GEOLOGICxVL EXPLORATIONS AND LITERATUKE— 1873. 49 



Tli(^ niaior liTliolt>i>'ical iiroiips recog-uized in the Marquette district are 

 the ores; the ferruginous, siHceous, and jaspery schists; the diorites, diorite- 

 schists, and rehited rocks; the magnesian (chloritie) schists; the quartzites, 

 inchidinu- conglomerates, breccias, and saudstoues; iuarl)h', argilUte or chiy- 

 slates, and related rocks; mica-schists; anthophyllite-schists: and carbona- 

 ceous shales. 



The ores comprise five varieties, viz: the red specular, including slaty 

 and granular aggregates of nrartite and magnetite; the magnetic ores; the 

 mixed ores, consisting of interbanded jasper and specular ore; the soft 

 hematites, which are the most ferruginous portions of a limonitic siliceous 

 schist, from which silica was probably removed by thermal waters ; and the 

 flao- ores, emln-acing ferruginous schists, in which silicate minerals are often 

 iiresent. These latter are often like the mixed ores, from which they ditfer 

 principally in geological occurrence. 



The diorites and their schists are obscurely bedded rocks, varying in 

 texture from aphauitic to eoarse-grainL-il and sometimes |)orphyritic. They 

 are composed of a nonmagnesian hornblende and a plagioclase. They 

 occur in beds, where they present "in mass precisely the same phenomena 

 as regards stratification as do the acconq)anying schists and (quartzites." 

 In the Laurentian area rocks similar to these occur as dikes and veins, and 

 probably as beds. In the Marquette district the greenstones are abundant 

 and are verj' closely associated with the iron ores. 



The magnesian schists are problematic rocks, consisting largely of talc 

 or chlorite. Their cleavage is distinct, but their bedding planes are obscure. 

 In color these schists vary from grayish to green. They are intercalated 

 •with the pure, hard, and mixed ores at mo.st of the mines worked; but iu 

 a few of the mines, and in the quartzite ridge north of the outlet of Teal 

 Lake, thevform "slate dikes." The authctr finds it impossible to draw the 

 line between the chloritie schists here considered and certain dioritic schists 

 like those mentioned above. At the Marquette quarries typical chlorite- 

 schists are found bedded with granular diorites. 



The quartzites are recognized as occurring in three principal horizons. 

 One of these is near the base of the Huronian, the most important one is just 

 above the hard-ore formation, and the third is near the top of the series. 



MON XXVIII i 



