The Iron Ores of the Mesabi Range. — Spurr. 343 
bands. Waters emerge heavily laden with iron into this zone 
of oxidation : here they precipitate their iron and are enabled 
to take into solution and carry away some small quantity <>i' 
silica. The effect of the underlying impermeable quartz3 T te is 
important, since it deflects the surface waters into the weak 
zone, instead of absorbing them. Owing to the crystalline 
nature of its quartz, it is hardly or not at all replaced by iron. 
and thus it forms an unaltered stratum, even in the weakened 
zone, upon which the ore-body may accumulate. 
In addition to this, smaller ore-bodies may be formed 
near the surface, at the bottom of the highly weathered 
zone, and resting upon the little-altered rock below. In- 
equalities in the decomposition produce small basins in the 
hard rock below, in which waters collect and finall} T deposit 
their iron. 
It may be noted that the impervious dikes, which have been 
shown by Irving and Van Hise to have been important in the 
concentration of the iron in the ranges of the South Shore, 
especially in the Penokee-Gogebic, have played no part upon 
the Mesabi. Upon the whole length of the Western Mesabi 
there has not yet been discovered a single dike or other igne- 
ous rock in the Animikie. 
It must be remembered that in no single case has an ore- 
body been mined out, or even explored sufficiently to give a 
complete knowledge of its features, so that much valuable in- 
formation must come to light in the future. 
Date of the Concentration. 
These is no positive evidence as to the exact time of the 
disturbances which ultimately brought about the formation 
of the great ore-bodies. It may be considered, however, 
pending evidence to the contrary, that they were contempora- 
neous with the monoclinal tilting, and that they occurred in 
later Keweenawan or immediately post-Keweenawan time. 
The Eastern Mesabi. 
The Eastern Mesabi differs in regard to it- iron from the 
Western Mesabi in that it contains a much larger proportion 
of magnetite, which is associated with somewhat more crye 
talline silica. As these peculiarities are associated with the 
presence of the igneous rocks of the Keweenawan, and since 
they fade out as the distance from the Keweenawan area in- 
