AMERICA.
some remarkable contrasts to that of the districts
in the old world which have supplied the types of
geological classification. None of these is more
striking than the enormous extent of country which
one formation occupies, and that without interrup-
tion. It has long been noticed that the rock-struc-
ture of islands is more varied than that of continents ;
and thus it is that the inhabitants of the British
Isles have been to some extent compelled to become
acquainted with geology. A journey of a few hours
presents to the traveller rocks which, as regards
both their mineral and fossil contents, are widely
different. In A., on the other hand, one may travel
for days over beds belonging to a single epoch.
American strata often stretch from the Atlantic
west beyond the Mississippi. They have, on the
whole, been subjected to few disturbing agencies ;
as is evidenced by the absence of any true mountain-
range, except the Appalachians, east of the Rocky
Mountains. The rocks of Britain, from their dis-
position and variety, have been, so to speak, the
‘primer’ and ‘pocket manual’ of this science, and
will always continue to be the ¢ vade mecum’ of the
geologist; but should he desire to peruse the large
¢ folios’ that contain the stony records of our earth’s |
l
!
|
history, in their order and natural vastness, he must |
betake himself to the new world.
It is not many years since attention was first
directed to American geology, but during the short
time that has intervened, its progress has been very
remarkable. This has resulted from the appoint-
ment of a geological staff in connection with nearly
every province of the United States, from the
vigorous operations of the Canadian survey under
Sir W. E. Logan, and from the observations of
arctic explorers, whose frequent visits to these
regions in search of the ill-fated Franklin have sup-
plied data for the exposition of their natural history.
Humboldt, though the first, is yet the most import-
ant of South American observers. The numerous
facts recorded by him have been confirmed and
added to by recent travellers. Data have been thus
supplied to form an approximate estimate of the
geological structure of this portion of the American
continent.
The names of North American observers are
almost past reckoning, yet the various systems may
be said to have been chiefly laid open by four sets of
observers—Morton for the Cretaceous, Conrad for
the Tertiary, Hall and the New York geologists for
the Palzozoic, and the Professors Rogers for the
Carboniferous strata and the Appalachians.
In the following rapid sketch of this subject,
we can do nothing more than glance at the various
formations, and must refer for details to the articles
under the different divisions of A.
The oldest strata are a range of CRYSTALLINE
Rocks which, in North A., occupy an area that
extends from the northern shores of Lake Superior,
and the banks of the St Lawrence, north-west to the
Arctic Ocean, and lies between the line of minor
lakes (Slave, Winnipeg, &c.) and Hudson’s Bay. The
average width of this area is about 200 miles, and its
length from Lake Superior to its termination on the
shores of the Arctic Sea is more than 1500 miles. The
rocks are chiefly gneiss, with granite and trap. They
form a flat plateau, very little elevated above the
surrounding country, and only in the Copper Moun-
tains rising to the altitude of hills, the highest of
which is 800 feet above the sea-level. In this
immense plain we have an example of the great
characteristic of American geology—the tranquil
operation of an upheaving force, exerted over ‘a
wide area, with limited and regulated intensity, and
constancy of direction. This series of rocks stretches
over nearly the whole of the eastern portion of
202
South A., extending from the northern shores to
the mouth of the La Plata, being, however, hidden in
the valley of the Amazon by its alluvial deposits.
The same rocks form the western slopes of the
Andes and Rocky Mountains, and the plains of
Russian A. In the central district, in which we first
traced them, they dip east and west under the
Silurian strata. They are themselves free from
superincumbent beds, shewing that even in the
Silurian age they formed dry land; and ever since,
although subject, like the rest of the world, to great
oscillation, it has apparently held its place with
wonderful stability, for it is now, as probably then,
not far above the level of the sea.
On either side of this tract there exists, as we have
said, a SILURIAN district. That on the eastern side,
reaching to Hudson’s Bay, has a low and uniformly
swampy aspect ; the strata are hid by superficial
deposits, chiefly boulder clay or drift, large boulders
from which are scattered along the shore. The
Silurian rocks under which the crystalline strata dip
on their western limits, cover a large extent of the
North American continent. They have been traced
from Canada and New England, bounding the
southern limits of the azoic rocks along the line of
the great lakes, and extending in a broad band of
some 200 miles parallel to the more ancient forma-
tion, probably till they reach the Arctic Ocean.
These rocks are only slightly developed in Southern
A., on the eastern slopes of the Andes.
The Silurians have been divided into Lower and
Upper, and each of these contains three periods.
| Beginning with the Lower, we have first the
Potsdam Period, comprising beds of slate and
sandstone, and containing fossils representative of
the three great divisions of the animal kingdom—
Molluscs, Articulates, and Radiates. Next follows
the
Trenton Period, a period of limestones indicating a
sea of greater depth, and teeming with life, for some
beds are composed entirely of shells and corals.
Another change, and rocks of a clayey and shaly
structure are deposited, containing numberless
zoophytes and other fossils, and forming the Hudson
Period.
The Upper Silurian division also comprises three
epochs : T'he Medina and Clinton, composed of sand-
stones and shales ; then 7%he Niagara and Onondaga,
with limestones and saline rocks; and, lastly, 7%e
Lower Helderberg Period, a richly fossiliferous series
of limestone rocks.
The Silurian beds on their southern and western
borders dip under the DEVONIAN rocks, which are
developed to a large extent north of lat. 72° N.,
where they appear to rest upon the azoic rocks.
They have been divided into five periods : Oriskany,
Upper Helderberg, Hamilton, Chemung, and Catskill.
Vast beds of conglomerate overlie the Devonian
rocks, and form the basis of the CARBONIFEROUS
strata. This formation covers large districts in New
Jersey and Pennsylvania, and in the Ohio and
Mississippi valleys, with an enormous thickness of
limestone, shale, and other beds, which still con-
tinue parallel to the previous. At the close of the
carboniferous epoch, the whole character of North A.
was altered by the formation of its mountain systems.
No hill higher than Copper Mountain seems to have
existed at this time, although the land occupied
much the same area, and had a similar outline as at
present. The Professors Rogers, having with perfect
success unravelled the contortions of the Appa-
lachians, have shewn that the Silurian, Devonian,
and Carboniferous strata, which were originally laid
out in horizontal layers, were afterwards pressed on
to the north-westward, and folded up till the folds
were of mountain height. To similar causes do the
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