432 THE PENOKEE IKON-BEAEIKG SERIES. 



Mingled frag mental and nonfragmental sediments. — If the above account 

 represents the facts, it explains how it is that the sharply separated frag- 

 mental and nonfragmental sediments of the Western area pass into the 

 mineled sediments so characteristic of the Eastern area. The time was 

 one in which the conditions throughout the great part of the Penokee basin 

 were favorable to nonfragmental sedimentation. When the sea was undis- 

 turbed, pure sediments of this class were formed ; V)ut in the Eastern area, 

 almost as soon ;is this period was inaugurated, came- a time of intennittent 

 eruptive activit}', which formed the masses of jjorphyrite and greenstone- 

 conglomerate there found. Even when these materials were not being 

 ejected with su^-h rapidity as to preponderate over other varieties of rock, 

 the sea was so disturbed that only for brief intervals, and locally, was it 

 sufficientlv clear to form pure nonfragmental sediments. In this part of the 

 basin, the nonfragmental sediments Ijelonging to the iron-bearing formation 

 are interbedded with layers of or mingled with mechanical sediments. Non- 

 fragmental sedinu'uts continued to dei)osit in gi-eater or lesser purity, how- 

 ever, at everv fiivoral)le occasion throughout the whole of the time of 

 deposition of the eastern rocks, later than the Quartz-slate. The mass of 

 rocks above these slates in the Eastern area is several times thicker than 

 the nonfragmental sediments of the iron-bearing formation to the west. 

 In estimating time equivalents, however, Ave must take into account the 

 fact that Avater- deposited mechanical sediments may accumulate much 

 moi'e rapidly than nonfragmental sediments, and volcanic products far more 

 rapidly than ordinary mechanical sediments. This great difterence of 

 thickness, then, is no obstacle to correlating the Iron-bearing member to 

 the west with the mass of mingled fragmental and nonfragmental sediments, 

 and tlie interbedded eruptive.s and greenstone-conglomerates. The fact 

 that at every cessation of volcanic activity nonfragmental sediments 

 were deposited, which are precisely like the sediments of the iron forma- 

 tion to the west, seems decisive evidence in favor of this correlation. 

 Further, the greenstone-conglomerate itself, as has been seen, has in places 

 phases which have many of the characteristics of the iron-bearing foi-ma- 

 tion, interleaved with other ])hases which are not nt all like the rocks of 

 that member. Finally, the greenstone-(;onglomerate is not always clearly 



