254 i?. D. Irving — Is there a Huroniam, Group ? 



times remaining in considerable bodies. Besides these two 

 classes of sediments, or alteration derivatives from sediments, 

 there are included also considerable thicknesses of diabasic 

 greenstones in the shape of eruptive sheets. These are of 



JV Jron J3earcn<? Series Iron, Series S 





Granite- CrystaZltrue. Schiets 



Fig. 4. — Generalized ideal section of the Marquette region in the vicinity of 

 Ishpcrning. In part after Brooks. Scale, 3 miles to the inch. 



course irregularly distributed, being almost absent on some 

 sections, and especially abundant on others, as for example on 

 the Ishpeming section herewith. West of the area mapped, 

 in the vicinity of Lake Michigamme, micaceous slates and 

 micaceous quartzites make up a considerable thickness in the 

 uppermost horizons of the series. These are often deserving 

 of the name of mica-schist, approaching very close to if not 

 quite reaching, the character of a genuine crystalline schist ; 

 but they run into kinds that are plainly detrital even to the 

 naked eye. They have apparently been produced, as have the 

 mica-schists of the Penokee region, by metasomatic processes, 

 from detrital rocks holding much fragmental feldspar ; but in 

 this case, unlike that of the Penokee country, there has been 

 strong lateral pressure involved and something of a genuine 

 foliation has been produced. These schists and slates do not 

 seem to be persistent, i. e., they are represented stratigraphically 

 in other portions of the district by unaltered detrital rocks. 



The accompanying map* and sections will serve to show the 

 distribution and relations of the granite, older schists and 

 younger iron-bearing series, for a distance of some five and 

 twenty miles westward from Marquette. Since the idea of a 

 separation between the older and newer rocks receives so great 

 support from the occurrence of basal conglomerates, I have 

 indicated on the map by letters the points at which the most 

 unmistakeable occurrences of this kind have been met with, 

 omitting all cases about which there seemed to me to be any 

 question. It will be observed that the places indicated furnish 

 points on each side of each of the detrital areas. In one case, 

 as for instance, that on the east line of Sec. 2, T. 47, E. 25 W., 

 we have merely a fine detritus from the contiguous granite; 

 but in most cases large blocks and pebbles of granite, gneiss 



* This map is not here reproduced. Jt was a large outline geological map, in 

 colors, of townships 47, 48 and 49, ranges 25, 26, 27, 28, in Michigan, drawn 

 to a scale of 1/5 miles to the inch, and showing the distributions of (1) the detrital 

 iron-bearing rocks, (2) the older schists, and (3) the granite which invades 

 the older schists at its contacts with them. 



