GEOLOGICAL EXPLORATIONS AND LITERATURE. 51 



iu the Penolvt'o ranse, wliich on the west side of the river is well to the uorthward of 

 its ijositiou oil the east side. It is also well marked by largt^ rock exposures on either 

 side, aud also hy an abrupt break in the line of magnetic attraction caused by the 

 magnetic Iielt of the formation. The facts with regard to it are all detailed on 

 Atlas Plate xxiii, and also in the descriptions of the following pages. 



The following tabulation indicates the succession of layers of the Hurouian series 

 iu the vicinity of Bad river, so far as made out, with the average thickness, surface 

 breadth, and other prominent points of each layer. Several of the lower layers, 

 including a total thickness of some 1,500 feet, have been traced across the entire 

 width of the district. The higher layers, on account of their comparative softness and 

 susceptibility to chemical changes, have been for the most part deeply eroded, and 

 are, moreover, largely buried beneath accumulations of drift materials, so that the 

 exposures are comparatively few and distant, and the task of making out the succes- 

 sion becomes much more difficult. (Pp. 10o-104.) . . . 



Synopsis of the SIratigraphii of the Huronian of the Penokee region, Wisconsin. 



Average 

 thickness- 

 Formation. Feet. 



I. Tromolitic crystalline limestone 90 



II. (A) Aiou.iceous white quartzite, often brecciated, 35 feet; (B) maguetitic quartz- 



SL^hist, 5 feet .' 40 



III. Siliceons slaty schists; including quartzite, " argillitic " mica-schist, and novacu- 



litc; all having much ([uartz, ami none ever showing any amorphous material.. 410 



IV. Magnetic bell; including: («) banded magnetic quartzite — gray to red quartzite, 

 free from or lean in iron oxides, banded with seams, from a fraction of an inch 

 to several inches iu width, of pure black granular magnetite, only rarely 

 mingled with the specular oxide; (6) maguetitic ([uartzitc, the magnetite in 

 • varying proportions, pretty well scattered throughout, and mingled with the 

 specular oxide in proportions varying from nothing to a predominating quan- 

 tity; (o) maguetitic (|uartz-slate, the magnetite pervading the whole, and 

 mingled with the specular oxide as before; (d) slate like (o) but largely charged 

 with treniolite or actiuolite ; (c) arenaceous to compact aud flaky <juartzite, free, 

 or nearly so, from iron oxides; (/) thin-laminated, soft, black maguetitic slate; 

 (f/) hematitic quartzite, the iron oxide the red variety; (/i) garnetiferous 

 actiuolite-schist, or eclogite; (t) diorite, which is restricted to the western end 

 of the Huronian belt. Kinds, (a) to (d), all carry much pyrolusite, or other 

 manganese oxide. These varitics Iiave no persistent stratigrapliical arrangement, 

 aud are named here in order of relative abuudanee. Total thickness, about.. . 780 



V. JUark feldspalhic islaic: consisting of orthoclase grains imbedded iu a paste of 



biotite, pyrite, liujouite, and carbon 180 



VI. Unknown, always drift-covered 880 



VII. Dark gray to black, aphanitic mica-slate, having a wholly crystalline base of 



quartz aud orthoclase, with disseminated biotite scales 120 



VIII. Unknown, l)ut probably in large part the same as VII 290 



IX. Chloritic, pyritiferous, massive diorite 150 



X. Black, aphanitic iiiiea-slatc, like VII..,..,.,,,.,, ,, 25 



