344 The American Geologist. May, 1894 
creases, it appears probable that, as has been suggested by 
H. V. Winchell,* the advent of* these rocks was in some way 
connected with the magnetic condition of the iron. If this be 
the case, we must conclude that most of the banded magne- 
tite of the Eastern Mesabi had become concentrated prior to 
Keweenawan time. But the lack of large ore-bodies in this 
region shows that up to this time the concentration had not 
occurred on a very large scale. It is probable that the same 
force which produced magnetization put a serious check upon 
the separation and concentration of the constituents of the 
rocks, causing the degree of concentration in that region at 
the present time to be behind that of the Western Mesabi. 
The Cretaceous Conglomerates. 
There are upon the Mesabi small patches of Cretaceous 
rocks, lying upon the Animikie strata. In the area examined, 
they are chiefly conglomeratic, and the fragments are mainly 
derived from the iron-bearing rocks. A study of these frag- 
ments shows two things: first, that at the time of the forma- 
tion of the conglomerate there existed hard iron ore in the 
iron-bearing member ; and, second, that "much of the rock has 
been decomposed and has had its iron concentrated subse- 
quently to being taken into the Cretaceous beds. 
We may conclude that the process of concentration has 
been going forward since early Keweenawan or pre-Keweena- 
wan time, and there is abundant evidence that it is going on 
at the present day. 
Summary. 
The more important points in regard to the ores of the 
Mesabi which this paper presents are these : 
1. The original rock is probably an altered greensand, in 
which the iron exists in the form of the hydrous silicate (jlaii- 
conite, a mineral which is supposed to be formed at the pres- 
ent day, on moderately deep areas of the sea beds adjoining 
coast lines, by the action of decaying organic matter upon 
fine mineral particles derived chiefly from subaerial erosion. 
2. The decomposition of this mineral, producing chiefly 
iron oxide and cryptocrystalline silica, and the concentration 
of these, together with the various phenomena consequent 
*Twentirtli Ann. Rep., Minn. Geol. Survey, )>. 136. 
