160 The American Geologist. March, 190% 
strata, their inclination, their alternation in composition and 
their having been broken and penetrated by igneous dikes, 
have had much to do, according to this hypothesis with the lo- 
calization of the chief iron deposits. 
Dr. M. E. Wadsworth advanced the idea that the jaspilyte 
seen at Marquette, Mich., which there constitutes the ore-bear- 
ing rock, is of igneous origin, the direct result of igneous in- 
trusion amongst the other rocks of the region. He appealed to 
certain structural features which to him indicated such forcible 
fracture and intrusion. 
Mr. J. E. Spurr, working for the Minnesota Geological 
Survey, with minute microscopical inspection and by means of 
a combination of field observation with chemical and petro- 
graphical research, traced the iron oxide back to greensand, 
which he took to have been glauconite. This supposed glau- 
conite was compared to that formed of foraminiferal remains 
in the Cretaceous formations and it led naturally to the sup- 
position that the sea in which the ore was formed was one that 
swarmed with microscopic organisms. 
The latest hypothesis of the origin of the iron ores of Min- 
nesota is that of the writer published in the fifth volume of the 
Minnesota report. Accepting the greensand of Mr. Spurr as 
the immediate source of the Mesabi ore, this hypothesis as- 
sumes that such greensand is not of the nature of glauconite, 
but of volcanic glass or basic obsidian. It presumes that an 
epoch of igneous activity began at or near the commencement 
of the Taconic, not only in Minnesota but throughout the lake 
Superior basin. This was accompanied by igneous eruption 
and lava flows. Such lavas were frequent near the ancient 
ocean shores and gave rise to much obsidian. They were also 
submarine, and heated the ocean adjacent, giving it more 
powerful attack on the pre-existing shores as well as on the 
lavas themselves. The result was the distribution of glass 
sands along the ancient shores in the same manner as Silica 
and other sands accumulate along the shore of lake Superior 
at the present time. Such sands, more or less mingled with 
the traps from which they were derived, constitute at the 
present the soft ores and the two sorts of taconyte mentioned 
above. The same explanation is applied to the ores of the 
Vermilion range, but it is necessary to understand that in the 
