336 The American Geologist. time, a 
The ores in the several districts do not all belong to the 
same geologic period. According to the latest results of the 
U. S. Geological Survey, there are ore-bearing horizons in the 
Archean, the Lower Huronian, and the Upper Huronian. 
Yet the appearance of the ores and the containing rocks in 
each district is such as to show a close relation and to suggest 
a common origin. 
It is true that even in a single district the ores and ore- 
bearing rocks exhibit a great variety. But everywhere a band- 
ed structure of crystalline or cryptocrystalline silica and iron 
is characteristic, and in the older and greatly altered forma- 
tions, such as those of the Vermilion (Archean), there is lit- 
tle else. In the eastern Menominee (Lower Huronian) the 
rocks consist chiefly of this banded silica and iron, but there 
are many apparently fragmental quartz grains. In the Lower 
Marquette (Lower Huronian) there is the same banded silica 
and iron, but chiefly at the top of the peculiar iron-bearing 
formation; further down there is ferruginous chert and slate, 
grunerite-magnetite slate, and sideritic slate, the last especially 
toward the bottom. In the Penokee-Gogebic district (Upper 
Huronian), the peculiar iron-bearing formation consists chief- 
ly of sideritic slate and slaty and cherty iron carbonate, ferru- 
ginous slate and chert (where the iron is mostly oxide), and 
actinolite and magnetite schist. Finally, the Mesabi range 
contains all of these rocks in its iron-bearing formation. There 
is the banded silica and iron (“‘jaspilyte”) of the older ranges, 
with the cherty siderite, the sideritic chert and slate, and the 
hematite-magnetite-actinolite schist and slate of the Marquette 
and Penokee-Gogebic. There is, besides, a large class of rocks 
not identified in the other ranges. They vary greatly in color, 
etc., but are all characterized by small, typically rounded gran- 
ules, often visible only under the microscope. They are the 
spotted-granular rocks of the present writer. 
The Origin of the Iron-Bearing Rocks. 
The banded silica and iron which is the chief rock in the 
older ranges offered to investigators little clue as to its or- 
igin; it was even at first supposed to be a primary rock. 
The earliest exploited of the districts was the Marquette.. 
Here the banded silica and iron was taken to be an unaltered 
or little altered rock, and an eruptive origin was assigned to 
