T. B. Brooks— Youngest Huronian Bocks. 207 



A careful consideration of all the facts to be observed in the 

 Menominee Eegion confirms me in this hypothesis,* which is 

 further supported, as it seems to me, by observations in the 

 Penokie Iron Eegion (Bad River), Wisconsin. 



Colonel Whittlesey's maps and sections, given in Owen's Re- 

 port, 1852, represent a belt of granite, syenite, and hornblende 

 rocks as dividing the Penokie series ( Huronian ) from the over- 

 lying Copper-bearing amygdaloidal traps and sandstones, which 

 lie to the north and nearer the lake. 



I observed these rocks at several points in 1871, and noted 

 their general lithological resemblance to the Laurentian, as well 

 as the almost insurmountable structural difficulties in assigning 

 to them that age, and recorded in my notes the probability of 

 their being Upper Huronian. Rowland Irving mentions these 

 rocksf as being coarsely crystalline aggregates " chiefly of lab- 

 radorite and orthoclase feldspar, hornblende, and some variety 

 of pyroxene," with occasional evidences of bedding, which points 

 toward their entire conformability with the underlying Huro- 

 nian. He regards them as of the period of the Copper-bearing 

 series, constituting its lowest and oldest portion. 



Having been, so far as I know, but little studied, it is perhaps 

 impossible at this time to determine their age : but what is 

 known can here be briefly surveyed, and an inference drawn, 

 which will not be without value in directing further investiga- 



1 The general lithological similarity of this granitoid belt 

 to the Laurentian, has been remarked. It has quite as much 

 similarity, if not more, to several members of the Huronian ; 

 and is, I believe, not identical with any rock known to belong 

 to the Copper seriea 



2. Its geographical extension is peculiar in this : it wedges 

 out rapidly to the east from the vicinity of Penokie Gap, en- 

 tirely disappearing at the Montreal River, which divides Mich- 

 igan from Wisconsin. Professor Pumpelly and myself traced 

 the boundary between the Copper and Huronian rocks 30 miles 

 farther eastward beyond Lake Gogebic, without again observ- 

 ing it, which we should certainly have done if it had existed 

 there ; for we often found the two series very near together, al- 

 though the actual contact was not seen. 



* Dr. H. Credner regarded the 

 lowest member (a quartzyte) of 



»orae out, as it seems to me, by the facts. He seems to have based this ge 

 nc reasoning largely on a rough section which I sketched for him ( and wh 

 has reproduced ) of the Negaunee District, where the Upper Huronian, so w 

 veloped at Michigamme Lake, is wanting. His great overestimate of the 

 ness of the Menominee rocks has also led him astray. (See Zeitschrif t d«r deu 

 geologischen Geseilschaft, Band xxi, 1867, p. 553.) No attempt was m 

 my Michigan Report to corrfllaiA the Marauette and Menominee series, eacl 

 provisionaUy numbere<J " ' 



f Am. Jour. ScL, vol. 



