C. B. Van Hise — Origin of the Mica-Schists, etc. 453 



Akt. XLIII. — Upon the Origin of the Mica-Schists and Black 

 Mica- Slates of the Penokee- Gogebic Iron- Bearing Series,' by 

 C. R. Van Hise. 



(Published by permission of the Director of the U. S. Geological Survey:)* 



The iron-bearing formation of the Penokee-Gogebic region 

 has been traced from Lake Numakagon in Wisconsin to Lake 

 Gogebic in Michigan, a distance of more than 80 miles. The 

 rock belts of this series traverse the country in a general east- 

 and-west direction. They dip quite uniformly to the north- 

 ward at an angle usually ranging from 60° to 70°, though 

 occasionally the dip is at a lower angle. They lie unconform- 

 ably upon a series of schists, gneisses and granites, and are 

 overlain unconformably by the Keweenaw Series.f 



At Penokee Gap, Wisconsin, the series is, in round numbers, 

 13,000 feet thick, of which the upper 11,000 feet are mica- 

 schists and black slates,;}; which thickness is retained to the 

 eastward until Michigan is reached. These upper members 

 have been traced continuously from English Lake (chiefly 

 in Sees. 5 and 8, T. 44 K, R. 3 W., Wis.) to the Black River 

 (Sec. 12, T. 47 N., R. 46 W., Mich.), a distance of more than 

 40 miles, their course in this distance being north of east, in 

 places as much as 30° north of east (see map).§ 



The present lithological condition of the upper members of 

 the formation is quite diverse in different localities. The group 

 comprises fine-grained, crystalline mica-schists, black mica- 

 slates, greywackesj greywacke-slates and quartzites. The ex- 

 posures at the eastern extremity of the belt at Black River are 

 red and white feldspathic quartzites. As we proceed westward 

 the exposures found show chloritic greywackes and greywacke- 

 slates. In section 14, T. 46 N., R. 2 E., Wis., the exposures 



* In advance of a full treatment of the subject to be included in a memoir on 

 the Penokee-Gogebic Iron-Bearing Series, by R. D. Irving and C. R. Van Hise. 



f This Journal, March, 1885. Divisibility of the Archaean in the Northwests 

 by R. D. Irving. 



% Geol. Wis., vol. hi, p 105. 



§ The accompanying map is taken from the article before referred to. For dis- 

 cussion of stratigraphical relations and geographical distribution of the Penokee- 

 Gogebic Series, see the same article, also Geol. Wis., vol. iii, pp. 100-166. 



|| The term greywacke is here used in a lithological sense, in accordance with 

 the definition of the term given by Geikie, Text Book of Geology, 2d ed., p. 162 : 

 " A compact aggregate of rounded or subangular grains of quartz, feldspar, slate, 

 or other minerals, or rocks, cemented by a paste which is usually siliceous, but 

 may be argillaceous, feldspathic, calcareous or anthracitic. Gray, as its name 

 denotes, is its prevailing color; but it passes into brown, brownish purple, and 

 sometimes, where anthracite predominates, iuto black. The rock is distinguished 

 from ordinary sandstone by its darker hue, its hardness, the variety of its compo- 

 nent grains, and above all by the compact cement in which the grains are im- 

 bedded." 



