Iron Ores of Minnesota.—IlW inchel!. 161 
Vermilion range the chief ore bodies are composed of the al- 
tered obsidian lava masses, instead of sands of detritus derived 
from them. In both ranges the chemical attack of the oceanic 
waters on the lavas resulted in the silicification of the obsidian, 
and the concentration of the contained iron locally in the lenses 
mentioned, while the alkaline elements were carried away in 
solution as carbonates. Along with this chemical change in 
the obsidian, the ocean itself deposited in the near vicinity a 
large amount of chemical silica and probably of iron, these 
substances, especially the former, forming the stratified jaspi- 
lyte associated with the ore bodies and furnishing also the fine 
silica which permeates the fine schists of the region. The de- 
tails of the evidence of this hypothesis cannot be given. Suf- 
fice it to say, that it satisfies all the conditions and depends on 
long examination in the field, and on microscopical examina- 
tion of the ores. It also throws light on some unsolved struc- 
tural problems connected with the eastern end of the Mesabi 
range. It appeals to the well known tendency of silica to re- 
place all non-crystalline substances when it is in solution in 
alkaline water, preserving their forms. Wherever these lavas 
became crystalline prior to cooling, they seem to have main- 
tained their composition, in the main, only having been pene- 
trated by interstitial silica and reddened by the entrance of a 
small amount of iron. When they were incipiently crystalline 
they have been changed to the masses of hard grayish-brown 
taconyte which replaces the ore on the Mesabi range. 
If with this hypothesis in mind, we attempt to forecast the 
future of the Mesabi iron range, we can restore in our mind’s 
eye the ancient shore line of the Archean across northeastern 
Minnesota. We can see the sands resulting from the commi- 
nution of the lavas, drifting westward along that shore, ever 
increasing toward the west, as the shore sands of lake Superior 
at the present drift westward and accumulate in greatest 
amount in the col at the western end of the lake. The Arch- 
ean lands of northern Minnesota may have formed a shallow 
strait, or even a Taconic col, somewhere to the westward from 
Duluth, and into that col the Taconic waves must have driven 
the sands in question. If we could remove the drift from 
northern Minnesota and could see the lines of the old Taconic 
shore, we could doubtless see the location of the greatest 
