340 The American Geologist. May, 1894 
brings about an almost complete separation of the two miner- 
als. 
The Concentration into Bands. 
Most often weak and firm points alternate closely in the 
rock, usually being arranged in narrow horizontal zones. If 
the weak zone is one of comparatively scanty oxidation, the 
iron may be deposited as carbonate; if of freer oxidation, as 
the hydrous sesqui-oxide. So in the final concentration we 
may have a very well defined banding of cherty silica with 
siderite ; or, more usually on the Western Mesabi, bands of 
silica (usually ferruginous), alternate with bands of iron 
oxide (usually siliceous). The crystallization of the latter 
gives the typical "jasper and ore," which is occasionally found 
on the Western Mesabi, but is not so common as in others of 
the Lake Superior iron regions. 
Results of Accumulated Strains. 
The change from the oxides of iron to the carbonate, and 
from the carbonates back to the oxides, has been found by 
study with the microscope to be very common, and the same 
iron ma}^ repeatedly undergo the metamorphosis, with chang- 
ing conditions. When the most of the rock is thus altered, 
there is an appreciable change of bulk, the oxidation of the 
carbonate being attended by contraction, and the carbon atati on 
of the oxides by expansion of the rock. The loss of the more 
soluble products of decomposition has also a marked contrac- 
tile effect, very earl} 7 in the rock's decay. These contractile 
and expansile tensions, occurring sometimes in different parts 
of the field, often successively in the same part, have given 
rise to many of the physical peculiarities of the rock. Among 
the effects of the contractile tensions there is a very common 
smooth, vertical, prismatic jointing, and the development 
of closely set horizontal joints, which somewhat simulate 
cleavage. It is these horizontal joint-cracks which become 
the seat of deposition of the iron from the percolating waters ; 
and thus the parallelism of the banding is explained. With 
the advance of decomposition, the horizontal cracks become 
more numerous, and the bands of iron which are formed along 
them often grow till they unite. It is to this cause that the 
beautifully bedded character of the ore-deposits must be 
chiefly ascribed. 
