456 THE PENOKEE IRON-BEAEIXG SERIES. 



plain that at the final stage of the formation of this member there was a 

 clear sea, only well rolled quartz g-rains being- deposited. This implies 

 sorting of the material and therefore less rapid accunnilation. It is prob- 

 able that this change of condition was accompanied by a gradual sinking of 

 the sea bed. Naturally following the clearing up of the water have come 

 the nonfragmental sediments of the iron belt. As has been shown (p[). "247- 

 248) they are analogous to limestone formations in man}' respects. The 

 uniformity in width of this belt is noticeable throughout its western three- 

 fourths. In its eastern part, where there are considerable variations in 

 thickness, the irregularities have been explained to be due to contempora- 

 neous volcanic activity. 



Above the Iron-bearing member next succeeded the great thickness of 

 fragmental Upper slate. At Tylers fork these slates show their jjresent 

 maxinuuu thickness, nearly 13,000 feet. East and west of this point the 

 belt gradually narrows until it is cut oft', at the west near Numakagon lake 

 and at the east near Sunday lake, by the overlying Keweenaw rocks. The 

 relations between the Iron-bearing formation and the upper slates are the 

 same as those between tlie latter and the underlying quartz-slates; tliat is, 

 they are two formations which are in perfect comformitv, as is shown by 

 the strike and dip of many exposures in both belts. The change from this 

 iron formation to the mechanical sediments of the Upper slate was rather 

 the abrupt, although not so sharp as the change from the Quartz-slate to 

 the Iron-bearing member. The three members mentioned with contemjwra- 

 neous eruptives constitute the Penokee series. By an examination of I'l. ii, 

 it is seen that the}- are not all continuous throughout the district travei-sed 

 by them; bui: wherever the series is present one formation follows another, 

 except in the Eastern area in the order mentioned in conformable succes- 

 sion, so that they are properly placed together as a group of formations. 



Tlie unconforniitij at the hasc of the Keivecnaio series. — North of this 

 Penokee series are found the eruptive and fragmental rocks of the Kewee- 

 naw series. The rocks of the latter immediately north of the former are 

 usually bedded surface eruptives; but from Black river, in the east j)art of 

 T. 47 N., R. 4() W., IVIichigaii, to near the Montreal river, in the basement 

 layers of the Keweenaw series, has been seen at various points a red sand- 



