APPENDIX—GEOLOGICAL SURVEY. 
429 
like indications of purer beds beneath these surface a out-crop- 
. pings. This slaty variety decomposes readily into a reddish 
brown clay, in which we sometimes find beds of ochre varying 
from a few inches in thickness to several feet. In these beds 
of ochre we sometimes meet with separate and distinct colors 
of brown, reddish brown, bright yellow, yellowish brown, and 
sometimes layers of pure white clay that look like chalk. It 
is certainly very difficult to account for this variety of form 
and color, unless it be by a process of seggregation, and aggre¬ 
gation, set up by strong chemical action. These argillaceous 
or slaty beds are found mostly either at or very near wffiere 
the Potsdam sandstone below, and the St. Peter’s sandstone 
above, unite with the lower magnesian limestone. 
In the sandstone, both in the St, Peters and Potsdam, where 
it is free from foreign matter, (that is lime, clay and the like) 
this ore assumes other forms. Where there is a good expos¬ 
ure of this rock f and especially where it has been exposed to 
atmospheric agencies for a great length of time, it assumes a 
banded structure. These bands are not like seams of iron ore 
which we sometimes meet with, spread out between beds of sand 
as though they were the result of deposition from water, but they 
resemble more the banded structure of wood, shown in a trans¬ 
verse section of a tree. These bands are sometimes very much 
contorted, as though they were bent by heavy pressure, while 
the sandstone remains perfectly undisturbed. In the iron belts 
of Lake Superior a similar banded structure in the Potsdam 
sandstone was noticed by Foster and Whitney, and in their 
report on this peculiar feature of iron ore in sandstone there, 
they say, “We know of no theory which affords so probable 
an explanation of this structure as that by which the action of 
the seggregating forces is brought into play.” 
Another, and I think the most important form in which this 
oxide of iron is found along this belt, is that of a bright red 
powder. In this condition it is sometimes found in regular 
flat openings in the sand rock, mixed with a very pure, coarse 
grained sand, but oftener disseminated through the mass in the 
shape of a cement. In this condition it varies from a ferru¬ 
ginous or irony sandstone, to a sandy ochre. The richest de- 
