842 The American Geologist. May, 1894 
name oi' an ore-deposit. These ore-deposits are often very 
large, being occasionally nearly a mile in their longest ex- 
tent. The ore is usually hematite, loose and granular, and 
when of best quality is of a blue or brown eolor. Typically 
there is a portion, near the surface, which has become 
hydrated into yellow limonite or gothite. Often these ore- 
bodies rest upon the basal quartzyte; often again they rest 
upon the hard and little-altered iron-bearing rock itself. The 
conditions under which they form seem identical with those 
necessary for the growth of the narrow bands of iron in the 
banded "jasper and ore." In both cases the iron has concen- 
trated in an area of especial weakness. In the case of the 
band the cause of the small area of weakness has already been 
explained. In considering the cause of the formation of the 
ore-bodies, it is only necessary to find the cause of the 
development of so great regions of weakness. 
The richest ore-producing region thus far developed is that 
which lies in T. 58-17, and surrounds the Virginia area, fol- 
lowing the supposed fault-lines. Immediately east of these 
lies another rich group, near Biwabik, in the somewhat dis- 
turbed strata adjacent to the upthrust area. At the Moun- 
tain Iron mine there is strong microscopic evidence of a dis- 
turbance, probably a fault, while at the other important mines 
there has not been sufficient exploration to enable one to 
determine their peculiarities. So the conclusion may be 
reached that the most important of the ore-bodies owe their 
existence to regional disturbances which have produced large 
areas of weakness. Faults, especially, are accompanied by 
the development of such areas, and in folded regions the sum- 
mits of anticlines, and to a less degree the troughs of syn- 
clines, are weakened. 
In the opening up of a great area of weakness, moreover, 
such as attends a fault-plane, a channel for the surface waters 
to find their way down is often afforded, and the shallow 
underground drainage for a considerable distance is deflected 
into the fault-fissure. The increased supply of water, emerg- 
ing saturated with iron from percolating through the decom- 
posing iron-bearing rocks, is an important factor in the for- 
mation of the ore. The chemistry of the process is practic- 
ally the same as for the small oxidized areas which produce 
