46 THE PENOKEE IRON-BEARING SERIES. 



em rial of tln' lake Superior trough, at an elevation of l.ouo to 1,100 feet above the 

 lake, lies but a few miles south of the northern boundary of the Laurentiau area, for 

 about 50 miles westward from the Moutreal, after which it passes on to the more north 

 erly and newer formations. 



The rocks of the Laurentian nucleus have already been partly described in former 

 reports. Where they apijroach lake Superior they are almost wholly gneiss and gran 

 ite. The prevailing rock along the northern border is a dark gray to black, often 

 greenish-black, hornblende gneiss, in which the hornblende has usually been more or 

 less completely altered to chlorite. This alteration, when carried to any considerable 

 extent, gives a greenish tinge and greasy feeling to the rock, and, in cases of extreme 

 alteration, there is a passage to a green chloritic schist. The associated granites are 

 usually light pinkish-tinted to gray, and highly (piartzose, a frequent gneissoid ten- 

 dency showing their sedimentary nature. These rocks have a nearly due E.-W. 

 strike, and, near the northern border, a high southerly dii>. They are, however, 

 beyond ([ucstion greatly folded, and have as certainly an enormous thickness. 



Huronian nystem. — Lying immediately against the Laurentiau, and very sharply 

 defined from it, we find, extending from the Montreal river westward for 50 miles to 

 lake Numakagon, a belt of schistose rocks which we refer unhesitatingly to the Can 

 ada. Huronian, and which are beyond (juestiou the direct westward extension of the 

 iron bearing series of the upper peuiusula of Michigan. This belt has a width, in 

 general, of from li to 2J miles, and includes an aggregate thickness of strata of nearly 

 i;5,000 feet, with a number of well marked subdivisions, several of which are persistent 

 throughout the entire length of the belt. These subdivisions may be briefly summa- 

 rized as follows, beginning below: (1) Crystalline treniolitic limestone, at times overlain 

 by a band of white arenaceous quartzite, and at times absent, the next formation above 

 them coming into contact with the Laui-entian, 130 feet; (2) straw-colored to greenish 

 quartz-schist, and argillitic mica-schist, often novacnlite, 410 feet; (3) treniolitic mag- 

 netite-schists, maguetitic and specular <piartzites, lean nuignctic and specular ores — 

 forming the " Penokee Iron range," 780 feet; (4) alternations of black mica-slates with 

 diorite and schistose quartzites, and unfilled gaps, 3,495 feet; (5) medium-grained to 

 a])lianitic, dark gray mica-schists, with coarse intrusive granite, 7,9iS5 feet — in all, 

 12,.S00 feet. These rocks all dip to the northward, the angle being usually high, 

 but lessening toward the west, and trend with the course of the belt, which has 

 numerous minor corrugations, while preserving one general direction. The strike direc- 

 tions are always oblique to the trends of the underlying Laurentian gneiss, proving 

 the unconformability of the two systems, the actual contact of which may indeed be 

 seen in several places. 



Westward from lake Numakagon the Huronian belt is lost sight of, the Lauren- 

 tian gneiss and Keweenawan gabbro and diabase coming apparently into direct con- 

 tact wiith each other. 



