The fossiliferous rocks are divided into seven systems, which are
readily distinguished by the order of superposition, lithological
characters and organic remains. These systems are the Silurian, the Old
Red Sandstone, the Carboniferous, the New Red Sandstone, the Oölitic,
the Cretaceous, and the Tertiary systems. There is also an eighth system
now in process of formation.
It is the opinion of some geologists that there is another system
situated between the metamorphic rocks and the silurian system. It has
been called by Dr. Emmons, who has studied it with much care, the
"Taconic System," the Taconic Mountains, in the western part of
Massachusetts, being composed of these rocks. It is the lower part of
what has been called, in England and Wales, the _Cambrian system_.
The strata of this system have a nearly vertical position, and consist
principally of black, greenish and purple slates, of great thickness.
Granular quartz rock, however, occurs in considerable quantity, and in
this country two thick and important beds of limestone are found. These
limestones are occasionally white and crystalline. Generally, however,
as a mass, they are a dark, nearly black rock, with a network of lines
of a lighter color. All the clouded marbles for architectural and
ornamental purposes are from these beds, and our roofing and writing
slates are all obtained from the argillaceous portion of this system.
The number of species of organic remains contained in this system is
very small, and these, so far as discovered, belong to the annelida,
with a few doubtful cases of mollusca. This system of rocks is found
coming to the surface in a large part of New England, and the eastern
part of New York, also in the western part of England and Wales.
Those geologists who deny the existence of this system consider these
rocks as parts of the silurian system which have been most disturbed by
subterranean forces, and most altered by proximity to igneous rocks. The
annexed sketch (Fig. 7) will exhibit the relations here referred to.
Certain portions of the silurian rocks are supposed to have been thrown
into folds by the upheaval of the primary rocks. The plications nearest
to the intrusive granite would be most altered. That part of the figure
below the line _a a_ represents the outcropping edges as they now
appear, the upper portion of the folds having been removed by some
abrading cause.
[Illustration: Fig. 7.]
As it is yet uncertain which of these views is correct, convenience will
justify us in retaining the name of Cambrian system till further
investigations shall settle the question.
1. _The Silurian System._--The following tabular arrangement exhibits
the divisions of the system as recognized in England, in New York, in
Pennsylvania and Virginia, and in Ohio.
Key to Divisions
----------------
C - Cambrian Rocks.
S - Silurian System.
D - Devonian.
Ch - Champlain Division.
On - Ontario Division.
He - Helderberg Division.
Er - Erie Division.
Divisions as recognized Divisions as recognized Pennsylvania Ohio.
by English Authors. by the New York and Virginia.
Geologists.
/-------------------\ /-----------------------\ /----------\ /--------\
{ Upper Cambrian { { Potsdam Sandstone. } No. 1.
{ { {
C { Rocks, of Sedgwick{ { Calciferous Sandrock. }
{ { { Birdseye Limestone. } No. 2. }
{ (probably). {Ch{ Trenton Limestone. } } Blue
{ { } Limestone
{ Llandeilo Flags. { { Utica Slate. } } and Marl.
{ { { } No. 3. }
{ { Hudson River Group. }
{ {
{ { { Gray Sandstone. } No. 4.
{ { { Oneida Conglomerate. }
{ {On{
{ Caradoc Sandstone.{ { Medina Sandstone. } No. 5.
{ { { Clinton Group. }
{ { Niagara Group.
{ { } }
{ { { Oneida Salt Group. } :
{ { { Water-lime Group. } :
{ { { Pentamerus Limestone. } No. 6. :
{ { { Delthyris Shaly } :
{ { { Limestone. } :
S { { { Encrinal Limestone. } : Cliff
{ Wenlock Rocks. {He{ Upper Pentamerus } :
{ { { Limestone. : Limestone.
{ { { Oriskany Sandstone. } :
{ { { } No. 7. :
{ { { Cauda-Galli Grit. } :
{ { { Schoharie Grit. } :
{ { { Onondaga Limestone. Wanting. :
{ { { Corniferous Limestone. Wanting. }
{ }
{ { { Marcellus Shales. } } Black
{ { { Hamilton Group. } } Slate.
{ Upper and Lower { { } No. 8.
{ { { Tully Limestone. }
{ Ludlow Rocks and {Er{ }
: { { Genesee Slate.
D : the Devonian { { }
: { { Portage Group. } No. 9. } Waver
: System. { { } }
{ { {Chemung Group. } } Sandstone.
This name, _Silurian_, was first used to designate the lowest
well-characterized fossiliferous rocks in England. But it is now used to
embrace the whole system as it occurs elsewhere. It is well exhibited in
New York, both in consequence of its great development there, and
because the whole system is only slightly acted upon by disturbing
forces, so that the outcropping edge of each division extends over a
large surface.
This system is of great thickness, amounting, in places where it is well
developed, to twenty thousand feet.
The Champlain division commences with a quartzose sandstone, passing
gradually into limestone, which is succeeded by a very thick
argillaceous deposit, the Utica slate and Hudson River group. The
Ontario division in the lower part is a mass of sandstone. Above this is
the Clinton group, consisting of shales and sandstones. The most
important part of this group, in an economical point of view, is a
fossiliferous, argillaceous iron ore, coextensive with the group in this
country, and is worked to supply a large number of furnaces. The last of
the division is the Niagara group, which commences with a mass of shale,
and becoming at length calcareous, it terminates in a firm compact
limestone. This limestone has withstood the action of denuding causes
better than the shales either above or below it. It therefore presents a
bold escarpment at its outcrop, and occasions waterfalls wherever
streams of water cross it. The falls of Niagara are formed by this rock.
The Niagara limestone, in its extension westward, becomes the
lead-bearing rock of Missouri, Iowa and Wisconsin. The Helderberg
division is a succession of highly fossiliferous limestones, with the
intervention of only occasional beds of grits and shales. One member of
the series is the Onondaga Salt group. The water obtained from this
group in New York annually furnishes immense quantities of salt. The
Erie division consists of a thick mass of shales and sandstones.
[Illustration: Fig. 8.]
[Illustration: Fig. 9.]
[Illustration: Fig. 10.]
The fossils of this system are very numerous, but consist mostly of the
lower forms of animal life. _Corals_ (Figs. 8 and 9) are abundant, and
constitute in some places a large proportion of the limestones. The
_Crinoidea_, or lily-shaped animals, consist of a jointed stem
permanently attached, and bearing at the free extremity of the stem an
expanded portion, which is the pelvis, or digestive cavity. The mouth is
surrounded with a series of leaf-like tentacula, which serve the purpose
of seizing and holding food. Fig. 10 represents the pelvis of one of the
silurian fossils. The general character of the animal is better
represented by Fig. 30. The most abundant fossils of this period are the
lowest orders of _bivalve mollusca_ (Fig. 11). The _Cephalopoda_ are
characterized by having the organs of locomotion attached to the head.
The shell of several species is peculiar in being divided into distinct
cells, or chambers (Fig. 12, _b d_), perforated by a tube (siphuncle
_a_). These fossil shells are sometimes straight, as the _Orthoceras_
(Fig. 13), or curved, as shown in the several forms of Fig. 14. The
_Trilobite_ was an articulated, crustaceous animal, having two lines
along the back dividing it into three lobes, from which circumstance its
name is derived. It is found in great numbers in the Silurian rocks
(Fig. 15). In a few instances remains of fishes have been found, but
they by no means characterize the system.
[Illustration: Fig. 11.]
[Illustration: Fig. 12.]
[Illustration: Fig. 13.]
[Illustration: Fig. 14.]
[Illustration: Fig. 15.]
The geographical range of this system is probably greater than that of
any system of rocks above it. It is found occupying a large part of the
territory west of the Alleghany Mountains, from Canada, through New
York, and the other states, to Alabama; and extending westward to and
beyond the Mississippi river. It occupies a large district in the west
of England, and is found in great force in the north and east of Europe.
2. _The Old Red Sandstone._--This formation consists almost entirely of
a sandstone of a red color. It admits of division into three parts,
though the characters vary in different places. The lowest is a
thin-bedded argillaceous sandstone, consisting of finely levigated
material, and easily splitting into thin sheets. From this circumstance
it has received the name of _tilestone_. The middle portion is composed
of nodules or concretions of limestone imbedded in a paste of red sand
and shale. This has been called by English geologists, _cornstone_, and
though very partially developed in some regions where the system is
found, it is yet a very persistent member. The highest member of this
formation is a mass of red sandstone, often passing into a coarse
_conglomerate_. In England the thickness of the Old Red Sandstone is not
less than ten thousand feet. In this country it is scarcely three
thousand feet.
[Illustration: Fig. 16.]
The fossils of this system are a few shells, a small number of vegetable
species, and in particular localities the remains of fishes in great
abundance. The system is characterized principally by fossils of this
last kind. The fishes of this system have a cartilaginous skeleton, but
are covered with plates of bone, which were faced externally with
enamel. The jaws, which consisted of solid bone, were not covered with
integument. The exterior bony covering seems to have been the true
skeleton, as is, in part, the case with the tortoise. In some of the
fishes of this period there is a wing-like expansion on each side of the
neck, which has given them the name of _Pterychthis_ (Fig. 17). In
others, as the _Cephalaspis_, the plate of bone on the back is so large
as to cover nearly the whole body, and make it resemble a trilobite
(Fig. 18).
This system has an extensive geographical range. In England, it occupies
a band of several miles in width, extending from the Welsh border
northward through Scotland to the Orkney Islands. In this country, it
forms the Catskill Mountains, in New York, and extends south and west so
as to underlie the coal-fields of Pennsylvania and Virginia.
3. _The Carboniferous System._--This system consists of three parts,
distinguished by lithological and fossil characters.
The _carboniferous limestone_ is a dark-colored, compact limestone,
forming the base of the system, and reposing on the old red sandstone.
Its thickness is from six hundred to one thousand feet, often with
scarcely any intermixture of other rock; but it sometimes loses its
character of a limestone, and becomes a sandstone, or conglomerate. It
generally contains the ores of lead in considerable quantity, and from
this circumstance has been called _metalliferous limestone_. In England
it is the principal repository of these ores. In the Western States it
is the upper portion of the lead-bearing strata.
[Illustration: Fig. 17.]
The fossils are marine, and very numerous. Corals and crinoidea are very
abundant. The crinoidea, in some localities, form so large a part of the
rock as to have given to it the name of encrinal limestone. The
orthoceras and trilobite are found, but become extinct with this
formation. Several species of bivalves, such as Delthyris and Leptæna,
are also common.
Next above the limestone lies the sandstone, sometimes called _millstone
grit_. It is generally drab-colored, but occasionally red. Its thickness
is often equal to that of the limestone. Sometimes it is fine-grained
and compact; but generally it is coarse-grained, and often passes into a
conglomerate. It contains but few fossils, and those of vegetable
origin.
The highest part of the system is the _coal measures_. They consist of
beds of sandstone, limestone, shale, clay, ironstone and coal, occurring
without much uniformity in their order of superposition. The coal
measures have a thickness of about three thousand feet. The sandstones
and limestones are not distinguishable from the sandstones and
limestones in the lower part of the system. The ironstone either occurs
in concretionary nodules, often formed around some organic nucleus, or
it is an argillaceous ore, having a slaty structure. In either case, it
consists of subordinate beds in the shale. The coal consists of several
beds distributed through the measures. The beds vary in thickness from a
few lines or inches to several feet. In a few cases beds have been found
measuring fifty or sixty feet in thickness. The workable beds are
ordinarily from three to six feet thick.
[Illustration: Fig. 18.]
The carboniferous formation is very much disturbed by dikes, faults
(Fig. 18; see also Fig. 50), and other dislocations. The amount of
change of position in the strata, by faults, is very various; frequently
but a few feet. In one case in England there is a fault of nearly a
thousand feet. There is a case of dislocation in Belgium where the
strata are bent into the form of the letter Z, so that a perpendicular
shaft would cut through the same bed of coal several times.
The characters and order of superposition which have now been given may
be regarded as the general type of the carboniferous formation. There
are, however, several important modifications. 1. Beds of coal sometimes
alternate with beds of millstone grit. Thus, in Scotland and in the
north of England, this intermediate member of the system disappears, or,
rather, is incorporated with the coal measures. The same is true, to
considerable extent, in this country. 2. Sometimes the carboniferous
limestone also disappears as a distinct member of the system, partly by
becoming arenaceous, and partly by the intercalation of beds of coal. In
this last case, the whole formation from the old to the new red
sandstone becomes a series of coal measures. In this country the
carboniferous limestone is found very generally to underlie the coal
strata. 3. The fractures and faults, which were formerly supposed to be
characteristic of the coal formation, are seldom found in the great
coal-fields of this country, except in those of the anthracite coal of
Pennsylvania; and even there they are much less common than in the
coal-fields of Europe.
There are three principal varieties of coal, distinguished by the
different proportions of bitumen which they contain. The common
bituminous coal kindles readily, emits much smoke, and throws out so
much liquid bitumen that the whole soon cakes into a solid mass. It
contains about forty per cent, of bitumen. The second kind, or cannel
coal, contains twenty per cent., and inflames easily, but does not
agglutinate. The stone-coal, or anthracite, contains scarcely any
bitumen, ignites with difficulty, emits but little smoke, and produces a
very intense heat. The bituminous varieties are always found in the
least disturbed portions of the coal districts; and the anthracite is
found in the more broken and convulsed portions, where we may suppose
that the subterranean heat has been sufficient to drive off the volatile
bituminous part, and reduce it to the anthracite form. Hence the eastern
Pennsylvania coal-fields, which lie near the principal axes of elevation
of the Appalachian Mountains, furnish only anthracite; while the same
coal-seams, in their extension to the western part of the state, are
bituminous.
Where coal is quarried in large quantity, a shaft is sunk through the
overlying strata to the coal-beds, and the coal is raised to 'the
surface by steam power. After the coal has been quarried to some
distance from the shaft, pillars of unquarried coal are left to support
the overlying strata. Fatal accidents have sometimes occurred by the
giving away of these supports. Over a large part of the coal-fields of
the United States it has not yet become necessary to sink shafts. The
quarrying is commenced at the outcrop of the coal-bed; and, till the
cover becomes of considerable thickness, it has been found economical to
"strip" off the overlying rock, rather than to work a subterranean
gallery.
Brine-springs are often found in the coal measures of sufficient
strength to be used in the manufacture of salt. This is now done to
considerable extent in Ohio. In the valley of the Kenhawa river,
Kentucky, the rocks of which belong to the carboniferous system, the
brine is nearly saturated with salt; and in some of the borings they
have even discovered beds of rock-salt of great thickness and purity.
There is no other part of the geological series so obviously connected
with national prosperity as the coal formation. While a country is new,
the forests furnish an abundant supply of fuel; but in the course of a
few years these are consumed. This country will soon be principally
dependent upon its coal-mines for fuel, even for domestic purposes; and,
in carrying on the great branches of national industry, such as the
smelting and working of iron, and in the formation of steam for the
purposes of manufacture and transportation, we are already mainly
dependent upon mineral coal. A nation which does not possess an abundant
supply of this mineral, _or which does not use it_, cannot long maintain
a high degree of national prosperity.
In these inexhaustible masses of coal, accumulated ages before the
existence of the human race, is a most obvious prospective arrangement
for securing our happiness and improvement. And this arrangement
embraces not only the accumulation of a combustible material in such
abundance, but also its juxtaposition with an equally inexhaustible
accumulation of iron ore, and the limestone which is necessary as a flux
in the reduction of the ore. So bulky and heavy materials as coal and
iron ore could neither of them have been transported to any
considerable distance for the manufacture of iron; and without the
manufacture of iron on a large scale, the present operations in
manufactures and transportation could never have been entered upon. A
large proportion of the iron furnaces in this country, and nearly all of
them in Great Britain, employ mineral coal for fuel, and obtain their
ore from the beds contained in the coal measures.
The fossils of the coal measures are almost entirely of vegetable
origin, and are very abundant. They are seldom found in the coal-beds,
but in the strata of shale immediately above or below the solid coal.
[Illustration: Fig. 19.]
The _Stigmaria_ (Fig. 19) is found most abundantly, and in a large
proportion of cases to the exclusion of every other form, in the lower
shales. It consisted of a large dome-shaped mass, often three or four
feet in diameter, with trailing branches, or roots, spreading off
horizontally to a distance of twenty feet. In a few instances tree ferns
have been found, petrified in a horizontal position, and being
apparently a mere continuation of the stigmaria. Hence the stigmaria has
been supposed to be the base of the tall tree ferns, the leaves of which
so abound in the upper shales. If this is not the case, there are no
forms of the existing flora of the earth analogous to the stigmaria. It
is always found in connection with the coal-beds of the carboniferous
formation, and never with the coal-beds which sometimes occur in the
later formations.
[Illustration: Fig. 20.]
[Illustration: Fig. 21.]
The tree ferns (Fig. 20) attained a height of fifty or sixty feet, and a
diameter of four feet. They have received the name of _Sigillaria_ in
consequence of the seal-like impressions (Fig. 21) with which the
surface is covered, and which are the scars left where the fronds have
fallen off. These fronds (fern leaves) are the most abundant fossil of
the series. They are distinguished by some peculiarity in form, as the
Sphenopteris (wedge-shaped fern leaf), Pachypteris (thick fern leaf),
&c. (Figs. 22 and 23.)
[Illustration: Fig. 22.]
[Illustration: Fig. 23.]
[Illustration: Fig. 24.]
[Illustration: Fig. 25.]
There was another kind of Sigillaria (Fig. 24), in which the surface was
fluted, and the markings are superficial, and occur on the ridges. It
reached as great a size as the tree ferns, but to what general class of
plants it belonged is still doubtful.
The _Lepidodendron_ (scale-covered tree) (Fig. 25) is the fossil which
most nearly resembled in general appearance our present forest trees.
Specimens are found four feet in diameter and seventy feet in height. In
botanical characters it resembled, in some respects, the trailing
club-mosses, while in others it was very similar to the Norfolk Island
pine.
[Illustration: Fig. 26.]
The _Calamite_ (Fig. 26) was a plant resembling, in its jointed and
striated surface, the equisetum (rush), but was sometimes twelve inches
in diameter.
The carboniferous formation exists more or less abundantly in all the
great divisions of the earth. It occurs in nearly all of the countries
of Europe. The largest deposits known are, however, in the United
States; especially in the States of Pennsylvania and Virginia, and in
Ohio.
4. _The New Red Sandstone._--The lower division of this formation,
called the Permian system, consists of a thick mass of sandstones,
generally of a red color, with occasional alternations of argillaceous
rock, succeeded by a series of magnesian limestones. The upper division,
or Triassic system, is composed of a red conglomerate, a limestone which
has received the name of Muschelkalk (shelly limestone), and a series of
variegated marls and sandstones.
The ores of copper are found, to considerable extent, in this formation.
The rich copper mines of Germany are in the magnesian limestone, or, as
it is there called, Zechstein (minestone). The Lake Superior copper
mines occur in a red sandstone formation, which will probably be found
to belong to this system.
The salt-beds, salt springs, and beds of gypsum, are so, generally found
in this rock in England, that it has been called by the English
geologists the "saliferous system." It is, however, found that in other
countries these minerals occur in equal abundance in formations of an
earlier and later date.
[Illustration: Fig. 27.]
The fossils of this system are not abundant. In the Permian portion,
impressions of fishes are found, always with the peculiarity that the
tail is _heterocercal_ (Fig. 27); that is, with the spine continued into
the upper lobe. The same peculiarity prevails in the carboniferous and
all the earlier formations. Fishes with the tail _homocercal_ begin to
appear in the Triassic portion of this system, and are found in all the
subsequent formations. The remains of saurians also occur in this
formation.
[Illustration: Fig. 28.]
The red sandstones seem to have been better adapted to retain the forms
which were impressed upon them than to preserve the organic remains
which were deposited in them. Hence, while they contain but few fossils,
the strata are often covered with ripple marks, with sun cracks,
occasioned by contraction while drying, or with depressions produced by
rain-drops, and the pits are sometimes so perfect as to show the
direction of the wind when the drops fell. (Fig. 28.) The tracks of
animals are also well preserved. Some of them were produced by reptiles
(Fig. 29, _c_), and some probably by marsupial animals, but most of them
by birds (_a_, _b_). President Hitchcock has distinguished the tracks of
more than thirty species in the sandstones of the Connecticut valley.
Birds, reptiles and marsupial animals, seem to have been first
introduced during this period.
[Illustration: Fig. 29.]
The new red sandstone is well developed in all its members on the
continent of Europe. In England, all the members are present, except the
Muschelkalk. The Triassic portion of it occurs in North America. It is
found in detached portions, probably as parts of a continuous formation,
in Nova Scotia, the eastern part of Maine, the Connecticut valley, and
from New Jersey southward through Pennsylvania, Maryland, &c., to South
Carolina.
5. _The Oölitic System._--The lower portion of this system is the Lias,
and consists of a series of fissile, argillaceous limestone, marl, and
clays. The _Oölite_ forms the intermediate member of the system, and
consists of alternations of clay, arenaceous rock and limestone. Some of
the limestones have an oölitic structure, and the whole system takes its
name from this circumstance, though this structure is not found in all
parts of it, and is often found in other formations. The central part of
the oölite, the coral rag, is principally a mass of corals and
comminuted shells. The _Wealden_, the highest member of the oölitic
system, is an estuary deposit, consisting of calcareous beds, followed
by sandstone, and terminated by the Wealden clay.
This system is throughout highly calcareous, and furnishes, wherever it
is developed, valuable materials for architectural and ornamental
purposes.
This system is distinguished for the great amount and variety of its
organic remains. The _vegetable productions_ were intermediate between
those of the coal period and those of the present time. The upper
oölite, in the south of England, contains the stumps of trees and other
plants, rooted in a black carbonaceous layer, evidently the soil from
which they grew. These stumps and prostrate trunks are the remains of
coniferous trees of large growth. (Fig. 30.)
Corals occur in great abundance; also encrinites (Fig. 31), mollusks
(Fig. 32), and cephalopoda.
[Illustration: Fig. 30.]
[Illustration: Fig. 31.]
[Illustration: Fig. 32.]
But this system is specially characterized by the remains of saurian
reptiles. The Ichthyosaurus (Fig. 33, _a_) was a marine animal, having
the general form of a fish, while its head, and especially its teeth,
resemble those of the crocodile. It was an air-breathing animal like
the cetacea, and was furnished with similar paddles. It was carnivorous,
and was undoubtedly the largest and most formidable animal existing in
the earlier part of the oölitic period. Its length could not have been
less than thirty or forty feet.
[Illustration: Fig. 33.]
The _Plesiosaurus_ (Fig. 33, _b_) was also a marine animal, and in ninny
respects similar to the Ichthyosaurus; but its general form was more
slender, its head was small, and its neck was of great length, the
cervical vertebræ exceeding in number those of the swan.
[Illustration: Fig. 34.]
The _Pterodactyle_ (Fig. 34) was a small saurian, of the size, probably,
of our largest eagle. The finger-bones, which in the other saurians form
the paddles, are in the Pterodactyle very much lengthened, so as to
support a membranous expansion, like that of the bat. These wings were
of sufficient size to enable it to sustain itself in the air, and to
make a rapid and easy flight.
The _Iguanodon_ is a Wealden fossil, remarkable for its great magnitude.
It is estimated that its length was seventy feet. It was a lizard,
adapted for motion on land, and was herbivorous.
This formation is well developed in England, and, with the exception of
the Wealden, on the continent of Europe. It has been supposed that no
part of the oölitic series was to be found in this country; but there is
a highly arenaceous rock occupying the valley of the James river, in the
vicinity of Richmond, Virginia, of considerable extent, and a thousand
feet in thickness, containing a bed of coal of forty feet in thickness,
which, from its fossils, must be referred to the oölitic series.
6. _The Cretaceous Formation._--The lower part of this formation
consists of _greensand_, interstratified with beds of clay. The
intermediate portion is a mixture of argillaceous greensand and _impure
chalk_. The upper part is composed of _chalk_, which is a friable,
nearly pure carbonate of lime. The strata of chalk are separated, at
intervals of from three to six feet, by layers of flint, either in the
form of nodules or of continuous strata.
These characters, by which the cretaceous system is known in England,
are but partially recognized elsewhere. Thus, in the Alps, the
"Neocomian System," consisting of crystalline limestones, is the
equivalent of the English greensand; while the greensand of this country
is the equivalent of the white chalk of England.
[Illustration: Fig. 35.]
[Illustration: Fig. 36.]
The fossils of the cretaceous formation are very different from those of
the oölite, and are such as to show that it was deposited in deep seas.
_Microscopic shells_ are often so abundant as to constitute a large
proportion of the mass. _Zoöphytes_ are very numerous, such as sponges,
corals, star-fishes (Fig. 35, _d e_), and a few crinoidea (_b_).
Mollusks were also abundant, and cephalopoda, consisting of
chamber-shells and belemnites (Fig. 36). The belemnite probably
resembled the existing cuttle-fish; but the remains consist, in most
cases, of a partially hollow calcareous substance (_b_), which was
contained within the animal, and formed its skeleton.
The chalk and greensand are largely developed in England; and the same
formation, with different lithological characters, is found in great
force flanking the principal mountain ranges of southern Europe, and
extending into Asia. In this country the system commences with the
greensand and friable limestones of New Jersey, and following the
Alleghany range to its southern termination, it bends around into a
north-western direction, and is continued into Missouri.
7. _The Tertiary System._--The tertiary strata embrace the formations
from the cretaceous to the human era. They consist of clay, sand,
sandstone, marl and limestone, and are distinguished from the lower
rocks by being less consolidated; though the limestones are in some
instances solidified, and resemble the strata of earlier origin. The
tertiary strata are generally of less thickness than the older
formations, and less continuous, being local deposits formed in lakes
and estuaries. In a few instances they have been thrown into inclined
positions, though in most cases they have been but slightly disturbed,
and raised but a few hundred feet above the present level of the sea.
The late tertiary strata seldom overlap the older, so as to indicate
their relative ages by superposition. They have therefore been separated
into groups according to the proportions of living and extinct species
of shells which they are found to contain. The oldest tertiary or
_Eocene formation_[A] contains only four per cent, of living species,
the _Miocene_ contains seventeen per cent., the Pleiocene forty per
cent., and the _Pleistocene_ ninety per cent.
[A] _Eös_, dawn, and _kainos_, recent. The formation
which commenced at _the dawn of the recent period_,
containing but a small number of living species.
Miocene (_meion_, less), less recent than the Pleiocene
(_pleion_, more). Pleistocene (_pleistos_, most), most
recent.
During the pleistocene period, peculiar conditions existed, by which a
great amount of loose material, known by the name of _drift_, was spread
over the northern portions of both hemispheres. In America it is found
from Nova Scotia nearly to the Rocky Mountains, and extending as far
south as Pennsylvania and the Ohio river. In Europe, it is found from
the Atlantic to the Ural Mountains, and reaching south into Germany and
Poland. It is also found in the colder portions of South America, and in
the vicinity of several mountains, as the Alps.
It consists of irregular accumulations of earthy substances of different
degrees of fineness, but characterized by containing masses of rock of
considerable size, often of many tons weight, called boulders. Rocks
having the same lithological characters exist in situ north of where the
boulders and other drift are now found, though at a distance often of
one or two hundred miles. There can be no doubt but that the drift has
been transported from these northern localities; and the polished,
striated and grooved condition of the rocky surface, wherever the drift
is distributed, has obviously been produced by the passage of the drift
materials over it.
Towards the close of this period, while the land was a few hundred feet
below its present level, there were deposited in the valleys of the
drift region beds of blue and gray clay, materials which are used in
making bricks and coarse pottery; also beds of sand, sometimes evenly
spread out, but often thrown into irregular mounds and ridges.
In regions which are not covered with drift,--as the south of Europe and
the United States,--the pleistocene deposits are succeeded, without
apparent change of conditions, by those which are now taking place.
The formations of the tertiary period are distinguished from those of
the cretaceous period by the absence of deep-sea fossils, and from the
oölite by the absence of its characteristic saurians. The mollusks are
also very different, such genera as the cerethium (Fig. 37), murex (Fig.
38), and conus (Fig. 39), which abound in the present seas, first
appearing in the tertiary period. The nummulite (Fig. 40), a peculiar
form of chambered shell, is so abundant as to constitute in some places
almost the entire rock.
The period is however characterized by the existence of a large number
of pachydermatous animals, of which the tapir, hog, horse and elephant,
are examples of living species.
[Illustration: Fig. 37.]
[Illustration: Fig. 38.]
[Illustration: Fig. 39.]
[Illustration: Fig. 40.]
The _Paleotherium_ (Fig. 41) resembled, in most respects, the tapir. It
was furnished with a short proboscis, and the foot was divided into
three toes. The length of the largest species was about that of the
horse; but its body was larger, and it was of less height.
[Illustration: Fig. 41.]
[Illustration: Fig. 42.]
The _Anoplotherium_ (Fig. 42) was a more slender animal, and resembled
in size and general form the gazelle.
The _Megatherium_, an animal of the late tertiary epoch, was larger than
the existing species of elephant, and in its general structure and
habits resembled the sloth.
The _Mastodon_ (Fig. 43) lived during the latest portion of the tertiary
epoch. Its remains are found most abundantly where the animal seems to
have perished by sinking into the soft marshy ground near the brackish
springs of New York and Kentucky. But they are found also in Europe and
Asia. It was larger than any existing land animal, and was nearly allied
in structure and habits to the elephant.
[Illustration: Fig. 43.]
The _Mammoth_ was a species of elephant, now extinct, of which remains
are found with those of the mastodon, but in the greatest abundance in
Europe and Asia. A large number of skeletons, many of them imperfect,
have been discovered in the low grounds in the south-east of England. It
was this animal which was found encased in ice and sand in Siberia, in
1804.
Contemporaneously with the existence of these huge animals, a near
approach was made to the present fauna of the earth, by the introduction
of ruminant animals resembling the ox and deer, and especially by the
existence of the class of animals which in anatomical characters stands
next to man, the apes and monkeys.
The tertiary system, though not generally so continuous over extended
areas as the older formations, yet constitutes the surface of a very
large part of Europe. (See Fig. 59.) In the United States the earlier
portion is found along the seaboard, from New Jersey to Louisiana, and
extending back towards the mountains to a distance varying from ten to
one hundred miles. The later deposits are found in detached portions
throughout the Eastern and Middle States. It covers a large surface in
South America, and is found in India.
8. _The Recent Formation._--It is intended to embrace in this term
strata which have been formed since the creation of man. It is, however,
impossible to separate them by any well-defined characters from those of
the tertiary period. The recent formation consists of land which is
forming by the filling up of lakes, and by the increase of deltas from
the accumulated sediment which rivers have furnished.
There is, however, no doubt but that formations on a large scale have
continued in progress over extensive areas of the bed of the sea; and
they have been no less rapid, we may presume, than they were in earlier
periods. But, though they are preserving the records of the present era,
they will probably remain in a great measure inaccessible for many ages.
These deposits, so far as they are accessible, are found to contain the
remains of plants and animals (including man) now living in the vicinity
where the deposits are forming.