Lake Superior Iron Ores.—S purr. 343 
So the differences between the views of the present writer 
and those of Messrs. Van Hise and Leith resolve themselves 
to still smaller dimensions. Each believes that the original 
source of the iron is a hydrous ferrous silicate of organic 
origin; and the remaining point is fortunately a slight one, 
the writer having decided to call the mineral glauconite, and 
his fellow-geologists objecting to this nomenclature. 
Just, then, what is glauconite and what are its limits? Can 
-our mineral be classed with the glauconites, or must we name 
it with a new name? 
, 
First, as to the chemical composition. Our mineral is, | 
repeat, ‘‘essentially a hydrous protosilicate of iron*with a small 
amount of alumina, variable small amounts of calcium and 
Magnesium, and trifling quantities of the alkalies.’* Dana 
defines glauconite as “essentially a hydrous silicate of iron 
and potassium; but the material is mostly, if not always, a 
mixture, and consequently varies much in composition.”+ In 
Dana's Manual of Geologyt the New Jersey glauconite is de- 
fined as “‘a soft, dark or light green silicate of alumina, iron, 
and potash, with water.’ Zirkel defines the mineral as “a 
hydrated silica of chiefly ferrous iron (or ferric iron), with 
potassium, also some alumina and calcium.”$ Neither Dana 
nor Zirkel gives a chenucal formula for the mineral—it has 
none. 
As regards the different elements present in glauconite, 
the writer finds in the few analyses at hand alumina varying’ 
from 15.21 per cent to I per cent; magnesia from 16.60 pe 
cent to .57 per cent, lime from 3.30 per cent to a trace; potash 
from 7.91 per cent to less than 1 per cent; soda from, 1.28 per 
cent to nothing; and water from 12.60 per cent to 4.71 per 
cent. As to the iron oxide, it varies from 30.69 per cent to 
16.30 per cent. The silica ranges from 40 per cent to 52.86 
per cent. 
In the course of his investigation of the Mesabi rocks the 
writer had two analyses made of the green silicate, by two dif- 
ferent methods. The only difficulty lay in the free silica, 
which was so disseminated in it that a thorough exclusion 
was impossible. It therefore became necessary to estimate the 
* Bull. X, Minn. Geol. and Nat. Hist. Surv., p. 235. 
+ The italics are mine. (J. E. S.) 
t Third edition (the only one I have access to), p. 58. 
§ Lehrbuch der Petrographie, 2d edition., vol. iii, p. 728. 
