30 THE PENOKEE IRON-BEAELNG SEEIES. 



1873. 



Brooks (T. B.) and Pumpelly (E. ). On the Age of the Copper-bearing Rocks 

 of Lake Superior. Am. Jour. Sci., 3d series, vol. ill, 1S7L', ])p. 428-432. 



Messrs. Brooks ami Pumpelly, in the tall of 1871, made a very rapid 

 trip from the passage of Bad river through the Penokee range in Wiscon- 

 sin eastward to the Ontonagon river in Michigan. 



The statements below quoted from this paper with regard to the coun- 

 try between the Montreal river and Gogebic lake contain the first pub- 

 lished announcement of the existence in Michigan of an eastward continua- 

 tion of the Penokee Iron series of Wisconsin. The width of the iron-be'ar- 

 ing series often greatly exceeds the " one-fourth to one-half mile" given in 

 this quotation, reaching as much as 2^ miles or more, while at other times 

 it is below their lowest figure, or is even cut out altogether by the overlap- 

 ping of the overlying Keweenawan rocks. 



During hist aiitnmn, traveling sometimes together and sometimes apart, we 

 made a reconnaissance of the country between Bad river in Wisconsin and the middle 

 branch of the Ontonagon east of lake Gogebic in Michigan. Our route was chiefly 

 confined to the surface of the upper member of the Michigan Azoic, which we have 

 provisionally considered to be the ei|ui\alent of the Huronian. 



From Penokie gap on Bad river to near lake Gogebic, a distance of nearly 60 

 miles, the (juartzites and schists of this formation are tilted at high angles and form 

 a belt one iburth to one half mile in width, bordered on the south by Laureiitian 

 gneiss and schists. On the north it is everywhere overlain by the bedded mehipliyre 

 (containing interstratified sandstones) of the Cupriferous series. These form ridges 

 and i)eaks which rise 200 to 300 feet above the surface of the Huronian belt. 



These ridges, forming the "South Mineral range," unite at their western end 

 witli the Jlincral range proper, which forms really throughits whole length the tongue 

 of lanil known as Keweenaw point. IJetween these two ranges lies the southwestern 

 paft of the Siluriaii trough, which has been mentioned l)efore as extending inland 

 from Keweenaw bay. 



Here, as tlicre, it is iillcd with the horizontally stratified Silurian sandstone, 

 forming a generally level country. For a distance of nearly 30 nules, between the 

 Montreal river in T. 17 and lake Gogebic, we found the Cupriferous series appar- 

 enlly coiifoniiing in strike and di|) with the Huronian schists, and both uniformly 

 dipping to tlic north at angles of ."iO' to 70^. But in ai»proachiiig lake Gogebic from 

 the west wc find that erosion of Silurian oi- i>reSilurian age has made a deep inden- 

 tation entirely across the Cu|)rifer(ms scries and the Huronian, and into the Lauren- 

 tiau, so that at a short distance west of the lake these rocks end in steep and high 



