The Iron Ores of the Mesabi Il<nnje. — Spurr. 341 
The expansile strains lead to breccias, and to local faulting 
and folding, especially in the ore bodies. They also give rise 
to shearing movements, which have in many places altered 
the rocks for a limited distance and induced a schistose or 
slaty structure. This shearing process forms one of the chief 
deviations from the normal process of change. 
Variations from the Normal Process. 
Another important modification of the usual manner of 
change follows the exposure of a considerable area to freely 
oxidizing forces. This condition is nearly always found at 
the surface, and often deep into the rock, following zones of 
weakness, induced chiefly by regional disturbances. In these 
parts of the rock the various stages of decomposition and 
concentration are hurried forward with such comparative 
swiftness that the phases of rock tbereb} 7 produced are differ- 
ent from those produced by the normal process. In these 
rocks the iron is concentrated into large bodies and impure 
disintegrated silica and clay is left behind. Thus, among the 
ferrated rocks, the ore-bodies are formed. Among the leached 
rocks the most common are the "paint rocks," which have, 
subsequent to their leaching, usually become iron-stained, so 
that they form a stiff clay, red, yellow or brown in color ; also 
a cream white residual clay, which has been mistaken for 
kaolin. On analysis the last proves to be mainly free silica, 
with a small amount of silicate of alumina and impurities; 
and, in one place at least on the Western Mesabi, it is a nearly 
pure silica powder. 
Least important among the causes of change lias been the 
impregnation of the rocks by minerals apparently derived 
from without the iron-bearing member and brought by pre 
colating waters. Qalcite is the chief of these minerals: and 
the calcitized portions of the iron-bearing rock are distributed 
near the contact with the calcareous stratum of the black 
slates. 
The Formation of thb Oke-Deposits. 
It has already been stated that the bands of iron owe their 
existence to previously formed /ones of weakness. From the 
narrowband there may be found every gradation upward in 
size, till the body of iron becomes large enough to merit the 
