THE ORDOVICIAN[2] AND SILURIAN
[2] 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.
[Illustration: Fig. 276. Hypothetical Map of the Eastern United
States in Ordovician Time
Shaded areas, probable sea; broken lines, approximate shore lines]
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.
[Illustration: Fig. 277. Diagram Illustrating the Conditions of
Accumulation of Oil and Gas
_a_, source; _b_, reservoir; _c_, cover. What would be the result
of boring to the reservoir rock at _d_? at _d´_? at _d´´_?]
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.
[Illustration: Fig. 278. Stromatopora]
=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 _Chætetes_, 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.
[Illustration: Fig. 279. Crinoid, a Jurassic Species]
[Illustration: Fig. 280. An Ordovician Starfish]
[Illustration: Fig. 281. An Ordovician Sea Urchin]
[Illustration: Fig. 282. Eurypterus]
=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 (Fig. 282).
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.
[Illustration: Fig. 283. A Bryozoan]
=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.
[Illustration: Fig. 284. Ordovician Brachiopods]
=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.
[Illustration: Fig. 285. A, Cyrtoceras; B, Trochoceras; C, Lituites]
[Illustration: Fig. 286. Nautilus]
=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.
[Illustration: Fig. 287. Hypothetical Map of Parts of North America
and Europe in Silurian Time.
Shaded areas, probably seas; broken lines, approximate shore lines]
=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.
[Illustration: Fig. 288. A Compound Cup Coral]
[Illustration: Fig. 289. A Simple Cup Coral]
[Illustration: Fig. 290. Honeycomb Corals]
[Illustration: Fig. 291. A Chain Coral]
[Illustration: Fig. 292. A Syringopora Coral]
_Honeycomb corals_ consist of masses of small, close-set prismatic
cells, each crossed by horizontal partitions, or _tabulæ_, while the
septa are rudimentary, being represented by faintly projecting ridges
or rows of spines.
_Chain corals_ are also marked by tabulæ. 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.
[Illustration: Fig. 293. A Blastoid: A, side view, showing
portion of the stem; B, summit of calyx (species
Carboniferous)]
[Illustration: Fig. 294. A Silurian Scorpion]
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_.
[Illustration: Fig. 295. Block of Limestone showing Interior Casts of
_Pentamerus oblongus_, a Common Silurian Brachiopod]