THE ORDOVICIAN AND SILURIAN
[Footnote: Often known as the Lower Silurian.]
THE ORDOVICIAN
In North America the Ordovician rocks lie conformably on the
Cambrian. The two periods, therefore, were not parted by any
deformation, either of mountain making or of continental uplift.
The general submergence which marked the Cambrian continued into
the succeeding period with little interruption.
SUBDIVISIONS AND DISTRIBUTION OF STRATA. The Ordovician series, as
they have been made out in New York, are given for reference in
the following table, with the rocks of which they are chiefly
composed:
5 Hudson . . . . . . . . shales
4 Utica . . . . . . . . shales
3 Trenton . . . . . . . limestones
2 Chazy . . . . . . . . limestones
1 Calciferous . . . . . sandy limestones
These marine formations of the Ordovician outcrop about the
Cambrian and pre-Cambrian areas, and, as borings show, extend far
and wide over the interior of the continent beneath more recent
strata. The Ordovician sea stretched from Appalachia across the
Mississippi valley. It seems to have extended to California,
although broken probably by several mountainous islands in the
west.
PHYSICAL GEOGRAPHY. The physical history of the period is recorded
in the succession of its formations. The sandstones of the Upper
Cambrian, as we have learned, tell of a transgressing sea which
gradually came to occupy the Mississippi valley and the interior
of North America. The limestones of the early and middle
Ordovician show that now the shore had become remote and the lands
had become more low. The waters now had cleared. Colonies of
brachiopods and other lime-secreting animals occupied the sea
bottom, and their debris mantled it with sheets of limy ooze. The
sandy limestones of the Calciferous record the transition stage
from the Cambrian when some sand was still brought in from shore.
The highly fossiliferous limestones of the Trenton tell of clear
water and abundant life. We need not regard this epicontinental
sea as deep. No abysmal deposits have been found, and the
limestones of the period are those which would be laid in clear,
warm water of moderate depth like that of modern coral seas.
The shales of the Utica and Hudson show that the waters of the sea
now became clouded with mud washed in from land. Either the land
was gradually uplifted, or perhaps there had arrived one of those
periodic crises which, as we may imagine, have taken place
whenever the crust of the shrinking earth has slowly given way
over its great depressions, and the ocean has withdrawn its waters
into deepening abysses. The land was thus left relatively higher
and bordered with new coastal plains. The epicontinental sea was
shoaled and narrowed, and muds were washed in from the adjacent
lands.
THE TACONIC DEFORMATION. The Ordovician was closed by a
deformation whose extent and severity are not yet known. From the
St. Lawrence River to New York Bay, along the northwestern and
western border of New England, lies a belt of Cambrian-Ordovician
rocks more than a mile in total thickness, which accumulated
during the long ages of those periods in a gradually subsiding
trough between the Adirondacks and a pre-Cambrian range lying west
of the Connecticut River. But since their deposition these ancient
sediments have been crumpled and crushed, broken with great
faults, and extensively metamorphosed. The limestones have
recrystallized into marbles, among them the famous marbles of
Vermont; the Cambrian sandstones have become quartzites, and the
Hudson shale has been changed to a schist exposed on Manhattan
Island and northward.
In part these changes occurred at the close of the Ordovician, for
in several places beds of Silurian age rest unconformably on the
upturned Ordovician strata; but recent investigations have made it
probable that the crustal movements recurred at later times, and
it was perhaps in the Devonian and at the close of the
Carboniferous that the greater part of the deformation and
metamorphism was accomplished. As a result of these movements,--
perhaps several times repeated,--a great mountain range was
upridged, which has been long since leveled by erosion, but whose
roots are now visible in the Taconic Mountains of western New
England.
THE CINCINNATI ANTICLINE. Over an oval area in Ohio, Indiana, and
Kentucky, whose longer axis extends from north to south through
Cincinnati, the Ordovician strata rise in a very low, broad swell,
called the Cincinnati anticline. The Silurian and Devonian strata
thin out as they approach this area and seem never to have
deposited upon it. We may regard it, therefore, as an island
upwarped from the sea at the close of the Ordovician or shortly
after.
PETROLEUM AND NATURAL GAS. These valuable illuminants and fuels
are considered here because, although they are found in traces in
older strata, it is in the Ordovician that they occur for the
first time in large quantities. They range throughout later
formations down to the most recent.
The oil horizons of California and Texas are Tertiary; those of
Colorado, Cretaceous; those of West Virginia, Carboniferous; those
of Pennsylvania, Kentucky, and Canada, Devonian; and the large
field of Ohio and Indiana belongs to the Ordovician and higher
systems.
Petroleum and natural gas, wherever found, have probably
originated from the decay of organic matter when buried in
sedimentary deposits, just as at present in swampy places the
hydrogen and carbon of decaying vegetation combine to form marsh
gas. The light and heat of these hydrocarbons we may think of,
therefore, as a gift to the civilized life of our race from the
humble organisms, both animal and vegetable, of the remote past,
whose remains were entombed in the sediments of the Ordovician and
later geological ages.
Petroleum is very widely disseminated throughout the stratified
rocks. Certain limestones are visibly greasy with it, and others
give off its characteristic fetid odor when struck with a hammer.
Many shales are bituminous, and some are so highly charged that
small flakes may be lighted like tapers, and several gallons of
oil to the ton may be obtained by distillation.
But oil and gas are found in paying quantities only when certain
conditions meet:
1. A SOURCE below, usually a bituminous shale, from whose organic
matter they have been derived by slow change.
2. A RESERVOIR above, in which they have gathered. This is either
a porous sandstone or a porous or creviced limestone.
3. Oil and gas are lighter than water, and are usually under
pressure owing to artesian water. Hence, in order to hold them
from escaping to the surface, the reservoir must have the shape of
an ANTICLINE, DOME, or LENS.
4. It must also have an IMPERVIOUS COVER, usually a shale. In
these reservoirs gas is under a pressure which is often enormous,
reaching in extreme cases as high as a thousand five hundred
pounds to the square inch. When tapped it rushes out with a
deafening roar, sometimes flinging the heavy drill high in air. In
accounting for this pressure we must remember that the gas has
been compressed within the pores of the reservoir rock by artesian
water, and in some cases also by its own expansive force. It is
not uncommon for artesian water to rise in wells after the
exhaustion of gas and oil.
LIFE OF THE ORDOVICIAN
During the ages of the Ordovician, life made great advances. Types
already present branched widely into new genera and species, and
new and higher types appeared.
Sponges continued from the Cambrian. Graptolites now reached their
climax.
STROMATOPORA--colonies of minute hydrozoans allied to corals--grew
in places on the sea floor, secreting stony masses composed of
thin, close, concentric layers, connected by vertical rods. The
Stromatopora are among the chief limestone builders of the
Silurian and Devonian periods.
CORALS developed along several distinct lines, like modern corals
they secreted a calcareous framework, in whose outer portions the
polyps lived. In the Ordovician, corals were represented chiefly
by the family of the CHOETETES, all species of which are long
since extinct. The description of other types of corals will be
given under the Silurian, where they first became abundant.
ECHINODERMS. The cystoid reaches its climax, but there appear now
two higher types of echinoderms,--the crinoid and the starfish.
The CRINOID, named from its resemblance to the lily, is like the
cystoid in many respects, but has a longer stem and supports a
crown of plumose arms. Stirring the water with these arms, it
creates currents by which particles of food are wafted to its
mouth. Crinoids are rare at the present time, but they grew in the
greatest profusion in the warm Ordovician seas and for long ages
thereafter. In many places the sea floor was beautiful with these
graceful, flowerlike forms, as with fields of long-stemmed lilies.
Of the higher, free-moving classes of the echinoderms, starfish
are more numerous than in the Cambrian, and sea urchins make their
appearance in rare archaic forms.
CRUSTACEANS. Trilobites now reach their greatest development and
more than eleven hundred species have been described from the
rocks of this period. It is interesting to note that in many
species the segments of the thorax have now come to be so shaped
that they move freely on one another. Unlike their Cambrian
ancestors, many of the Ordovician trilobites could roll themselves
into balls at the approach of danger. It is in this attitude,
taken at the approach of death, that trilobites are often found in
the Ordovician and later rocks. The gigantic crustaceans called
the EURYPTERIDS were also present in this period.
The arthropods had now seized upon the land. Centipedes and
insects of a low type, the earliest known land animals, have been
discovered in strata of this system.
BRYOZOANS. No fossils are more common in the limestones of the
time than the small branching stems and lacelike mats of the
bryozoans,--the skeletons of colonies of a minute animal allied in
structure to the brachiopod.
BRACHIOPODS. These multiplied greatly, and in places their shells
formed thick beds of coquina. They still greatly surpassed the
mollusks in numbers.
CEPHALOPODS. Among the mollusks we must note the evolution of the
cephalopods. The primitive straight Orthoceras has now become
abundant. But in addition to this ancestral type there appears a
succession of forms more and more curved and closely coiled, as
illustrated in Figure 285. The nautilus, which began its course in
this period, crawls on the bottom of our present seas.
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.