Lake Superior Iron Ores.—S purr. 339 
pyrite and clayey matter. In the least altered rocks the granules 
were found to be made up almost exclusively of the green fer- 
rous silicate. The other forms of iron,—the carbonates as well 
as the oxides—were found to be derived from alteration of 
this original silicate. It was shown that this silicate is un- 
stable, and that decomposition begins by the appearance of 
very tiny transparent rings of silica scattered through the 
mineral. These rings steadily grow, till in the last stage they 
meet and form a continuous mass of silica. In proportion as 
the silica separates out, the remaining portion of the green 
mineral becomes darker and opaque from the separation of 
iron oxide, so that finally the original mineral has disappeared, 
leaving a mixture of hydrous tron oxide and silica. 
From this condition the silica changes somewhat by coars- 
ening of texture and by changing of position; and the iron 
may change into carbonate or magnetite, with or without the 
assumption of crystalline form. Subsequent changes often 
bring this siderite or magnitite back to the hematitic or limon- 
itic condition, and thus the fluctuation is kept up indefinitely. 
All these changes and many more were demonstrated by mic- 
roscopic work, and the alteration of the spotted-granular rock, 
not only to sideritic chert, but also to banded iron and silica, 
and even into iron-ore bodies, was followed in all its stages. 
The evidence put forward as to the derivation of the 
Mesabi iron from an original green hydrous ferrous silicate 
was accepted by the state geologist and others of the Minne- 
sota survey, and by outside scientific writers in general. It 
was not fully accepted by the United States Geological Survey 
until its parties had made a thorough study of the Mesabi; 
but on emerging from this study they also have agreed that 
the iron ores “have resulted from the alteration of certain 
rocks containing green granules, which, on analysis, prove to 
be essentially ferrous silicate.’’* 
The present writer agrees with the geologists of the United 
States Geological Survey, with those of the Minnesota survey, 
and so far as he knows, most others, in believing that the 
greater part of the ores of the Lake Superior region have had 
a common origin. The iron-bearing formation of the Peno- 
kee-Gogebic region lies, like that of the Mesabi, in the Upper 
* Eng. and Min, Jour., Feb. 22; 1902, p. 277. 
