340 The American Geologist. June, 1902. 
Huronian. It is 800 to 1000 feet thick; the present write: 
estimated the thickness of the Mesabi iron-bearing formation 
at from 500 to 1000 feet. The Gogebic iron-bearing forma- 
tion is underlain by a persistent quartzyte 500 feet thick, and 
overlain by a very thick (12,800 feet and less) series of slates, 
etc. The Mesabi formation is likewise underlain by a persis- 
tent quartzyte of approximately 500 to 800 feet in thickness, 
and overlain by a very thick series of slate. The Mesabi and the 
Gogebic are only a hundred miles apart, and the strata of the 
two districts dip toward each other, those of the Mesabi south, 
those of the Gogebic north. It is therefore probable that the 
iron-bearing member is identical in these ranges. Most of the 
other iron ranges are provedly of different age, some being 
Lower Huronian and some Archean; but the same peculiar 
types of rocks as those recognized in the Mesabi and Gogebic, 
although far more altered and with the fresher types lacking, 
are associated with the ore deposits, and point most strongly 
to a common origin. 
The Nature of the Original Ferrous Silicate. 
There remains but one question on which there is not 
a fairly general agreement.—the nature of the hydrous green 
ferrous silicate which the writer has shown to be the source 
of the iron. 
In his original investigation the writer summed up his 
inquiries as follows :* 
“Chemically, it is essentially a hydrous protosilicate of iron, with a 
small amount of alumina, variable small amounts of calcium and mag- 
nesium, and trifling quantities of the alkalies. Chemically it seems 
more closely related to glauconitz than to any other mineral, and 
differs chiefly in the absence of the usual larger amount of potash. 
Another way in which it differs from the ordinary glauconite ‘s that the 
iron here is normally in the protoxide state, while in nearly all the 
reported analyses of glauconite it is mainly in the sesquioxide condi- 
Oia rcr ly. & o> wee 
“The specific gravity of glauconite is given by Dana as from 2.2 
to 2.35: while we have found that of our mineral as higher than 2.8. 
But the glauconite of the St. Lawrence limestone (Upper Cambrian) 
of Minnesota, analyzed by professor S. F, Peckham, has according 
to him, a specific gravity of 3.634; and from the chemical composition 
of the mineral it must be that in many cases the density rises above 3. 
* Bull. X, Minn. Geol. and Nat. Hist. Survey, p. 235-7. 
—r 
