338 The American Geologist. June, 1902: 
the latter to the protoxide,* and by decomposition furnished 
the carbon dioxide to unite with the protoxide, and thus repro- 
duce iron carbonate.” 
Concerning the silica, they wrote: 
“Our conclusion ts then: First, that the chert was mainly 
deposited simultaneously with the iron carbonate with which 
it is so closely associated; and second, that it is probable that 
the chert is of orgamc origin,t although we have no positive 
proof that it is not an original chemical sediment, while it may 
in part be from both sources.” 
In 1893, the present writer perceived that most of the mul- 
titudinous rock phases of the Mesabi iron-bearing formation 
were secondary and were derived from one another by rear- 
rangement and crystallization. He saw this and worked out 
most of the changes even before he had the opportunity, on 
returning from the field, to read the literature of the iron-de- 
posits on the south shore of the lake. The discovery, that 
what he feared might be deemed a too radical conclusion had 
already been sustained for the Penokee-Gogebic range, en- 
couraged him, and he set about to continue his investigations 
with the aid of the microscope. Further work showed that 
while the variety of rocks in the iron-bearing formation was 
great, they were not so dissimilar in their internal structure 
as in their outward appearance. ‘There were sideritic cherts 
and cherty carbonates, banded hematite and cherty silica, mag- 
netite-hematite slates, actinolite slates, etc., like the rocks which 
had been described from the Penokee-Gogebic. But these 
were all evidently greatly altered rocks and further removed 
from the original source than certain abundant kinds which 
he called the spotted granular rocks, from their being under 
the microscope made up of many rounded or angular or ir- 
‘regular granular bodies in a generally silicious matrix. These 
granules give a mottled and fragmental appearance to the 
the granules are composed of silica, generally of finer texture 
than the silica of the ground-mass, with siderite, hematite, lim- 
onite, magnetite, and a green hydrous ferrous silicate, in all 
combinations and proportions.$ ‘There are besides various sub- 
ordinate minerals, such as actinolite, calcite, apatite, epidote, 
* The italics are mine. (J. E. S.) 
+ Op. cit., p. 397. 
i The italics are mine (J. E. S.) 
§ Bull. X, Minn. Geol. and Nat. Hist. Surv., pp. 138-139. 
