1248 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1902. [bull. 213. 
concentrating the ores, but other factors, the cross fracturing of the 
formation affording trunk channels for water circulation and the 
ponding of water by impervious slate layers within and above the 
iron formation, have had strong modifying effects on the circulation 
and have locally been dominant. So important are these modifying 
factors and so complex their effect that the ore deposits have most 
irregular shapes and erratic distribution, and it is scarcely possible 
to indicate from the structure of the iron formation an area which is 
more likely than any other to reveal ore on exploration. 
The ore deposits are shallow, seldom exceeding 350 feet in depth, 
but have great horizontal dimensions, sometimes a mile or more. 
The ore bodies come to the rock surface for most of their area, but 
are covered with glacial drift to a depth varying from a few feet to 
100 feet. The rich ore deposits thus far discovered are confined to 
the central part of the district. At the east end of the district the 
ores are hard and magnetic and associated with amphiboles, and have 
not been shown to occur in bodies large enough to warrant mining. 
In the west end of the district some good ore has been found, but 
here for the most part the ores are of low grade and contain abundant 
chert particles resulting from the disintegration of associated chert, 
making it necessary to wash the ores before using them. 
The iron formation is overlain to the south by a thick slate forma- 
tion. Exploration has not yet shown whether or not ore will be found 
beneath this slate, but certain geologic facts indicate small proba- 
bility. The boundary between the slate and iron formation as siiown 
on the geologic map is, because of the heavy covering of glacial drift, 
only approximately correct. Much more exploration will have to be 
done before the true limits of the iron formation on the south can be 
mapped. The iron formation itself contains interbedded slate hiyers 
which closely resemble the slate overlying the iron formation, making 
it difficult in individual areas to determine whether the area should 
be mapped as iron formation or as overlying slate. 
The total tonnage of high-grade ore thus far discovered in the 
Mesabi district has been estimated at from 500,000,000 to 700,000,000 
tons, a common estimate being 000,000,000 tons, of which nearly 70 per 
cent is of Bessemer grade. This tonnage is over twice that of all the 
other Lake Superior iron districts combined. The United States 
Steel Corporation in 1902 owned and controlled 85 per cent or more of 
the Mesabi ore. Shipments from the Mesabi district first began in 1801, 
and in 1902 the shipments were nearly half of the total for the Lake 
Superior region, namely, 13,329,953 tons, and more than one-third of 
the total of the United States. 
The ores are mined in open ruts by steam shovel, or underground 
by either ordinary or " milling "« methods. In 1 902 about 40 per cent 
a Chutes, known as mills, are run from the level of the bottom of the shaft up to the working 
levels. The ore is loosened and dumped into the chute, falling into ears at the bottom, being 
trammed to the shaft and hoisted. 
