Books: The Elements of Geology
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William Harmon Norton >> The Elements of Geology
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VERTEBRATES. The most important record of the Ordovician is that
of the appearance of a new and higher type, with possibilities of
development lying hidden in its structure that the mollusk and the
insect could never hope to reach. Scales and plates of minute
fishes found in the Ordovician rocks near Canon City, Colorado,
show that the humblest of the vertebrates had already made its
appearance. But it is probable that vertebrates had been on the
earth for ages before this in lowly types, which, being destitute
of hard parts, would leave no record.
THE SILURIAN
The narrowing of the seas and the emergence of the lands which
characterized the closing epoch of the Ordovician in eastern North
America continue into the succeeding period of the Silurian. New
species appear and many old species now become extinct.
THE APPALACHIAN REGION. Where the Silurian system is most fully
developed, from New York southward along the Appalachian
Mountains, it comprises four series:
4 Salina . . . shales, impure limestones, gypsum, salt
3 Niagara . . . chiefly limestones
2 Clinton . . . sandstones, shales, with some limestones
1 Medina . . . conglomerates, sandstones
The rocks of these series are shallow-water deposits and reach the
total thickness of some five thousand feet. Evidently they were
laid over an area which was on the whole gradually subsiding,
although with various gentle oscillations which are recorded in
the different formations. The coarse sands of the heavy Medina
formations record a period of uplift of the oldland of Appalachia,
when erosion went on rapidly and coarse waste in abundance was
brought down from the hills by swift streams and spread by the
waves in wide, sandy flats. As the lands were worn lower the waste
became finer, and during an epoch of transition--the Clinton--
there were deposited various formations of sandstones, shales, and
limestones. The Niagara limestones testify to a long epoch of
repose, when low-lying lands sent little waste down to the sea.
The gypsum and salt deposits of the Salina show that toward the
close of the Silurian period a slight oscillation brought the sea
floor nearer to the surface, and at the north cut off extensive
tracts from the interior sea. In these wide lagoons, which now and
then regained access to the open sea and obtained new supplies of
salt water, beds of salt and gypsum were deposited as the briny
waters became concentrated by evaporation under a desert climate.
Along with these beds there were also laid shales and impure
limestones.
In New York the "salt pans" of the Salina extended over an area
one hundred and fifty miles long from east to west and sixty miles
wide, and similar salt marshes occurred as far west as Cleveland,
Ohio, and Goderich on Lake Huron. At Ithaca, New York, the series
is fifteen hundred feet thick, and is buried beneath an equal
thickness of later strata. It includes two hundred and fifty feet
of solid salt, in several distinct beds, each sealed within the
shales of the series.
Would you expect to find ancient beds of rock salt inclosed in
beds of pervious sandstone?
The salt beds of the Salina are of great value. They are reached
by well borings, and their brines are evaporated by solar heat and
by boiling. The rock salt is also mined from deep shafts.
Similar deposits of salt, formed under like conditions, occur in
the rocks of later systems down to the present. The salt beds of
Texas are Permian, those of Kansas are Permian, and those of
Louisiana are Tertiary.
THE MISSISSIPPI VALLEY. The heavy near-shore formations of the
Silurian in the Appalachian region thin out toward the west. The
Medina and the Clinton sandstones are not found west of Ohio,
where the first passes into a shale and the second into a
limestone. The Niagara limestone, however, spreads from the Hudson
River to beyond the Mississippi, a distance of more than a
thousand miles. During the Silurian period the Mississippi valley
region was covered with a quiet, shallow, limestone-making sea,
which received little waste from the low lands which bordered it.
The probable distribution of land and sea in eastern North America
and western Europe is shown in Figure 287. The fauna of the
interior region and of eastern Canada are closely allied with that
of western Europe, and several species are identical. We can
hardly account for this except by a shallow-water connection
between the two ancient epicontinental seas. It was perhaps along
the coastal shelves of a northern land connecting America and
Europe by way of Greenland and Iceland that the migration took
place, so that the same species came to live in Iowa and in
Sweden.
THE WESTERN UNITED STATES. So little is found of the rocks of the
system west of the Missouri River that it is quite probable that
the western part of the United States had for the most part
emerged from the sea at the close of the Ordovician and remained
land during the Silurian. At the same time the western land was
perhaps connected with the eastern land of Appalachia across
Arkansas and Mississippi; for toward the south the Silurian
sediments indicate an approach to shore.
LIFE OF THE SILURIAN
In this brief sketch it is quite impossible to relate the many
changes of species and genera during the Silurian.
CORALS. Some of the more common types are familiarly known as cup
corals, honeycomb corals, and chain corals. In the CUP CORALS the
most important feature is the development of radiating vertical
partitions, or SEPTA, in the cell of the polyp. Some of the cup
corals grew in hemispherical colonies (Fig. 288), while many were
separate individuals (Fig. 289), building a single conical, or
horn-shaped cell, which sometimes reached the extreme size of a
foot in length and two or three inches in diameter.
HONEYCOMB CORALS consist of masses of small, close-set prismatic
cells, each crossed by horizontal partitions, or TABULAE, while
the septa are rudimentary, being represented by faintly projecting
ridges or rows of spines.
CHAIN CORALS are also marked by tabulae. Their cells form
elliptical tubes, touching each other at the edges, and appearing
in cross section like the links of a chain. They became extinct at
the end of the Silurian.
The corals of the SYRINGOPORA family are similar in structure to
chain corals, but the tubular columns are connected only in
places.
To the echinoderms there is now added the BLASTOID (bud-shaped).
The blastoid is stemmed and armless, and its globular "head" or
"calyx," with its five petal-like divisions, resembles a flower
bud. The blastoids became more abundant in the Devonian,
culminated in the Carboniferous, and disappeared at the end of the
Paleozoic.
The great eurypterids--some of which were five or six feet in
length--and the cephalopods were still masters of the seas. Fishes
were as yet few and small; trilobites and graptolites had now
passed their prime and had diminished greatly in numbers.
Scorpions are found in this period both in Europe and in America.
The limestone-making seas of the Silurian swarmed with corals,
crinoids, and brachiopods.
With the end of the Silurian period the AGE OF INVERTEBRATES comes
to a close, giving place to the Devonian, the AGE OF FISHES.
CHAPTER XVIII
THE DEVONIAN
In America the Silurian is not separated from the Devonian by any
mountain-making deformation or continental uplift. The one period
passed quietly into the other. Their conformable systems are so
closely related, and the change in their faunas is so gradual,
that geologists are not agreed as to the precise horizon which
divides them.
SUBDIVISIONS AND PHYSICAL GEOGRAPHY. The Devonian is represented
in New York and southward by the following five series. We add the
rocks of which they are chiefly composed.
5 Chemung . . . . . . sandstones and sandy shales
4 Hamilton . . . . . . shales and sandstones
3 Corniferous . . . . . . limestones
2 Oriskany . . . . . . sandstones
1 Helderberg . . . . . . limestones
The Helderberg is a transition epoch referred by some geologists
to the Silurian. The thin sandstones of the Oriskany mark an epoch
when waves worked over the deposits of former coastal plains. The
limestones of the Corniferous testify to a warm and clear wide sea
which extended from the Hudson to beyond the Mississippi. Corals
throve luxuriantly, and their remains, with those of mollusks and
other lime-secreting animals, built up great beds of limestone.
The bordering continents, as during the later Silurian, must now
have been monotonous lowlands which sent down little of even the
finest waste to the sea.
In the Hamilton the clear seas of the previous epoch became
clouded with mud. The immense deposits of coarse sandstones and
sandy shales of the Chemung, which are found off what was at the
time the west coast of Appalachia, prove an uplift of that ancient
continent.
The Chemung series extends from the Catskill Mountains to
northeastern Ohio and south to northeastern Tennessee, covering an
area of not less than a hundred thousand square miles. In eastern
New York it attains three thousand feet in thickness; in
Pennsylvania it reaches the enormous thickness of two miles; but
it rapidly thins to the west. Everywhere the Chemung is made of
thin beds of rapidly alternating coarse and fine sands and clays,
with an occasional pebble layer, and hence is a shallow-water
deposit. The fine material has not been thoroughly winnowed from
the coarse by the long action of strong waves and tides. The sands
and clays have undergone little more sorting than is done by
rivers. We must regard the Chemung sandstones as deposits made at
the mouths of swift, turbid rivers in such great amount that they
could be little sorted and distributed by waves.
Over considerable areas the Chemung sandstones bear little or no
trace of the action of the sea. The Catskill Mountains, for
example, have as their summit layers some three thousand feet of
coarse red sandstones of this series, whose structure is that of
river deposits, and whose few fossils are chiefly of fresh-water
types. The Chemung is therefore composed of delta deposits, more
or less worked over by the sea. The bulk of the Chemung equals
that of the Sierra Nevada Mountains. To furnish this immense
volume of sediment a great mountain range, or highland, must have
been upheaved where the Appalachian lowland long had been. To what
height the Devonian mountains of Appalachia attained cannot be
told from the volume of the sediments wasted from them, for they
may have risen but little faster than they were worn down by
denudation. We may infer from the character of the waste which
they furnished to the Chemung shores that they did not reach an
Alpine height. The grains of the Chemung sandstones are not those
which would result from mechanical disintegration, as by frost on
high mountain peaks, but are rather those which would be left from
the long chemical decay of siliceous crystalline rocks; for the
more soluble minerals are largely wanting. The red color of much
of the deposits points to the same conclusion. Red residual clays
accumulated on the mountain sides and upland summits, and were
washed as ocherous silt to mingle with the delta sands. The iron-
bearing igneous rocks of the oldland also contributed by their
decay iron in solution to the rivers, to be deposited in films of
iron oxide about the quartz grains of the Chemung sandstones,
giving them their reddish tints.
LIFE OF THE DEVONIAN
PLANTS. The lands were probably clad with verdure during Silurian
times, if not still earlier; for some rare remains of ferns and
other lowly types of vegetation have been found in the strata of
that system. But it is in the Devonian that we discover for the
first time the remains of extensive and luxuriant forests. This
rich flora reached its climax in the Carboniferous, and it will be
more convenient to describe its varied types in the next chapter.
RHIZOCARPS. In the shales of the Devonian are found microscopic
spores of rhizocarps in such countless numbers that their weight
must be reckoned in hundreds of millions of tons. It would seem
that these aquatic plants culminated in this period, and in widely
distant portions of the earth swampy flats and shallow lagoons
were filled with vegetation of this humble type, either growing
from the bottom or floating free upon the surface. It is to the
resinous spores of the rhizocarps that the petroleum and natural
gas from Devonian rocks are largely due. The decomposition of the
spores has made the shales highly bituminous, and the oil and gas
have accumulated in the reservoirs of overlying porous sandstones.
INVERTEBRATES. We must pass over the ever-changing groups of the
invertebrates with the briefest notice. Chain corals became
extinct at the close of the Silurian, but other corals were
extremely common in the Devonian seas. At many places corals
formed thin reefs, as at Louisville, Kentucky, where the hardness
of the reef rock is one of the causes of the Falls of the Ohio.
Sponges, echinoderms, brachiopods, and mollusks were abundant. The
cephalopods take a new departure. So far in all their various
forms, whether straight, as the Orthoceras, or curved, or close-
coiled as in the nautilus, the septum, or partition dividing the
chambers, met the inner shell along a simple line, like that of
the rim of a saucer. There now begins a growth of the septum by
which its edges become sharply corrugated, and the suture, or line
of juncture of the septum and the shell, is thus angled. The group
in which this growth of the septum takes place is called the
GONIATITE (Greek GONIA, angle).
VERTEBRATES. It is with the greatest interest that we turn now to
study the backboned animals of the Devonian; for they are believed
to be the ancestors of the hosts of vertebrates which have since
dominated the earth. Their rudimentary structures foreshadowed
what their descendants were to be, and give some clue to the
earliest vertebrates from which they sprang. Like those whose
remains are found in the lower Paleozoic systems, all of these
Devonian vertebrates were aquatic and go under the general
designation of fishes.
The lowest in grade and nearest, perhaps, to the ancestral type of
vertebrates, was the problematic creature, an inch or so long, of
Figure 297. Note the circular mouth not supplied with jaws, the
lack of paired fins, and the symmetric tail fin, with the column
of cartilaginous, ringlike vertebrae running through it to the
end. The animal is probably to be placed with the jawless lampreys
and hags,--a group too low to be included among true fishes.
OSTRACODERMS. This archaic group, long since extinct, is also too
lowly to rank among the true fishes, for its members have neither
jaws nor paired fins. These small, fishlike forms were cased in
front with bony plates developed in the skin and covered in the
rear with scales. The vertebrae were not ossified, for no trace of
them has been found.
DEVONIAN FISHES. The TRUE FISHES of the Devonian can best be
understood by reference to their descendants now living. Modern
fishes are divided into several groups: SHARKS and their allies;
DIPNOANS; GANOIDS, such as the sturgeon and gar; and TELEOSTS,--
most common fishes, such as the perch and cod.
SHARKS. Of all groups of living fishes the sharks are the oldest
and still retain most fully the embryonic characters of their
Paleozoic ancestors. Such characters are the cartilaginous
skeleton, and the separate gill slits with which the throat wall
is pierced and which are arranged in line like the gill openings
of the lamprey. The sharks of the Silurian and Devonian are known
to us chiefly by their teeth and fin spines, for they were
unprotected by scales or plates, and were devoid of a bony
skeleton. Figure 299 is a restoration of an archaic shark from a
somewhat higher horizon. Note the seven gill slits and the
lappetlike paired fins. These fins seem to be remnants of the
continuous fold of skin which, as embryology teaches, passed from
fore to aft down each side of the primitive vertebrate.
Devonian sharks were comparatively small. They had not evolved
into the ferocious monsters which were later to be masters of the
seas.
DIPNOANS, OR LUNG FISHES. These are represented to-day by a
few peculiar fishes and are distinguished by some high structures
which ally them with amphibians. An air sac with cellular spaces
is connected with the gullet and serves as a rudimentary lung. It
corresponds with the swim bladder of most modern fishes, and
appears to have had a common origin with it. We may conceive that
the primordial fishes not only had gills used in breathing air
dissolved in water, but also developed a saclike pouch off the
gullet. This sac evolved along two distinct lines. On the line of
the ancestry of most modern fishes its duct was closed and it
became the swim bladder used in flotation and balancing. On
another line of descent it was left open, air was swallowed into
it, and it developed into the rudimentary lung of the dipnoans and
into the more perfect lungs of the amphibians and other air-
breathing vertebrates.
One of the ancient dipnoans is illustrated in Figure 300. Some of
the members of this order were, like the ostracoderms, cased in
armor, but their higher rank is shown by their powerful jaws and
by other structures. Some of these armored fishes reached twenty-
five feet in length and six feet across the head. They were the
tyrants of the Devonian seas.
GANOIDS. These take their name from their enameled plates or
scales of bone. The few genera now surviving are the descendants
of the tribes which swarmed in the Devonian seas. A restoration of
one of a leading order, the FRINGE-FINNED ganoids, is given in
Figure 301. The side fins, which correspond to the limbs of the
higher vertebrates, are quite unlike those of most modern fishes.
Their rays, instead of radiating from a common base, fringe a
central lobe which contains a cartilaginous axis. The teeth of the
Devonian ganoids show a complicated folded structure.
GENERAL CHARACTERISTICS OF DEVONIAN FISHES. THE NOTOCHORD IS
PERSISTENT. The notochord is a continuous rod of cartilage, or
gristle, which in the embryological growth of vertebrate animals
supports the spinal nerve cord before the formation of the
vertebrae. In most modern fishes and in all higher vertebrates the
notochord is gradually removed as the bodies of the vertebrae are
formed about it; but in the Devonian fishes it persists through
maturity and the vertebrae remain incomplete.
THE SKELETON IS CARTILAGINOUS. This also is an embryological
characteristic. In the Devonian fishes the vertebrae, as well as
the other parts of the skeleton, have not ossified, or changed to
bone, but remain in their primitive cartilaginous condition.
THE TAIL FIN IS VERTEBRATED. The backbone runs through the fin and
is fringed above and below with its vertical rays. In some fishes
with vertebrated tail fins the fin is symmetric, and this seems to
be the primitive type. In others the tail fin is unsymmetric: the
backbone runs into the upper lobe, leaving the two lobes of
unequal size. In most modern fishes (the teleosts) the tail fin is
not vertebrated: the spinal column ends in a broad plate, to which
the diverging fin rays are attached.
But along with these embryonic characters, which were common to
all Devonian fishes, there were other structures in certain groups
which foreshadowed the higher structures of the land vertebrates
which were yet to come: air sacs which were to develop into lungs,
and cartilaginous axes in the side fins which were a prophecy of
limbs. The vertebrates had already advanced far enough to prove
the superiority of their type of structure to all others. Their
internal skeleton afforded the best attachment for muscles and
enabled them to become the largest and most powerful creatures of
the time. The central nervous system, with the predominance given
to the ganglia at the fore end of the nerve cord,--the brain,--
already endowed them with greater energy than the invertebrates;
and, still more important, these structures contained the
possibility of development into the more highly organized land
vertebrates which were to rule the earth.
TELEOSTS. The great group of fishes called the teleosts, or those
with complete bony skeletons, to which most modern fishes belong,
may be mentioned here, although in the Devonian they had not yet
appeared. The teleosts are a highly specialized type, adapted most
perfectly to their aquatic environment. Heavy armor has been
discarded, and reliance is placed instead on swiftness. The
skeleton is completely ossified and the notochord removed. The
vertebrae have been economically withdrawn from the tail, and the
cartilaginous axis of the side fins has been fotfoid unnecessary.
The air sac has become a swim bladder. In this complete
specialization they have long since lost the possibility of
evolving into higher types.
It is interesting to note that the modern teleosts in their
embryological growth pass through the stages which characterized
the maturity of their Devonian ancestors; their skeleton is
cartilaginous and their tail fin vertebrated.
CHAPTER XIX
THE CARBONIFEROUS
The Carboniferous system is so named from the large amount of
coal which it contains. Other systems, from the Devonian on, are
coal bearing also, but none so richly and to so wide an extent.
Never before or since have the peculiar conditions been so
favorable for the formation of extensive coal deposits.
With few exceptions the Carboniferous strata rest on those of the
Devonian without any marked unconformity; the one period passed
quietly into the other, with no great physical disturbances.
The Carboniferous includes three distinct series. The lower is
called the MISSISSIPPIAN, from the outcrop of its formations along
the Mississippi River in central and southern Illinois and the
adjacent portions of Iowa and Missouri. The middle series is
called the PENNSYLVANIAN (or Coal Measures), from its wide
occurrence over Pennsylvania. The upper series is named the
PERMIAN, from the province of Perm in Russia.
THE MISSISSIPPIAN SERIES. In the interior the Mississippian is
composed chiefly of limestones, with some shales, which tell of a
clear, warm, epicontinental sea swarming with crinoids, corals,
and shells, and occasionally clouded with silt from the land.
In the eastern region, New York had been added by uplift to the
Appalachian land which now was united to the northern area. From
eastern Pennsylvania southward there were laid in a subsiding
trough, first, thick sandstones (the Pocono sandstone), and later
still heavier shales,--the two together reaching the thickness of
four thousand feet and more. We infer a renewed uplift of
Appalachia similar to that of the later epochs of the Devonian,
but as much less in amount as the volume of sediments is smaller.
THE PENNSYLVANIAN SERIES
The Mississippian was brought to an end by a quiet oscillation
which lifted large areas slightly above the sea, and the
Pennsylvanian began with a movement in the opposite direction. The
sea encroached on the new land, and spread far and wide a great
basal conglomerate and coarse sandstones. On this ancient beach
deposit a group of strata rests which we must now interpret. They
consist of alternating shales and sandstones, with here and there
a bed of limestone and an occasional seam of coal. A stratum of
fire clay commonly underlies a coal seam, and there occur also
beds of iron ore. We give a typical section of a very small
portion of the series at a locality in Pennyslvania. Although some
of the minor changes are omitted, the section shows the rapid
alternation of the strata:
Feet
9 Sandstone and shale . . . . . . . . 25
8 Limestone . . . . . . . . . . . . . 18
7 Sandstone . . . . . . . . . . . . . 10
6 Coal . . . . . . . . . . . . . . . 1-6
5 Shale . . . . . . . . . . . . . . . 0-2
4 Sandstone . . . . . . . . . . . . . 40
3 Limestone . . . . . . . . . . . . . 10
2 Coal . . . . . . . . . . . . . . . 5-12
1 Fire clay . . . . . . . . . . . . . 3
This section shows more coal than is usual; on the whole, coal
seams do not take up more than one foot in fifty of the Coal
Measures. They vary also in thickness more than is seen in the
section, some exceptional seams reaching the thickness of fifty
feet.
HOW COAL WAS MADE.
1. Coal is of vegetable origin. Examined under the microscope even
anthracite, or hard coal, is seen to contain carbonized vegetal
tissues. There are also all gradations connecting the hardest
anthracite--through semibituminous coal, bituminous or soft coal,
lignite (an imperfect coal in which sometimes woody fibers may be
seen little changed)--with peat and decaying vegetable tissues.
Coal is compressed and mineralized vegetal matter. Its varieties
depend on the perfection to which the peculiar change called
bituminization has been carried, and also, as shown in the table
below, on the degree to which the volatile substances and water
have escaped, and on the per cent of carbon remaining.
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