718 THE JOURNAL OF GEOLOGY. 
hard and little altered iron-bearing rock, in areas of especial weakness 
or disturbance, as (1) actual fault lines, (2) incipient fault lines, (3) 
apices of anticlinal folds, and the troughs of synclines. These are. 
places of fracture and where abundant waters were converged, often 
form wide areas, and therefore where large quantities of iron were 
supphed. ‘The downward percolating water, taking iron carbonate 
in solution, precipitated the iron as oxide in those places where there 
was an abundance of oxygen, and at the same time took the silica in 
solution, thus forming the ore bodies. Between those of the largest 
‘size and the small local concentrations there are all gradations. The 
larger deposits of ore occur where they are protected from glacial 
erosion on the north by a hard ridge of the Keewatin rocks, and espe- 
‘cially when the hard rocks give slight elevations on either side, so as to 
present a basin-like depression. 
The glauconite in origin is believed to be the same as modern 
glauconites, that is, it has developed within foraminifera and other 
minute shells, as a result of a reaction between the organic matter 
within the shells and fine ferriferous clay. As the formation contains 
only a small quantity of ordinary fragmental quartz grains, it formed 
in water at a depth beyond which much of these material was deposited. 
As its upper horizon grades into limestone, this indicates a further 
subsidence of the area, so that the distance from the shore line became 
so great that very little mechanical detritus was furnished, and the 
deposit was made up of calcareous matter. 
In the eastern Mesabi district the Animikie strata are pierced and 
intermingled with the northern border of the Keweenawan rocks, so 
that their normal attitude is often much disturbed. With this change 
the iron of the iron-bearing member becomes largely magnetic and the 
silica hard and crystalline. It is concluded that the iron before 
Keweenawan time was here in the state of sesquioxide, and that the 
heat of the igneous Keweenawan rocks and the disturbances of the 
Animikie series produced by them are the causes of the change of the 
sesquioxide of iron to its magnetic form. Thus the normal process of 
decomposition and concentration was brought to aclose, and this prob- 
ably explains the poverty of this part of the district in large ore deposits. 
At the base of the Cretaceous are ferriferous detrital deposits 
derived from the Animikie. A study of these indicates that the meta- 
somatic processes had gone far before Cretaceous time, although they 
have since continued to the present time. 
