually bear strong marks of strati- 

 fication, make it usually not difficult to determine the dip of the beds 

 at auy point. . . . Descending to the oldest or bottom rocks of the 

 Lake Superior coimtry, the granites and associated beds (Laurentian), 

 wo find the bedding indications still more obscure, and often entirely 

 wanting." {I. c, pp. 74, 75, 7G.) 



" In subdividing the Ilui'onian or iron-bearing series which we have 

 particularly to study, the rocks have been grouped (1) lltkologically , i, e., 

 according to their mineral composition, and (2) stratiyraphically, L <-., ac- 

 cording to relative age. As this system was first described and named 

 by the Canadian geologists, their namcfe have been employed as far as pos- 

 sible in the body of this report; the identity in composition of many of 

 our rocks with theirs, having been established by an examination of a large 

 number of Marquette specimens by Dr. T. Sterry Hunt." (/. c, p. 82^) 



u fp 



The several beds or layers of the Iluronian system, as developed in tlie 



I\rarquctte region, are nuinbcred upwards from I. to XLX L, IT., 



IIL, ly., are composed of beds of siliceous ferruginous schist, alternating 

 with chloritic schists and dioritcs, the relations of which have not been 

 fully made out; V. is a quartzite, sometimes containing marble and 

 beds of argillite and novaculite ; VL, VIIL and X. are siliceous fer- 

 ruginous schists; VTL, LX. and XL arc dioritic rocks, varying mucli 

 in character; XI FL is the bed which contains all the rich specular and 

 magnetic ore, associated with mixed ore and magnesian schist; XIY. is 

 a quartzite, often conglomeritic ; XV. is argillite or clay slate ; XVL is 

 uncertain, it contains some soft hcuiatite ; XVII. is anthophyllitic schist, 

 containing iron and manganese; XVIIL is doubtful ; XIX. is mica 

 schist, ^containing stanrolite, audalu^ite, and garnets. This classifica- 

 tion, it will be borne in mind, applies oidy to the Marquette region. 

 . These beds a])pcar to be metamorphosed sedimentary strata, havin- 



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o 



many folds or corrugations, thereby forming in the Marquette region an 



irregidar trough or basin Wliile some of the beds pi-escnt litho- 



loglcal characters so constant, that they can be identified wherever seen 



