H. D. Irving — Is there a Huronian Group f 253 



formation with them. The matter has been yet further com- 

 plicated — though this difficulty mainly disappears on micro- 

 scopic study — by a resemblance, partly close and partly only 

 macroscopic or very vague, between certain of the older schists 

 that have been produced by the intense squeezing and altera- 

 tion of some sort of basic eruptives, and certain interleaved 

 basic eruptives of the upper series. Again, some of the later 

 basic eruDtives have intruded indifferently the detrital areas 

 and those of the older schists. Notwithstanding these difficul- 

 ties, and the fact that there may be with regard to some par- 

 ticular exposures doubt as to whether they should be referred 

 to one or the other series of rocks, it remains sufficiently evi- 

 dent that there are the two distinct sets of rocks in the Mar- 

 quette region. 



The upper series is mainly composed of detrital rocks usually 

 little more altered than in the Lake Huron Huronian; the 

 most abundant being quartzites. These are of various phases, 

 ranging from the most vitreous quartzite to only slightly indu- 

 rated sandstone ; but all are genuine fragmental rocks. A 

 clayey admixture is often present, and then at times pressure 

 has developed a schistose structure and a secondary sericitic 

 ingredient; but even in this case there is no difficulty whatever 

 in detecting the essentially fragmental character of the rock. 

 By increase of this clayey ingredient we have a gradation 

 into the clayey shales and slates, which form the next constit- 

 uent of the series in order of abundance. These present many 

 phases, ranging from arenaceous to earthy in texture ; being 

 at times cross-cleaved and again not so, when they are little 

 more than clay shales. Tbey include also some very highly car- 

 bonaceous kinds. Next to the detrital rocks in importance, 

 and presenting often gradation phases into them, is a series of 

 what we may speak of as chemical sediments, though these have 

 often undergone very great changes since their first deposition. 

 Here belong the various calcareous and dolomitic rocks, the 

 cherty or jaspery ferruginous schists and iron ores, and the 

 actinolitic ferruginous schists. Most, if not all, of these 

 ferruginous rocks have been derived from bedded iron carbon- 



Qrantte- Cry**,. ScJvoste GrcuiiXe 



Fig. 3. — Generalized ideal section of the Marquette region in the vicinity of 

 Marquette, showing the relations of the younger detrital rocks to the older 

 granite-invaded schists. Scale, 2 miles to the inch. 



ates by a process of silicification, as I have endeavored to show 

 elsewhere, the unaltered or little altered iron carbonate [at 



