The Elements of Geology
by
William Harmon Norton
Part 5 out of 7
ARAGONITE, a crystalline form of carbonate of lime, that mineral
is usually replaced by CALCITE, a more stable form of the same
substance. The most common petrifying materials are calcite,
silica, and pyrite.
Often the organic substance has neither been preserved nor
replaced, but the FORM has been retained by means of molds and
casts. Permanent impressions, or molds, may be made in sediments
not only by the hard parts of organisms, but also by such soft and
perishable parts as the leaves of plants, and, in the rarest
instances, by the skin of animals and the feathers of birds. In
fine-grained limestones even the imprints of jellyfish have been
retained.
The different kinds of molds and casts may be illustrated by means
of a clam shell and some moist clay, the latter representing the
sediments in which the remains of animals and plants are entombed.
Imbedding the shell in the clay and allowing the clay to harden,
we have a MOLD OF THE EXTERIOR of the shell, as is seen on cutting
the clay matrix in two and removing the shell from it. Filling
this mold with clay of different color, we obtain a CAST OF THE
EXTERIOR, which represents accurately the original form and
surface markings of the shell. In nature, shells and other relics
of animals or plants are often removed by being dissolved by
percolating waters, and the molds are either filled with sediments
or with minerals deposited from solution.
Where the fossil is hollow, a CAST OF THE INTERIOR is made in the
same way. Interior casts of shells reproduce any markings on the
inside of the valves, and casts of the interior of the skulls of
ancient vertebrates show the form and size of their brains.
IMPERFECTION OF THE LIFE RECORD. At the present time only the
smallest fraction of the life on earth ever gets entombed in rocks
now forming. In the forest great fallen tree trunks, as well as
dead leaves, decay, and only add a little to the layer of dark
vegetable mold from which they grew. The bones of land animals
are, for the most part, left unburied on the surface and are soon
destroyed by chemical agencies. Even where, as in the swamps of
river, flood plains and in other bogs, there are preserved the
remains of plants, and sometimes insects, together with the bones
of some animal drowned or mired, in most cases these swamp and bog
deposits are sooner or later destroyed by the shifting channels of
the stream or by the general erosion of the land.
In the sea the conditions for preservation are more favorable than
on land; yet even here the proportion of animals and plants whose
hard parts are fossilized is very small compared with those which
either totally decay before they are buried in slowly accumulating
sediments or are ground to powder by waves and currents.
We may infer that during each period of the past, as at the
present, only a very insignificant fraction of the innumerable
organisms of sea and land escaped destruction and left in
continental and oceanic deposits permanent records of their
existence. Scanty as these original life records must have been,
they have been largely destroyed by metamorphism of the rocks in
which they were imbedded, by solution in underground waters, and
by the vast denudation under which the sediments of earlier
periods have been eroded to furnish materials for the sedimentary
records of later times. Moreover, very much of what has escaped
destruction still remains undiscovered. The immense bulk of the
stratified rocks is buried and inaccessible, and the records of
the past which it contains can never be known. Comparatively few
outcrops have been thoroughly searched for fossils. Although new
species are constantly being discovered, each discovery may be
considered as the outcome of a series of happy accidents,--that
the remains of individuals of this particular species happened to
be imbedded and fossilized, that they happened to escape
destruction during long ages, and that they happened to be exposed
and found.
SOME INFERENCES FROM THE RECORDS OF THE HISTORY OF LIFE UPON THE
PLANET. Meager as are these records, they set forth plainly some
important truths which we will now briefly mention.
1. Each series of the stratified rocks, except the very deepest,
contains vestiges of life. Hence THE EARTH WAS TENANTED BY LIVING
CREATURES FOR AN UNCALCULATED LENGTH OF TIME BEFORE HUMAN HISTORY
BEGAN.
2. LIFE ON THE EARTH HAS BEEN EVERCHANGING. The youngest strata
hold the remains of existing species of animals and plants and
those of species and varieties closely allied to them. Strata
somewhat older contain fewer existing species, and in strata of a
still earlier, but by no means an ancient epoch, no existing
species are to be found; the species of that epoch and of previous
epochs have vanished from the living world. During all geological
time since life began on earth old species have constantly become
extinct and with them the genera and families to which they
belong, and other species, genera, and families have replaced
them. The fossils of each formation differ on the whole from those
of every other. The assemblage of animals and plants (the FAUNA-
FLORA) of each epoch differs from that of every other epoch.
In many cases the extinction of a type has been gradual; in other
instances apparently abrupt. There is no evidence that any
organism once become extinct has ever reappeared. The duration of
a species in time, or its "vertical range" through the strata,
varies greatly. Some species are limited to a stratum a few feet
in thickness; some may range through an entire formation and be
found but little modified in still higher beds. A formation may
thus often be divided into zones, each characterized by its own
peculiar species. As a rule, the simpler organisms have a longer
duration as species, though not as individuals, than the more
complex.
3. THE LARGER ZOOLOGICAL AND BOTANICAL GROUPINGS SURVIVE LONGER
THAN THE SMALLER. Species are so short-lived that a single
geological epoch may be marked by several more or less complete
extinctions of the species of its fauna-flora and their
replacement by other species. A genus continues with new species
after all the species with which it began have become extinct.
Families survive genera, and orders families. Classes are so long-
lived that most of those which are known from the earliest
formations are represented by living forms, and no sub-kingdom has
ever become extinct.
Thus, to take an example from the stony corals,--the ZOANTHARIA,--
the particular characters--which constituted a certain SPECIES--
Facosites niagarensis--of the order are confined to the Niagara
series. Its GENERIC characters appeared in other species earlier
in the Silurian and continued through the Devonian. Its FAMILY
characters, represented in different genera and species, range
from the Ordovician to the close of the Paleozoic; while the
characters which it shares with all its order, the Zoantharia,
began in the Cambrian and are found in living species.
4. THE CHANGE IN ORGANISMS HAS BEEN GRADUAL. The fossils of each
life zone and of each formation of a conformable series closely
resemble, with some explainable exceptions, those of the beds
immediately above and below. The animals and plants which tenanted
the earth during any geological epoch are so closely related to
those of the preceding and the succeeding epochs that we may
consider them to be the descendants of the one and the ancestors
of the other, thus accounting for the resemblance by heredity. It
is therefore believed that the species of animals and plants now
living on the earth are the descendants of the species whose
remains we find entombed in the rocks, and that the chain of life
has been unbroken since its beginning.
5. THE CHANGE IN SPECIES HAS BEEN A GRADUAL DIFFERENTIATION.
Tracing the lines of descent of various animals and plants of the
present backward through the divisions of geologic time, we find
that these lines of descent converge and unite in simpler and
still simpler types. The development of life may be represented by
a tree whose trunk is found in the earliest ages and whose
branches spread and subdivide to the growing twigs of present
species.
6. THE CHANGE IN ORGANISMS THROUGHOUT GEOLOGIC TIME HAS BEEN A
PROGRESSIVE CHANGE. In the earliest ages the only animals and
plants on the earth were lowly forms, simple and generalized in
structure; while succeeding ages have been characterized by the
introduction of types more and more specialized and complex, and
therefore of higher rank in the scale of being. Thus the Algonkian
contains the remains of only the humblest forms of the
invertebrates. In the Cambrian, Ordovician, and Silurian the
invertebrates were represented in all their subkingdoms by a
varied fauna. In the Devonian, fishes--the lowest of the
vertebrates--became abundant. Amphibians made their entry on the
stage in the Carboniferous, and reptiles came to rule the world in
the Mesozoic. Mammals culminated in the Tertiary in strange forms
which became more and more like those of the present as the long
ages of that era rolled on; and latest of all appeared the noblest
product of the creative process, man.
Just as growth is characteristic of the individual life, so
gradual, progressive change, or evolution, has characterized the
history of life upon the planet. The evolution of the organic
kingdom from its primitive germinal forms to the complex and
highly organized fauna-flora of to-day may be compared to the
growth of some noble oak as it rises from the acorn, spreading
loftier and more widely extended branches as it grows.
7. While higher and still higher types have continually been
evolved, until man, the highest of all, appeared, THE LOWER AND
EARLIER TYPES HAVE GENERALLY PERSISTED. Some which reached their
culmination early in the history of the earth have since changed
only in slight adjustments to a changing environment. Thus the
brachiopods, a type of shellfish, have made no progress since the
Paleozoic, and some of their earliest known genera are represented
by living forms hardly to be distinguished from their ancient
ancestors. The lowest and earliest branches of the tree of life
have risen to no higher levels since they reached their climax of
development long ago.
8. A strange parallel has been found to exist between the
evolution of organisms and the development of the individual. In
the embryonic stages of its growth the individual passes swiftly
through the successive stages through which its ancestors evolved
during the millions of years of geologic time. THE DEVELOPMENT OF
THE INDIVIDUAL RECAPITULATES THE EVOLUTION OF THE RACE.
The frog is a typical amphibian. As a tadpole it passes through a
stage identical in several well-known features with the maturity
of fishes; as, for example, its aquatic life, the tail by which it
swims, and the gills through which it breathes. It is a fair
inference that the tadpole stage in the life history of the frog
represents a stage in the evolution of its kind,--that the
Amphibia are derived from fishlike ancestral forms. This inference
is amply confirmed in the geological record; fishes appeared
before Amphibia and were connected with them by transitional
forms.
THE GREAT LENGTH OF GEOLOGIC TIME INFERRED FROM THE SLOW CHANGE OF
SPECIES. Life forms, like land forms, are thus subject to change
under the influence of their changing environment and of forces
acting from within. How slowly they change may be seen in the
apparent stability of existing species. In the lifetime of the
observer and even in the recorded history of man, species seem as
stable as the mountain and the river. But life forms and land
forms are alike variable, both in nature and still more under the
shaping hand of man. As man has modified the face of the earth
with his great engineering works, so he has produced widely
different varieties of many kinds of domesticated plants and
animals, such as the varieties of the dog and the horse, the apple
and the rose, which may be regarded in some respects as new
species in the making. We have assumed that land forms have
changed in the past under the influence of forces now in
operation. Assuming also that life forms have always changed as
they are changing at present, we come to realize something of the
immensity of geologic time required for the evolution of life from
its earliest lowly forms up to man.
It is because the onward march of life has taken the same general
course the world over that we are able to use it as a UNIVERSAL
TIME SCALE and divide geologic time into ages and minor
subdivisions according to the ruling or characteristic organisms
then living on the earth. Thus, since vertebrates appeared, we
have in succession the Age of Fishes, the Age of Amphibians, the
Age of Reptiles, and the Age of Mammals.
The chart given on page 295 is thus based on the law of
superposition and the law of the evolution of organisms. The first
law gives the succession of the formations in local areas. The
fossils which they contain demonstrate the law of the progressive
appearance of organisms, and by means of this law the formations
of different countries are correlated and set each in its place in
a universal time scale and grouped together according to the
affinities of their imbedded organic remains.
GEOLOGIC TIME DIVISIONS COMPARED WITH THOSE OF HUMAN HISTORY. We
may compare the division of geologic time into eras, periods, and
other divisions according to the dominant life of the time, to the
ill-defined ages into which human history is divided according to
the dominance of some nation, ruler, or other characteristic
feature. Thus we speak of the DARK AGES, the AGE OF ELIZABETH, and
the AGE OF ELECTRICITY. These crude divisions would be of much
value if, as in the case of geologic time, we had no exact
reckoning of human history by years.
And as the course of human history has flowed in an unbroken
stream along quiet reaches of slow change and through periods of
rapid change and revolution, so with the course of geologic
history. Periods of quiescence, in which revolutionary forces are
perhaps gathering head, alternate with periods of comparatively
rapid change in physical geography and in organisms, when new and
higher forms appear which serve to draw the boundary line of new
epochs. Nevertheless, geological history is a continuous progress;
its periods and epochs shade into one another by imperceptible
gradations, and all our subdivisions must needs be vague and more
or less arbitrary.
HOW FOSSILS TELL OF THE GEOGRAPHY OF THE PAST. Fossils are used
not only as a record of the development of life upon the earth,
but also in testimony to the physical geography of past epochs.
They indicate whether in any region the climate was tropical,
temperate, or arctic. Since species spread slowly from some center
of dispersion where they originate until some barrier limits their
migration farther, the occurrence of the same species in rocks of
the same system in different countries implies the absence of such
barriers at the period. Thus in the collection of antarctic
fossils referred to on page 294 there were shallow-water marine
shells identical in species with Mesozoic shells found in India
and in the southern extremity of South America. Since such
organisms are not distributed by the currents of the deep sea and
cannot migrate along its bottom, we infer a shallow-water
connection in Mesozoic times between India, South America, and the
antarctic region. Such a shallow-water connection would be offered
along the marginal shelf of a continent uniting these now widely
separated countries.
CHAPTER XV
THE PRE-CAMBRIAN SYSTEMS
THE EARTH'S BEGINNINGS. The geological record does not tell us of
the beginnings of the earth. The history of the planet, as we have
every reason to believe, stretches far back beyond the period of
the oldest stratified rocks, and is involved in the history of the
solar system and of the nebula,--the cloud of glowing gases or of
cosmic dust,--from which the sun and planets are believed to have
been derived.
THE NEBULAR HYPOTHESIS. It is possible that the earth began as a
vaporous, shining sphere, formed by the gathering together of the
material of a gaseous ring which had been detached from a cooling
and shrinking nebula. Such a vaporous sphere would condense to a
liquid, fiery globe, whose surface would become cold and solid,
while the interior would long remain intensely hot because of the
slow conductivity of the crust. Under these conditions the
primeval atmosphere of the earth must have contained in vapor the
water now belonging to the earth's crust and surface. It held also
all the oxygen since locked up in rocks by their oxidation, and
all the carbon dioxide which has since been laid away in
limestones, besides that corresponding to the carbon of
carbonaceous deposits, such as peat, coal, and petroleum. On this
hypothesis the original atmosphere was dense, dark, and noxious,
and enormously heavier than the atmosphere at present.
THE ACCRETION HYPOTHESIS. On the other hand, it has been recently
suggested that the earth may have grown to its present size by the
gradual accretion of meteoritic masses. Such cold, stony bodies
might have come together at so slow a rate that the heat caused by
their impact would not raise sensibly the temperature of the
growing planet. Thus the surface of the earth may never have been
hot and luminous; but as the loose aggregation of stony masses
grew larger and was more and more compressed by its own
gravitation, the heat thus generated raised the interior to high
temperatures, while from time to time molten rock was intruded
among the loose, cold meteoritic masses of the crust and outpoured
upon the surface.
It is supposed that the meteorites of which the earth was built
brought to it, as meteorites do now, various gases shut up within
their pores. As the heat of the interior increased, these gases
transpired to the surface and formed the primitive atmosphere and
hydrosphere. The atmosphere has therefore grown slowly from the
smallest beginnings. Gases emitted from the interior in volcanic
eruptions and in other ways have ever added to it, and are adding
to it now. On the other hand, the atmosphere has constantly
suffered loss, as it has been robbed of oxygen by the oxidation of
rocks in weathering, and of carbon dioxide in the making of
limestones and carbonaceous deposits.
While all hypotheses of the earth's beginnings are as yet unproved
speculations, they serve to bring to mind one of the chief lessons
which geology has to teach,--that the duration of the earth in
time, like the extension of the universe in space, is vastly
beyond the power of the human mind to realize. Behind the history
recorded in the rocks, which stretches back for many million
years, lies the long unrecorded history of the beginnings of the
planet; and still farther in the abysses of the past are dimly
seen the cycles of the evolution of the solar system and of the
nebula which gave it birth.
We pass now from the dim realm of speculation to the earliest era
of the recorded history of the earth, where some certain facts may
be observed and some sure inferences from them may be drawn.
THE ARCHEAN.
The oldest known sedimentary strata, wherever they are exposed by
uplift and erosion, are found to be involved with a mass of
crystalline rocks which possesses the same characteristics in all
parts of the world. It consists of foliated rocks, gneisses, and
schists of various kinds, which have been cut with dikes and other
intrusions of molten rock, and have been broken, crumpled, and
crushed, and left in interlocking masses so confused that their
true arrangement can usually be made out only with the greatest
difficulty if at all. The condition of this body of crystalline
rocks is due to the fact that they have suffered not only from the
faultings, foldings, and igneous intrusions of their time, but
necessarily, also, from those of all later geological ages.
At present three leading theories are held as to the origin of
these basal crystalline rocks.
1. They are considered by perhaps the majority of the geologists
who have studied them most carefully to be igneous rocks intruded
in a molten state among the sedimentary rocks involved with them.
In many localities this relation is proved by the phenomena of
contact; but for the most part the deformations which the rocks
have since suffered again and again have been sufficient to
destroy such evidence if it ever existed.
2. An older view regards them as profoundly altered sedimentary
strata, the most ancient of the earth.
3. According to a third theory they represent portions of the
earth's original crust; not, indeed, its original surface, but
deeper portions uncovered by erosion and afterwards mantled with
sedimentary deposits. All these theories agree that the present
foliated condition of these rocks is due to the intense
metamorphism which they have suffered.
It is to this body of crystalline rocks and the stratified rocks
involved with it, which form a very small proportion of its mass,
that the term ARCHEAN (Greek, ARCHE, beginning) is applied by many
geologists.
THE ALGONKIAN
In some regions there rests unconformably on the Archean an
immense body of stratified rocks, thousands and in places even
scores of thousands of feet thick, known as the ALGONKIAN. Great
unconformities divide it into well-defined systems, but as only
the scantiest traces of fossils appear here and there among its
strata, it is as yet impossible to correlate the formations of
different regions and to give them names of more than local
application. We will describe the Algonkian rocks of two typical
areas.
THE GRAND CANYON OF THE COLORADO. We have already studied a very
ancient peneplain whose edge is exposed to view deep on the walls
of the Colorado Canyon. The formation of flat-lying sandstone
which covers this buried land surface is proved by its fossils to
belong to the Cambrian,--the earliest period of the Paleozoic era.
The tilted rocks on whose upturned edges the Cambrian sandstone
rests are far older, for the physical break which separates them
from it records a time interval during which they were upheaved to
mountainous ridges and worn down to a low plain. They are
therefore classified as Algonkian. They comprise two immense
series. The upper is more than five thousand feet thick and
consists of shales and sandstones with some limestones. Separated
from it by an unconformity which does not appear in Figure 207,
the lower division, seven thousand feet thick, consists chiefly of
massive reddish sandstones with seven or more sheets of lava
interbedded. The lowest member is a basal conglomerate composed of
pebbles derived from the erosion of the dark crumpled schists
beneath,--schists which are supposed to be Archean. As shown in
Figure 207, a strong unconformity parts the schists and the
Algonkian. The floor on which the Algonkian rests is remarkably
even, and here again is proved an interval of incalculable length,
during which an ancient land mass of Archean rocks was baseleveled
before it received the cover of the sediments of the later age.
THE LAKE SUPERIOR REGION. In eastern Canada an area of pre-
Cambrian rocks, Archean and Algonkian, estimated at two million
square miles, stretches from the Great Lakes and the St. Lawrence
River northward to the confines of the continent, inclosing Hudson
Bay in the arms of a gigantic U. This immense area, which we have
already studied as the Laurentian peneplain, extends southward
across the Canadian border into northern Minnesota, Wisconsin, and
Michigan. The rocks of this area are known to be pre-Cambrian; for
the Cambrian strata, wherever found, lie unconformably upon them.
The general relations of the formations of that portion of the
area which lies about Lake Superior are shown in Figure 262. Great
unconformities, UU' separate the Algonkian both from the Archean
and from the Cambrian, and divide it into three distinct systems,
--the LOWER HURONIAN, the UPPER HURONIAN, and the KEWEENAWAN. The
Lower and the Upper Huronian consist in the main of old sea muds
and sands and limy oozes now changed to gneisses, schists,
marbles, quartzites, slates, and other metamorphic rocks. The
Keweenawan is composed of immense piles of lava, such as those of
Iceland, overlain by bedded sandstones. What remains of these rock
systems after the denudation of all later geologic ages is
enormous. The Lower Huronian is more than a mile thick, the Upper
Huronian more than two miles thick, while the Keweenawan exceeds
nine miles in thickness. The vast length of Algonkian time is
shown by the thickness of its marine deposits and by the cycles of
erosion which it includes. In Figure 262 the student may read an
outline of the history of the Lake Superior region, the
deformations which it suffered, their relative severity, the times
when they occurred, and the erosion cycles marked by the
successive unconformities.
OTHER PRE-CAMBRIAN AREAS IN NORTH AMERICA. Pre-Cambrian rocks are
exposed in various parts of the continent, usually by the erosion
of mountain ranges in which their strata were infolded. Large
areas occur in the maritime provinces of Canada. The core of the
Green Mountains of Vermont is pre-Cambrian, and rocks of these
systems occur in scattered patches in western Massachusetts. Here
belong also the oldest rocks of the Highlands of the Hudson and of
New Jersey. The Adirondack region, an outlier of the Laurentian
region, exposes pre-Cambrian rocks, which have been metamorphosed
and tilted by the intrusion of a great boss of igneous rock out of
which the central peaks are carved. The core of the Blue Ridge and
probably much of the Piedmont Belt are of this age. In the Black
Hills the irruption of an immense mass of granite has caused or
accompanied the upheaval of pre-Cambrian strata and metamorphosed
them by heat and pressure into gneisses, schists, quartzites, and
slates. In most of these mountainous regions the lowest strata are
profoundly changed by metamorphism, and they can be assigned to
the pre-Cambrian only where they are clearly overlain
unconformably by formations proved to be Cambrian by their
fossils. In the Belt Mountains of Montana, however, the Cambrian
is underlain by Algonkian sediments twelve thousand feet thick,
and but little altered.
MINERAL WEALTH OF THE PRE-CAMBRIAN ROCKS. The pre-Cambrian rocks
are of very great economic importance, because of their extensive
metamorphism and the enormous masses of igneous rock which they
involve. In many parts of the country they are the source of
supply of granite, gneiss, marble, slate, and other such building
materials. Still more valuable are the stores of iron and copper
and other metals which they contain.
At the present time the pre-Cambrian region about Lake Superior
leads the world in the production of iron ore, its output for 1903
being more than five sevenths of the entire output of the whole
United States, and exceeding that of any foreign country. The ore
bodies consist chiefly of the red oxide of iron (hematite) and
occur in troughs of the strata, underlain by some impervious rock.
A theory held by many refers the ultimate source of the iron to
the igneous rocks of the Archean. When these rocks were upheaved
and subjected to weathering, their iron compounds were decomposed.
Their iron was leached out and carried away to be laid in the
Algonkian water bodies in beds of iron carbonate and other iron
compounds. During the later ages, after the Algonkian strata had
been uplifted to form part of the continent, a second
concentration has taken place. Descending underground waters
charged with oxygen have decomposed the iron carbonate and
deposited the iron, in the form of iron oxide, in troughs of the
strata where their downward progress was arrested by impervious
floors.
The pre-Cambrian rocks of the eastern United States also are rich
in iron. In certain districts, as in the Highlands of New Jersey,
the black oxide of iron (magnetite) is so abundant in beds and
disseminated grains that the ordinary surveyor's compass is
useless.
The pre-Cambrian copper mines of the Lake Superior region are
among the richest on the globe. In the igneous rocks copper, next
to iron, is the most common of all the useful metals, and it was
especially abundant in the Keweenawan lavas. After the Keweenawan
was uplifted to form land, percolating waters leached out much of
the copper diffused in the lava sheets and deposited it within
steam blebs as amygdules of native copper, in cracks and fissures,
and especially as a cement, or matrix, in the interbedded gravels
which formed the chief aquifers of the region. The famous Calumet
and Hecla mine follows down the dip of the strata to the depth of
nearly a mile and works such an ancient conglomerate whose matrix
is pure copper.
THE APPEARANCE OF LIFE. Sometime during the dim ages preceding the
Cambrian, whether in the Archean or in the Algonkian we know not,
occurred one of the most important events in the history of the
earth. Life appeared for the first time upon the planet. Geology
has no evidence whatever to offer as to whence or how life came.
All analogies lead us to believe that its appearance must have
been sudden. Its earliest forms are unknown, but analogy suggests
that as every living creature has developed from a single cell, so
the earliest organisms upon the globe--the germs from which all
later life is supposed to have been evolved--were tiny,
unicellular masses of protoplasm, resembling the amoeba of to-day
in the simplicity of their structure.
Such lowly forms were destitute of any hard parts and could leave
no evidence of their existence in the record of the rocks. And of
their supposed descendants we find so few traces in the pre-
Cambrian strata that the first steps in organic evolution must be
supplied from such analogies in embryology as the following. The
fertilized ovum, the cell with which each animal begins its life,
grows and multiplies by cell division, and develops into a hollow
globe of cells called the BLASTOSPHERE. This stage is succeeded by
the stage of the GASTRULA,--an ovoid or cup-shaped body with a
double wall of cells inclosing a body cavity, and with an opening,
the primitive mouth. Each of these early embryological stages is
represented by living animals,--the undivided cell by the
PROTOZOA, the blastosphere by some rare forms, and the gastrula in
the essential structure of the COELENTERATES,--the subkingdom to
which the fresh-water hydra and the corals belong. All forms of
animal life, from the coelenterates to the mammals, follow the
same path in their embryological development as far as the
gastrula stage, but here their paths widely diverge, those of each
subkingdom going their own separate ways.
We may infer, therefore, that during the pre-Cambrian periods
organic evolution followed the lines thus dimly traced. The
earliest one-celled protozoa were probably succeeded by many-
celled animals of the type of the blastosphere, and these by
gastrula-like organisms. From the gastrula type the higher sub-
divisions of animal life probably diverged, as separate branches
from a common trunk. Much or all of this vast differentiation was
accomplished before the opening of the next era; for all the
subkingdoms are represented in the Cambrian except the
vertebrates.
EVIDENCES OF PRE-CAMBRIAN LIFE. An indirect evidence of life
during the pre-Cambrian periods is found in the abundant and
varied fauna of the next period; for, if the theory of evolution
is correct, the differentiation of the Cambrian fauna was a long
process which might well have required for its accomplishment a
large part of pre-Cambrian time.
Other indirect evidences are the pre-Cambrian limestones, iron
ores, and graphite deposits, since such minerals and rocks have
been formed in later times by the help of organisms. If the
carbonate of lime of the Algonkian limestones and marbles was
extracted from sea water by organisms, as is done at present by
corals, mollusks, and other humble animals and plants, the life of
those ancient seas must have been abundant. Graphite, a soft black
mineral composed of carbon and used in the manufacture of lead
pencils and as a lubricant, occurs widely in the metamorphic pre-
Cambrian rocks. It is known to be produced in some cases by the
metamorphism of coal, which itself is formed of decomposed vegetal
tissues. Seams of graphite may therefore represent accumulations
of vegetal matter such as seaweed. But limestone, iron ores, and
graphite can be produced by chemical processes, and their presence
in the pre-Cambrian makes it only probable, and not certain, that
life existed at that time.
PRE-CAMBRIAN FOSSILS. Very rarely has any clear trace of an
organism been found in the most ancient chapters of the geological
record, so many of their leaves have been destroyed and so far
have their pages been defaced. Omitting structures whose organic
nature has been questioned, there are left to mention a tiny
seashell of one of the most lowly types,--a DISCINA from the pre-
Cambrian rocks of the Colorado Canyon,--and from the pre-Cambrian
rocks of Montana trails of annelid worms and casts of their
burrows in ancient beaches, and fragments of the tests of
crustaceans. These diverse forms indicate that before the
Algonkian had closed, life was abundant and had widely
differentiated. We may expect that other forms will be discovered
as the rocks are closely searched.
PRE-CAMBRIAN GEOGRAPHY. Our knowledge is far too meager to warrant
an attempt to draw the varying outlines of sea and land during the
Archean and Algonkian eras. Pre-Cambrian time probably was longer
than all later geological time down to the present, as we may
infer from the vast thicknesses of its rocks and the
unconformities which part them. We know that during its long
periods land masses again and again rose from the sea, were worn
low, and were submerged and covered with the waste of other lands.
But the formations of separated regions cannot be correlated
because of the absence of fossils, and nothing more can be made
out than the detached chapters of local histories, such as the
outline given of the district about Lake Superior.
The pre-Cambrian rocks show no evidence of any forces then at work
upon the earth except the forces which are at work upon it now.
The most ancient sediments known are so like the sediments now
being laid that we may infer that they were formed under
conditions essentially similar to those of the present time. There
is no proof that the sands of the pre-Cambrian sandstones were
swept by any more powerful waves and currents than are offshore
sands to-day, or that the muds of the pre-Cambrian shales settled
to the sea floor in less quiet water than such muds settle in at
present. The pre-Cambrian lands were, no doubt, worn by wind and
weather, beaten by rain, and furrowed by streams as now, and, as
now, they fronted the ocean with beaches on which waves dashed and
along which tidal currents ran.
Perhaps the chief difference between the pre-Cambrian and the
present was the absence of life upon the land. So far as we have
any knowledge, no forests covered the mountain sides, no verdure
carpeted the plains, and no animals lived on the ground or in the
air. It is permitted to think of the most ancient lands as deserts
of barren rock and rock waste swept by rains and trenched by
powerful streams. We may therefore suppose that the processes of
their destruction went on more rapidly than at present.
CHAPTER XVI
THE CAMBRIAN
THE PALEOZOIC ERA. The second volume of the geological record,
called the Paleozoic (Greek, PALAIOS, ancient; ZOE, life), has
come down to us far less mutilated and defaced than has the first
volume, which contains the traces of the most ancient life of the
globe. Fossils are far more abundant in the Paleozoic than in the
earlier strata, while the sediments in which they were entombed
have suffered far less from metamorphism and other causes, and
have been less widely buried from view, than the strata of the
pre-Cambrian groups. By means of their fossils we can correlate
the formations of widely separated regions from the beginning of
the Paleozoic on, and can therefore trace some outline of the
history of the continents.
Paleozoic time, although shorter than the pre-Cambrian as measured
by the thickness of the strata, must still be reckoned in millions
of years. During this vast reach of time the changes in organisms
were very great. It is according to the successive stages in the
advance of life that the Paleozoic formations are arranged in five
systems,--the CAMBRIAN, the ORDOVICIAN, the SILURIAN, the
DEVONIAN, and the CARBONIFEROUS. On the same basis the first three
systems are grouped together as the older Paleozoic, because they
alike are characterized by the dominance of the invertebrates;
while the last two systems are united in the later Paleozoic, and
are characterized, the one by the dominance of fishes, and the
other by the appearance of amphibians and reptiles.
Each of these systems is world-wide in its distribution, and may
be recognized on any continent by its own peculiar fauna. The
names first given them in Great Britain have therefore come into
general use, while their subdivisions, which often cannot be
correlated in different countries and different regions, are
usually given local names.
The first three systems were named from the fact that their strata
are well displayed in Wales. The Cambrian carries the Roman name
of Wales, and the Ordovician and Silurian the names of tribes of
ancient Britons which inhabited the same country. The Devonian is
named from the English county Devon, where its rocks were early
studied. The Carboniferous was so called from the large amount of
coal which it was found to contain in Great Britain and
continental Europe.
THE CAMBRIAN
DISTRIBUTION OF STRATA. The Cambrian rocks outcrop in narrow belts
about the pre-Cambrian areas of eastern Canada and the Lake
Superior region, the Adirondacks and the Green Mountains. Strips
of Cambrian formations occupy troughs in the pre-Cambrian rocks of
New England and the maritime provinces of Canada; a long belt
borders on the west the crystalline rocks of the Blue Ridge; and
on the opposite side of the continent the Cambrian reappears in
the mountains of the Great Basin and the Canadian Rockies. In the
Mississippi valley it is exposed in small districts where uplift
has permitted the stripping off of younger rocks. Although the
areas of outcrop are small, we may infer that Cambrian rocks were
widely deposited over the continent of North America.
PHYSICAL GEOGRAPHY. The Cambrian system of North America comprises
three distinct series, the LOWER CAMBRIAN, the MIDDLE CAMBRIAN,
and the UPPER CAMBRIAN, each of which is characterized by its own
peculiar fauna. In sketching the outlines of the continent as it
was at the beginning of the Paleozoic, it must be remembered that
wherever the Lower Cambrian formations now are found was certainly
then sea bottom, and wherever the Lower Cambrian are wanting, and
the next formations rest directly on pre-Cambrian rocks, was
probably then land.
EARLY CAMBRIAN GEOGRAPHY. In this way we know that at the opening
of the Cambrian two long, narrow mediterranean seas stretched from
north to south across the continent. The eastern sea extended from
the Gulf of St. Lawrence down the Champlain-Hudson valley and
thence along the western base of the Blue Ridge south at least to
Alabama. The western sea stretched from the Canadian Rockies over
the Great Basin and at least as far south as the Grand Canyon of
the Colorado in Arizona.
Between these mediterraneans lay a great central land which
included the pre-Cambrian U-shaped area of the Laurentian
peneplain, and probably extended southward to the latitude of New
Orleans. To the east lay a land which we may designate as
APPALACHIA, whose western shore line was drawn along the site of
the present Blue Ridge, but whose other limits are quite unknown.
The land of Appalachia must have been large, for it furnished a
great amount of waste during the entire Paleozoic era, and its
eastern coast may possibly have lain even beyond the edge of the
present continental shelf. On the western side of the continent a
narrow land occupied the site of the Sierra Nevada Mountains.
Thus, even at the beginning of the Paleozoic, the continental
plateau of North America had already been left by crustal
movements in relief above the abysses of the great oceans on
either side. The mediterraneans which lay upon it were shallow, as
their sediments prove. They were EPICONTINENTAL SEAS; that is,
they rested UPON (Greek, EPI) the submerged portion of the
continental plateau. We have no proof that the deep ocean ever
occupied any part of where North America now is.
The Middle and Upper Cambrian strata are found together with the
Lower Cambrian over the area of both the eastern and the western
mediterraneans, so that here the sea continued during the entire
period. The sediments throughout are those of shoal water. Coarse
cross-bedded sandstones record the action of strong shifting
currents which spread coarse waste near shore and winnowed it of
finer stuff. Frequent ripple marks on the bedding planes of the
strata prove that the loose sands of the sea floor were near
enough to the surface to be agitated by waves and tidal currents.
Sun cracks show that often the outgoing tide exposed large muddy
flats to the drying action of the sun. The fossils, also, of the
strata are of kinds related to those which now live in shallow
waters near the shore.
The sediments which gathered in the mediterranean seas were very
thick, reaching in places the enormous depth of ten thousand feet.
Hence the bottoms of these seas were sinking troughs, ever filling
with waste from the adjacent land as fast as they subsided.
LATE CAMBRIAN GEOGRAPHY. The formations of the Middle and Upper
Cambrian are found resting unconformably on the pre-Cambrian rocks
from New York westward into Minnesota and at various points in the
interior, as in Missouri and in Texas. Hence after earlier
Cambrian time the central land subsided, with much the same effect
as if the Mississippi valley were now to lower gradually, and the
Gulf of Mexico to spread northward until it entered Lake Superior.
The Cambrian seas transgressed the central land and strewed far
and wide behind their advancing beaches the sediments of the later
Cambrian upon an eroded surface of pre-Cambrian rocks.
The succession of the Cambrian formations in North America records
many minor oscillations and varying conditions of physical
geography; yet on the whole it tells of widening seas and lowering
lands. Basal conglomerates and coarse sandstones which must have
been laid near shore are succeeded by shaly sandstones, sandy
shales, and shales. Toward the top of the series heavy beds of
limestone, extending from the Blue Ridge to Missouri, speak of
clear water, and either of more distant shores or of neighboring
lands which were worn or sunk so low that for the most part their
waste was carried to the sea in solution.
In brief, the Cambrian was a period of submergence. It began with
the larger part of North America emerged as great land masses. It
closed with most of the interior of the continental plateau
covered with a shallow sea.
THE LIFE OF THE CAMBRIAN PERIOD
It is now for the first time that we find preserved in the
offshore deposits of the Cambrian seas enough remains of animal
life to be properly called a fauna. Doubtless these remains are
only the most fragmentary representation of the life of the time,
for the Cambrian rocks are very old and have been widely
metamorphosed. Yet the five hundred and more species already
discovered embrace all the leading types of invertebrate life, and
are so varied that we must believe that their lines of descent
stretch far back into the pre-Cambrian past.
PLANTS. No remains of plants have been found in Cambrian strata,
except some doubtful markings, as of seaweed.
SPONGES. The sponges, the lowest of the multicellular animals,
were represented by several orders. Their fossils are recognized
by the siliceous spicules, which, as in modern sponges, either
were scattered through a mass of horny fibers or were connected in
a flinty framework.
COELENTERATES. This subkingdom includes two classes of interest to
the geologist,--the HYDROZOA, such as the fresh-water hydra and
the jellyfish, and the CORALS. Both classes existed in the
Cambrian.
The Hydrozoa were represented not only by jellyfish but also by
the GRAPTOLITE, which takes its name from a fancied resemblance of
some of its forms to a quill pen. It was a composite animal with a
horny framework, the individuals of the colony living in cells
strung on one or both sides along a hollow stem, and communicating
by means of a common flesh in this central tube. Some graptolites
were straight, and some curved or spiral; some were single
stemmed, and others consisted of several radial stems united.
Graptolites occur but rarely in the Upper Cambrian. In the
Ordovician and Silurian they are very plentiful, and at the close
of the Silurian they pass out of existence, never to return.
CORALS are very rarely found in the Cambrian, and the description
of their primitive types is postponed to later chapters treating
of periods when they became more numerous.
ECHINODERMS. This subkingdom comprises at present such familiar
forms as the crinoid, the starfish, and the sea urchin. The
structure of echinoderms is radiate. Their integument is hardened
with plates or particles of carbonate of lime.
Of the free echinoderms, such as the starfish and the sea urchin,
the former has been found in the Cambrian rocks of Europe, but
neither have so far been discovered in the strata of this period
in North America. The stemmed and lower division of the
echinoderms was represented by a primitive type, the CYSTOID, so
called from its saclike form, A small globular or ovate "calyx" of
calcareous plates, with an aperture at the top for the mouth,
inclosed the body of the animal, and was attached to the sea
bottom by a short flexible stalk consisting of disks of carbonate
of lime held together by a central ligament.
ARTHOPODS. These segmented animals with "jointed feet," as their
name suggests, may be divided in a general way into water
breathers and air breathers. The first-named and lower division
comprises the class of the CRUSTACEA,--arthropods protected by a
hard exterior skeleton, or "crust,"--of which crabs, crayfish, and
lobsters are familiar examples. The higher division, that of the
air breathers, includes the following classes: spiders, scorpions,
centipedes, and insects.
THE TRILOBITE. The aquatic arthropods, the Crustacea, culminated
before the air breathers; and while none of the latter are found
in the Cambrian, the former were the dominant life of the time in
numbers, in size, and in the variety of their forms. The leading
crustacean type is the TRILOBITE, which takes its name from the
three lobes into which its shell is divided longitudinally. There
are also three cross divisions,--the head shield, the tail shield,
and between the two the thorax, consisting of a number of distinct
and unconsolidated segments. The head shield carries a pair of
large, crescentic, compound eyes, like those of the insect. The
eye varies greatly in the number of its lenses, ranging from
fourteen in some species to fifteen thousand in others. Figure
268, C, is a restoration of the trilobite, and shows the
appendages, which are found preserved only in the rarest cases.
During the long ages of the Cambrian the trilobite varied greatly.
Again and again new species and genera appeared, while the older
types became extinct. For this reason and because of their
abundance, trilobites are used in the classification of the
Cambrian system. The Lower Cambrian is characterized by the
presence of a trilobitic fauna in which the genus Olenellus is
predominant. This, the OLENELLUS ZONE, is one of the most
important platforms in the entire geological series; for, the
world over, it marks the beginning of Paleozoic time, while all
underlying strata are classified as pre-Cambrian. The Middle
Cambrian is marked by the genus Paradoxides, and the Upper
Cambrian by the genus Olenus. Some of the Cambrian trilobites were
giants, measuring as much as two feet long, while others were the
smallest of their kind, a fraction of an inch in length.
Another type of crustacean which lived in the Cambrian and whose
order is still living is illustrated in Figure 269.
WORMS. Trails and burrows of worms have been left on the sea
beaches and mud flats of all geological times from the Algonkian
to the present.
BRACHIOPODS. These soft-bodied animals, with bivalve shells and
two interior armlike processes which served for breathing,
appeared in the Algonkian, and had now become very abundant. The
two valves of the brachiopod shell are unequal in size, and in
each valve a line drawn from the beak to the base divides the
valve into two equal parts. It may thus be told from the pelecypod
mollusk, such as the clam, whose two valves are not far from equal
in size, each being divided into unequal parts by a line dropped
from the beak.
Brachiopods include two orders. In the most primitive order--that
of the INARTICULATE brachiopods--the two valves are held together
only by muscles of the animal, and the shell is horny or is
composed of phosphate of lime. The DISCINA, which began in the
Algonkian, is of this type, as is also the LINGULELLA of the
Cambrian. Both of these genera have lived on during the millions
of years of geological time since their introduction, handing down
from generation to generation with hardly any change to their
descendants now living off our shores the characters impressed
upon them at the beginning.
The more highly organized ARTICULATE brachiopods have valves of
carbonate of lime more securely joined by a hinge with teeth and
sockets (Fig. 270). In the Cambrian the inarticulates predominate,
though the articulates grow common toward the end of the period.
MOLLUSKS. The three chief classes of mollusks--the PELECYPODS
(represented by the oyster and clam of to-day), the GASTROPODS
(represented now by snails, conches, and periwinkles), and the
CEPHALOPODS (such as the nautilus, cuttlefish, and squids)--were
all represented in the Cambrian, although very sparingly.
Pteropods, a suborder of the gastropods, appeared in this age.
Their papery shells of carbonate of lime are found in great
numbers from this time on.
Cephalopods, the most highly organized of the mollusks, started
into existence, so far as the record shows, toward, the end of the
Cambrian, with the long extinct ORTHOCERAS (STRAIGHTHORN) and the
allied genera of its family. The Orthoceras had a long, straight,
and tapering shell, divided by cross partitions into chambers. The
animal lived in the "body chamber" at the larger end, and walled
off the other chambers from it in succession during the growth of
the shell. A central tube, the SIPHUNCLE, passed through from the
body chamber to the closed tip of the cone.
The seashells, both brachiopods and mollusks, are in some respects
the most important to the geologist of all fossils. They have been
so numerous, so widely distributed, and so well preserved because
of their durable shells and their station in growing sediments,
that better than any other group of organisms they can be used to
correlate the strata of different regions and to mark by their
slow changes the advance of geological time.
CLIMATE. The life of Cambrian times in different countries
contains no suggestion of any marked climatic zones, and as in
later periods a warm climate probably reached to the polar
regions.
CHAPTER XVII
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.
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.
Peat Lignite Bituminous Coal
Anthracite
Dismal Swamp Texas Penn.
Penn.
Moisture . . . . 78.89 14.67 1.30 2.74
Volatile matter . 13.84 37.32 20.87 4.25
Fixed carbon . . 6.49 41.07 67.20 81.51
Ash . . . . . . . 0.78 6.69 8.80 10.87
2. The vegetable remains associated with coal are those of land
plants.
3. Coal accumulated in the presence of water; for it is only when
thus protected from the air that vegetal matter is preserved.
4. The vegetation of coal accumulated for the most part where it
grew; it was not generally drifted and deposited by waves and
currents. Commonly the fire clay beneath the seam is penetrated
with roots, and the shale above is packed with leaves of ferns and
other plants as beautifully pressed as in a herbarium. There often
is associated with the seam a fossil forest, with the stumps,
which are still standing where they grew, their spreading roots,
and the soil beneath, all changed to stone. In the Nova Scotia
field, out of seventy-six distinct coal seams, twenty are
underlain by old forest grounds.
The presence of fire clay beneath a seam points in the same
direction. Such underclays withstand intense heat and are used in
making fire brick, because their alkalies have been removed by the
long-continued growth of vegetation.
Fuel coal is also too pure to have been accumulated by driftage.
In that case we should expect to find it mixed with mud, while in
fact it often contains no more ash than the vegetal matter would
furnish from which it has been compressed.
These conditions are fairly met in the great swamps of river
plains and deltas and of coastal plains, such as the great Dismal
Swamp, where thousands of generations of forests with their
undergrowths contribute their stems and leaves to form thick beds
of peat. A coal seam is a fossil peat bed.
GEOGRAPHICAL CONDITIONS DURING THE PENNSYLVANIAN. The
Carboniferous peat swamps were of vast extent. A map of the Coal
Measures (Fig. 260) shows that the coal marshes stretched, with
various interruptions of higher ground and straits of open water,
from eastern Pennsylvania into Alabama, Texas, and Kansas. Some
individual coal beds may still be traced over a thousand square
miles, despite the erosion which they have suffered. It taxes the
imagination to conceive that the varied region included within
these limits was for hundreds of thousands of years a marshy plain
covered with tropical jungles such as that pictured in Figure 304.
On the basis that peat loses four fifths of its bulk in changing
to coal, we may reckon the thickness of these ancient peat beds.
Coal seams six and ten feet thick, which are not uncommon,
represent peat beds thirty and fifty feet in thickness, while
mammoth coal seams fifty feet thick have been compressed from peat
beds two hundred and fifty feet deep.
At the same time, the thousands of feet of marine and freshwater
sediments, with their repeated alternations of limestones,
sandstones, and shales, in which the seams of coal occur, prove a
slow subsidence, with many changes in its rate, with halts when
the land was at a stillstand, and with occasional movements
upward.
When subsidence was most rapid and long continued the sea
encroached far and wide upon the lowlands and covered the coal
swamps with sands and muds and limy oozes. When subsidence
slackened or ceased the land gained on the sea. Bays were barred,
and lagoons as they gradually filled with mud became marshes.
River deltas pushed forward, burying with their silts the sunken
peat beds of earlier centuries, and at the surface emerged in
broad, swampy flats,--like those of the deltas of the Mississippi
and the Ganges,--which soon were covered with luxuriant forests.
At times a gentle uplift brought to sea level great coastal
plains, which for ages remained mantled with the jungle, their
undeveloped drainage clogged with its debris, and were then again
submerged.
PHYSICAL GEOGRAPHY OF THE SEVERAL REGIONS. THE ACADIAN REGION lay
on the eastern side of the northern land, where now are New
Brunswick and Nova Scotia, and was an immense river delta. Here
river deposits rich in coal accumulated to a depth of sixteen
thousand feet. The area of this coal field is estimated at about
thirty-six thousand square miles.
THE APPALACHIAN REGION skirts the Appalachian oldland on the west
from the southern boundary of New York to northern Alabama,
extending west into eastern Ohio. The Cincinnati anticline was now
a peninsula, and the broad gulf which had lain between it and
Appalachia was transformed at the beginning of the Pennsylvanian
into wide marshy plains, now sinking beneath the sea and now
emerging from it. This area subsided during the Carboniferous
period to a depth of nearly ten thousand feet.
THE CENTRAL REGION lay west of the peninsula of the Cincinnati
anticline, and extended from Indiana west into eastern Nebraska,
and from central Iowa and Illinois southward about the ancient
island in Missouri and Arkansas into Oklahoma and Texas. On the
north the subsidence in this area was comparatively slight, for
the Carboniferous strata scarcely exceed two thousand feet in
thickness. But in Arkansas and Indian Territory the downward
movement amounted to four and five miles, as is proved by shoal
water deposits of that immense thickness.
The coal fields of Indiana, and Illinois are now separated by
erosion from those lying west of the Mississippi River. At the
south the Appalachian land seems still to have stretched away to
the west across Louisiana and Mississippi into Texas, and this
westward extension formed the southern boundary of the coal
marshes of the continent.
The three regions just mentioned include the chief Carboniferous
coal fields of North America. Including a field in central
Michigan evidently formed in an inclosed basin (Fig. 260), and one
in Rhode Island, the total area of American coal fields has been
reckoned at not less than two hundred thousand square miles. We
can hardly estimate the value of these great stores of fossil fuel
to an industrial civilization. The forests of the coal swamps
accumulated in their woody tissues the energy which they received
from the sun in light and heat, and it is this solar energy long
stored in coal seams which now forms the world's chief source of
power in manufacturing.
THE WESTERN AREA. On the Great Plains beyond the Missouri River
the Carboniferous strata pass under those of more recent systems.
Where they reappear, as about dissected mountain axes or on
stripped plateaus, they consist wholly of marine deposits and are
devoid of coal. The rich coal fields of the West are of later
date.
On the whole the Carboniferous seems to have been a time of
subsidence in the West. Throughout the period a sea covered the
Great Basin and the plateaus of the Colorado River. At the time of
the greatest depression the sites of the central chains of the
Rockies were probably islands, but early in the period they may
have been connected with the broad lands to the south and east.
Thousands of feet of Carboniferous sediments were deposited where
the Sierra Nevada Mountains now stand.
THE PERMIAN. As the Carboniferous period drew toward its close the
sea gradually withdrew from the eastern part of the continent.
Where the sea lingered in the deepest troughs, and where inclosed
basins were cut off from it, the strata of the Permian were
deposited. Such are found in New Brunswick, in Pennsylvania and
West Virginia, in Texas, and in Kansas. In southwestern Kansas
extensive Permian beds of rock salt and gypsum show that here lay
great salt lakes in which these minerals were precipitated as
their brines grew dense and dried away.
In the southern hemisphere the Permian deposits are so
extraordinary that they deserve a brief notice, although we have
so far omitted mention of the great events which characterized the
evolution of other continents than our own. The Permian fauna-
flora of Australia, India, South Africa, and the southern part of
South America are so similar that the inference is a reasonable
one that these widely separated regions were then connected
together, probably as extensions of a great antarctic continent.
Interbedded with the Permian strata of the first three countries
named are extensive and thick deposits of a peculiar nature which
are clearly ancient ground moraines. Clays and sand, now hardened
to firm rock, are inset with unsorted stones of all sizes, which
often are faceted and scratched. Moreover, these bowlder clays
rest on rock pavements which are polished and scored with glacial
markings. Hence toward the close of the Paleozoic the southern
lands of the eastern hemisphere were invaded by great glaciers or
perhaps by ice sheets like that which now shrouds Greenland.
These Permian ground moraines are not the first traces of the work
of glaciers met with in the geological record. Similar deposits
prove glaciation in Norway succeeding the pre-Cambrian stage of
elevation, and Cambrian glacial drift has recently been found in
China.
THE APPALACHIAN DEFORMATION. We have seen that during Paleozoic
times a long, narrow trough of the sea lay off the western coast
of the ancient land of Appalachia, where now are the Appalachian
Mountains. During the long ages of this era the trough gradually
subsided, although with many stillstands and with occasional
slight oscillations upward. Meanwhile the land lying to the east
was gradually uplifted at varying rates and with long pauses. The
waste of the rising land was constantly transferred to the sinking
marginal sea bottom, and on the whole the trough was filled with
sediments as rapidly as it subsided. The sea was thus kept
shallow, and at times, especially toward the close of the era,
much of the area was upbuilt or raised to low, marshy, coastal
plains. When the Carboniferous was ended the waste which had been
removed from the land and laid along its margin in the successive
formations of the Paleozoic had reached a thickness of between
thirty and forty thousand feet.
Both by sedimentation and by subsidence the trough had now become
a belt of weakness in the crust of the earth. Here the crust was
now made of layers to the depth of six or seven miles. In
comparison with the massive crystalline rocks of Appalachia on the
east, the layered rock of the trough was weak to resist lateral
pressure, as a ream of sheets of paper is weak when compared with
a solid board of the same thickness. It was weaker also than the
region to the west, since there the sediments were much thinner.
Besides, by the long-continued depression the strata of the trough
had been bent from the flat-lying attitude in which they were laid
to one in which they were less able to resist a horizontal thrust.
There now occurred one of the critical stages in the history of
the planet, when the crust crumples under its own weight and
shrinks down upon a nucleus which is diminishing in volume and no
longer able to support it. Under slow but resistless pressure the
strata of the Appalachian trough were thrust against the rigid
land, and slowly, steadily bent into long folds whose axes ran
northeast-southwest parallel to the ancient coast line. It was on
the eastern side next the buttress of the land that the
deformation was the greatest, and the folds most steep and close.
In central Pennsylvania and West Virginia the folds were for the
most part open. South of these states the folds were more closely
appressed, the strata were much broken, and the great thrust
faults were formed which have been described already. In eastern
Pennsylvania seams of bituminous coal were altered to anthracite,
while outside the region of strong deformation, as in western
Pennyslvania, they remained unchanged. An important factor in the
deformation was the massive limestones of the Cambrian-Ordovician.
Because of these thick, resistant beds the rocks were bent into
wide folds and sheared in places with great thrust faults. Had the
strata been weak shales, an equal pressure would have crushed and
mashed them.
Although the great earth folds were slowly raised, and no doubt
eroded in their rising, they formed in all probability a range of
lofty mountains, with a width of from fifty to a hundred and
twenty-five miles, which stretched from New York to central
Alabama.
From their bases lowlands extended westward to beyond the Missouri
River. At the same time ranges were upridged out of thick
Paleozoic sediments both in the Bay of Fundy region and in the
Indian Territory. The eastern portion of the North American
continent was now well-nigh complete.
The date of the Appalachian deformation is told in the usual way.
The Carboniferous strata, nearly two miles thick, are all infolded
in the Appalachian ridges, while the next deposits found in this
region--those of the later portion of the first period (the Trias)
of the succeeding era--rest unconformably on the worn edges of the
Appalachian folded strata. The deformation therefore took place
about the close of the Paleozoic. It seems to have begun in the
Permian, in, eastern Pennsylvania,--for here the Permian strata
are wanting,--and to have continued into the Trias, whose earlier
formations are absent over all the area.
With this wide uplift the subsidence of the sea floor which had so
long been general in eastern North America came to an end.
Deposition now gave place to erosion. The sedimentary record of
the Paleozoic was closed, and after an unknown lapse of time, here
unrecorded, the annals of the succeeding era were written under
changed conditions.
In western North America the closing stages of the Paleozoic were
marked by important oscillations. The Great Basin, which had long
been a mediterranean sea, was converted into land over western
Utah and eastern Nevada, while the waves of the Pacific rolled
across California and western Nevada.
The absence of tuffs and lavas among the Carboniferous strata of
North America shows that here volcanic action was singularly
wanting during the entire period. Even the Appalachian deformation
was not accompanied by any volcanic outbursts.
LIFE OF THE CARBONIFEROUS
PLANTS. The gloomy forests and dense undergrowths of the
Carboniferous jungles would appear unfamiliar to us could we see
them as they grew, and even a botanist would find many of their
forms perplexing and hard to classify. None of our modern trees
would meet the eye. Plants with conspicuous flowers of fragrance
and beauty were yet to come. Even mosses and grasses were still
absent.
Tree ferns lifted their crowns of feathery fronds high in air on
trunks of woody tissue; and lowly herbaceous ferns, some belonging
to existing families, carpeted the ground. Many of the fernlike
forms, however, have distinct affinities with the cycads, of which
they may be the ancestors, and some bear seeds and must be classed
as gymnosperms.
Dense thickets, like cane or bamboo brakes, were composed of thick
clumps of CALAMITES, whose slender, jointed stems shot up to a
height of forty feet, and at the joints bore slender branches set
with whorls of leaves. These were close allies of the Equiseta or
"horsetails," of the present; but they bore characteristics of
higher classes in the woody structures of their stems.
There were also vast monotonous forests, composed chiefly of trees
belonging to the lycopods, and whose nearest relatives to-day are
the little club mosses of our eastern woods. Two families of
lycopods deserve special mention,--the Lepidodendrons and the
Sigillaria.
The LEPIDODENDRON, or "scale tree," was a gigantic club moss fifty
and seventy-five feet high, spreading toward the top into stout
branches, at whose ends were borne cone-shaped spore cases. The
younger parts of the tree were clothed with stiff needle-shaped
leaves, but elsewhere the trunk and branches were marked with
scalelike scars, left by the fallen leaves, and arranged in spiral
rows.
The SIGILLARIA, or "seal tree," was similar to the Lepidodendron,
but its fluted trunk divided into even fewer branches, and was
dotted with vertical rows of leaf scars, like the impressions of a
seal.
Back to Full Books
|