84                    GENERAL GEOLOGY.
series now measures nearly 8,000 feet in thickness, making the entire
group of the region embrace, as above stated, about 13,000 feet of
strata.
It will be observed, in glancing over the whole, that the great
mass of the series was formed from the ordinary sediments arising
from rock disintegration, and that they were unquestionably derived
from the adjacent Laurentian land. The exceptions to this state-
ment are found, (1) in the limestone, probably derived from the
remains of marine life, (2) in the iron ores, a portion, at least, of
which probably arose through organic action, and (3) possibly the
diorites, which may have had an igneous origin.
IfHronian of the ilffenominee Region. Waiving, for the present
the subject of the tilting and transformation of these beds, let us
pass to a comparison with the deposits that took place in the Menom-
inee region, where the accumulations gathered in what seems to
have been a strait, or bay, even more protected from violent action
than the Penokee region, as will be observed by glancing at the
map. The deposits here, while corresponding in general with those
of the Penokee region, differ from them in precisely those respects
which might be anticipated in view of the above conditions.
There were (1) larger ingredients of fine detrital material, as indi-
cated by the great clay beds, slates, and fine textured schists, (2) a
larger proportion of calcareous and magnesian deposits, (3) a larger
proportion also of iron ore, so far as present developments show,'
and (4) more particularly, a vast increase of carbonaceous material.
The Iron Ores. The iron ores are here associated with clay-slates
and carbonaceous schists, as well as with the quartzite, and exist in
beds of great thickness and richness. Industrially considered, they
constitute much the most important element of the formation, and
have recently given rise to a settlement and development of the
district quite extraordinary. A region that five years ago was an
almost unbroken wilderness is now dotted with thriving towns, and
resounds with the scarcely interrupted rumble of passing trains.
Ore shipments that began in 1877 reached in 1880, 592,288 tons, or
nearly half as much as the famous Marquette district itself.2
The ore occurs in definitely stratified beds, which show a pro-
1 The supposed equivalents of the higher iron-bearing horizons of the Mar-
quette and Menominee districts are concealed in the Penokee region, and have
not yet been uncovered, and, therefore, the limitation of this statement
to present
development is not to be overlooked.
2 Report of the Commissioner of Mineral Statistics for Michigan (C. E. Wright).
This statement embraces the production of the Menominee range in Michigan
as well as in Wisconsin.