I 
APPENDIX-GEOLOGICAL SURVEY. 433 
were composed, for the most part, of finely comminuted materials, princi¬ 
pally silicious and argillaceous, in some cases consisting of almost pure 
silex, like the purest portion of the Potsdam sandstone which was after¬ 
wards deposited upon these strata. 
“ During the deposition of these strata, at various intervals sheets of 
plastic mineral matter were poured forth from below, and spread out upon 
the surface of the pre-existing strata. These igneous rocks are exceedingly 
compact and uniform in their texture, which would seem to indicate that 
they were under heavy pressure, probably at the bottom of a deep ocean. 
The same depth of water is also inferred from the comparative absence of 
ripple-marked surfaces throughout'the whole series. During this period, 
the interior of the earth was the source of constant emanations of iron, 
which appeared at the surface in the form of a plastic mass in combination 
with oxygen, or rose in metallic vapor or as a sublimate, perhaps as a 
chloride; in the one case it covered over the surface like a lava sheet; in 
^the other it was absorbed into the adjacent rocks.” 
In the closing remarks of the same report on the origin of 
the iron deposits in the azoic rocks of Lake Superior, we have 
the following conclusions: 
“ On the whole, we are disposed to regard the specular and magnetic 
oxides of iron as a purely igneous product, in some instances poured out, 
but in others sublimed from the interior of the earth. The supposition en¬ 
tertained by some that they may be a secondary product, resulting from 
decomposition of the pyritous ores, or from the metamorphism of bog-iron, 
is inadequate to account for the accumulation of such mountain masses, or 
to explain their relation to the associated rocks. 
“ Where these ores occur in a state of almost absolute purity, in the form 
of vast, irregular masses occupying pre-existing depressions; or, where the 
incumbent strata are metamorphosed and folded over them; or, where they 
are traversed by long lines of ferruginous matter in the form of dykes, there 
can be little doubt that these ores have risen up, in a plastic state from 
below. 
“ Where they are found impregnating metamorphic products, such as 
jasper, hornstone or chert, quartz, chlorite and talcos slate, not only inter¬ 
posed between the laminae, but intimately incorporated with the mass, giv¬ 
ing it a banded structure, we are disposed to regard it as the result of sub¬ 
limation from the interior. 
“ Where they are included in the metamorphic strata in the form of beds 
of variable w’idths, with a conformable range and dip, and with minute par¬ 
ticles of the associated rock mechanically mixed with the ore, we are dis¬ 
posed to regard them as the result of aqueous deposition, although the ma¬ 
terials may have been derived from the ruins of purely igneous products.’’ 
Part II. Pages 68 and 69. 
28—Ag. Te. 
