Editorial Coiunient. 387 
The succession of formations above given differs from that which T 
supposed would be found before the Vermilion district was studied in 
detail. I had supposed that the Aniniikie series was equivalent to the 
series here placed in the Lower Huronian. When the relations v-ere 
first studied in the field by the members of the United States Geolog- 
ical Survey, Mr. Clements and Mr. Leith thought that the relations 
of the two series was those of unconformity. When I reached the field 
later and saw the same facts, I was inclined to believe that the phenom- 
ena were more likely to be explained by overlap. However, in the 
season of 1900, when Mr. Leith began studies upon the Mesabi district, 
he showed that the Mesabi series, which is undoubtedly the equivalent 
of the Animikie series, with gentle inclination rests unconfoririably 
upon vertical slates and conglomerates equivalent to the Knife Lake 
slates and Ogishke conglomerates. I have therefore no doubt that the 
correct interpretation of the phenomena in the Vermilion district is 
that of unconformity between the Animikie series and the Lower Hu- 
ronian series. 
"Before closely studying the Vermilion district in the field, suppos- 
ing the two series to be the same, I also thought it probable that the 
iron-bearing formation of the Vermilion would turn out to be the 
equivalent of the Lower Huronian iron-bearing formation of the south 
shore of lake Superior in the Marquette district. But very careful \.ork 
by Messrs. Clements, Merriam, Leith and myself has failed to discover 
any great structural break between the Soudan formation and the great 
Ely greenstone formation which is undoubtedly Archean. Therefore 
we now recognize an iron-bearing formation in the Archean. Here, 
as elsewhere in the Lake Superior region, there is great unconformity 
between the basal Archean complex, consisting mainly of granites, 
gneisses, greenstone schists, and greenstones, but containing subordinate 
amounts of sediments, and the sedimentary series of the Lower Hu- 
ronian." 
These modifications, barring differences of nomenclature, 
constitute an important step toward concordance respecting the 
general geological stntctttre of northeastern Minnesota, be- 
tween the United States and the Minnesota surveys. The s-'udy 
and interpretation of the geology of the Lake Superior region, 
and of the iron ore-s found on both sides of that great depress- 
ion, is a very arduous and complicated undertaking. There is 
no reason to expect complete agreement between the observers 
who work on the different parts of the problem, even when they 
examine the same facts at distant points. Northern Minnesota 
is a district in which the oldest rocks can be studied with ex- 
ceptional case. The classification which may be ultim.-i.tcly 
evolved when they arc fully worked up, in connection with the 
adjacent portions of Canada, will be likely to stand for many 
