250 The American Geologist. October, 1904. 
mingled with detrital sediment, thus warranting the assump- 
tion of the action of organic agencies and the general cotem- 
porary prevalence of ordinary detrital sedimentation at other 
points. The writer believes that all the facts that have been 
brought to ligh.t bearing on the origin of the lake Superior 
ores warrant the conclusion that these ores in their first con- 
dition resulted from the chemical action of oceanic waters on 
volcanic rocks, and that they were cotemporary, from the old- 
est to the youngest, with epochs of unwonted volcanic ac- 
tivity. The discussion of this view as to the older iron ranges 
has been presented, in an incomplete manner, in volume five of 
the final report of the Minnesota survey, and earlier in Bulletin 
No. 6 of the same survey. 
Referring to that report for reasons for this belief, it may 
be well to consider reasons for assuming that the Baraboo 
ore should be assigned to- a similar volcanic epoch. 
1. In the final discussion of the Keweenawan igneous 
rocks the writer divided them into two great divisions, to 
which he gave the names Cabotian and Manitou, the former 
being the older. The Cabotian contains the gabbro and anor- 
thos)i;es, the so-called red-rocks, i. e., the red quartz porphy- 
ries, the red granites and the felsytes, and numerous red-rock 
surface lavas. In the Cabotian are also many important basic 
lava flow rocks now much rotted. In the Manitou are only 
basic lavas and alternatmg sandstones and shales, with rare 
conglomerates, the lavas gradually fading out and giving place 
to a formation of great thickness, of shale and shaly sandstone. 
2. In the course of about 15 years of study and field 
examination in the Archean and Taconic rocks of the lake 
Superior region the writer has found no red quartz porphyry 
below the base of the Animikie. It seems to be confined to the 
Keweenawan. It. is true that in Wisconsin are several isolated 
knobs of quartz porphyry that have been assigned to the Arche- 
an, but there is no evidence whatever that they are not co- 
temporary with similar knobs further north that are well 
known to belong to the Keeweenawan. 
3. The writer has shown conclusively that the conglom- 
erate lying at the base of the New Ulm quartzyte is of later 
date than the Animikie and hence later than the Mesabi ore. He 
accepts the unanimous assumption that the Baraboo quartzyte, 
