Lake Superior Iron Ores.—S purr. 337 
it by Foster and Whitney (1851) and by Wadsworth (1880).* 
Credner (1869) and Brooks (1873) favored the idea that the 
iron ores were old limonite beds in a sedimentary series, sub- 
sequently metamorphosed. Likewise the banded silica and 
iron of the Vermilion range was supposed by N. H. and H. 
V. Winchell (1889-1891) to have been orginally formed in 
practically its present condition, but instead of adopting the 
eruptive theory of the geologists on the south shore of lake 
_ Superior they advanced the hypothesis of direct chemical pre- 
cipitation from the waters of a hot primordial ocean, the silica 
and iron being supposed to have been alternately precipitated. 
It was chiefly by observations in the geologically younger 
districts, where, besides the banded silica and iron, there were 
found many other rock-types which were transitional into the 
banded rock, that the idea originated that not only this band- 
ed rock, but also many of the associated phases, were second- 
ary, and were derived from some original rock by a thorough 
internal alteration and chemical interchange. Irving and Van 
Hise, by their work in the comparatively little-altered rocks of 
the Penokee-Gogebic district (about 1888-1892), advanced the 
question enormously. They showed that the ferruginous 
slates and cherts, where the iron is in the oxidized state and 
is more or less banded with silica, and even the actinolite and 
magnetite schists, have resulted from the alteration and re- 
crystallization of the slaty and cherty iron carbonate. At first 
Irving adopted the theory that this iron carbonate had been 
formed by the replacement of an original limestone,+ but he 
afterwards abandoned it, and considered the cherty carbon- 
ate to have been the original rock. In regard to the iron the 
following statement was made := 
“Whether the iron was originally precipitated as a car- 
bonate, or was decomposed and precipitated as a hydrated 
sesquioxide, just as limonite now forms from iron carbon- 
ate in places where bog ore is depositing, is uncertain. If 
the latter is taken to be the case—and it is perhaps the most 
probable supposition—it ts necessary to believe that the or- 
ganic matter with which the limonite was associated reduced 
* Later (1893) Wadsworth came to believe that the iron-bearing rocks 
were metamorphosed sediments. 
+ Am. Jour. Sci., (III), vol. xxxii, Oct., 1886. 
tIRVING and VAN HIsE, Tenth Ann. Rept., U. S. Geol. Surv., The Penokee 
Iron-Bearing Series, p. 396. 
