50 The American Geologist. Janugey seer 
Lower Huronian (Upper Keewatin), i. ¢., in the Archean 
(Lower Keewatin), has necessitated a modification of the de- 
finition of the term Archean as first proposed by Van Hise, 
which modification has been noted in these columns.* The 
broad and important conclusions, summarized above, concern- 
ing the stratigraphy of northeastern Minnesota are identical 
with those reached by the Minnesota survey, and it is a subject 
for congratulation to the geologists of that survey that the 
later and more detailed work of the United States Geological 
Survey confirms their conclusions. But of much more impor- 
tance is the fact that, because of the agreement in the conclu- 
sions of geologists who have worked in the district at different 
times and who have approached it from different points of 
view, we can now feel assured that the correct solutions of the 
broader problems in the stratigraphy of the district have been 
reached and will be generally adopted. There are, however, 
certain points in regard to details of stratigraphy, the origin 
of certain rocks and the genesis of the ore bodies, concerning 
which the conclusions reached in the present paper differ from 
the views of some of the Minnesota geologists, especially from 
those of the state geologist. 
The Vermillion district is one of very complicated and close 
folding, and a regional cleavage is commonly present. In fact 
the geology of this district is so complicated that heretofore 
only the broadest outlines of the stratigraphy have been pre- 
sented. The ore bodies occur in, but at or near the bottom 
of, the Soudan formation, which lies upon the Ely greenstone, 
the two constituting the essential part of the Archean. The ore 
bodies lie in troughs in the greenstone, and these bodies, as 
well as the various phases of the Soudan formation, are be- 
lieved to have been derived from a siliceous iron carbonate in 
the same manner as in the iron districts on the south side of 
lake Superior. ; 
The Mesabi range—the latest to be discovered and the most 
important of all—is treated in some detail. The three forma- 
tions which constitute the Upper Huronian in this district are 
a lower quartzyte (Pokegama) formation, a middle or iron- 
bearing (here named Biwabik) formation and an upper or 
slate (here named Virginia) formation. The rocks of the 
*AMEKICAN GROLOGIST, Vol, xxviii. p. 385-388, Dec., 1901. 
