THE GEOLOGICAL RECORD
=What a formation records.= We have already learned that each
individual body of stratified rock, or formation, constitutes a record
of the time when it was laid. The structure and the character of the
sediments of each formation tell whether the area was land or sea at
the time when they were spread; and if the former, whether the land
was river plain, or lake bed, or was covered with wind-blown sands, or
by the deposits of an ice sheet. If the sediments are marine, we may
know also whether they were laid in shoal water near the shore or in
deeper water out at sea, and whether during a period of emergence, or
during a period of subsidence when the sea transgressed the land. By
the same means each formation records the stage in the cycle of
erosion of the land mass from which its sediments were derived (p.
185). An unconformity between two marine formations records the fact
that between the periods when they were deposited in the sea the area
emerged as land and suffered erosion (p. 227). The attitude and
structure of the strata tell also of the foldings and fractures,
the deformation and the metamorphism, which they have suffered; and
the igneous rocks associated with them as lava flows and igneous
intrusions add other details to the story. Each formation is thus a
separate local chapter in the geological history of the earth, and its
strata are its leaves. It contains an authentic record of the physical
conditions--the geography--of the time and place when and where its
sediments were laid.
=Past cycles of erosion.= These chapters in the history of the planet
are very numerous, although much of the record has been destroyed in
various ways. A succession of different formations is usually seen in
any considerable section of the crust, such as a deep canyon or where
the edges of upturned strata are exposed to view on the flanks of
mountain ranges; and in any extensive area, such as a state of the
Union or a province of Canada, the number of formations outcropping on
the surface is large.
It is thus learned that our present continent is made up for the most
part of old continental deltas. Some, recently emerged as the strata
of young coastal plains, are the records of recent cycles of erosion;
while others were deposited in the early history of the earth, and in
many instances have been crumpled into mountains, which afterwards
were leveled to their bases and lowered beneath the sea to receive a
cover of later sediments before they were again uplifted to form land.
The cycle of erosion now in progress and recorded in the layers of
stratified rock being spread beneath the sea in continental deltas has
therefore been preceded by many similar cycles. Again and again
movements of the crust have brought to an end one cycle--sometimes
when only well under way, and sometimes when drawing toward its
close--and have begun another. Again and again they have added to the
land areas which before were sea, with all their deposition records of
earlier cycles, or have lowered areas of land beneath the sea to
receive new sediments.
=The age of the earth.= The thickness of the stratified rocks now
exposed upon the eroded surface of the continents is very great. In
the Appalachian region the strata are seven or eight miles thick, and
still greater thicknesses have been measured in several other mountain
ranges. The aggregate thickness of all the formations of the
stratified rocks of the earth's crust, giving to each formation its
maximum thickness wherever found, amounts to not less than forty
miles. Knowing how slowly sediments accumulate upon the sea floor
(p. 184), we must believe that the successive cycles which the earth
has seen stretch back into a past almost inconceivably remote, and
measure tens of millions and perhaps even hundreds of millions of
years.
=How the formations are correlated and the geological record made up.=
Arranged in the order of their succession, the formations of the
earth's crust would constitute a connected record in which the
geological history of the planet may be read, and therefore known as
the _geological record_. But to arrange the formations in their
natural order is not an easy task. A complete set of the volumes of
the record is to be found in no single region. Their leaves and
chapters are scattered over the land surface of the globe. In one area
certain chapters may be found, though perhaps with many missing
leaves, and with intervening chapters wanting, and these absent parts
perhaps can be supplied only after long search through many other
regions.
Adjacent strata in any region are arranged according to the _law of
superposition_, i.e. any stratum is younger than that on which it was
deposited, just as in a pile of paper, any sheet was laid later than
that on which it rests. Where rocks have been disturbed, their
original attitude must be determined before the law can be applied.
Nor can the law of superposition be used in identifying and comparing
the strata of different regions where the formations cannot be traced
continuously from one region to the other.
The formations of different regions are arranged in their true order
by the _law of included organisms_; i.e. formations, however widely
separated, which contain a similar assemblage of fossils are
equivalent and belong to the same division of geological time.
The correlation of formations by means of fossils may be explained by
the formations now being deposited about the north Atlantic.
Lithologically they are extremely various. On the continental shelf of
North America limestones of different kinds are forming off Florida,
and sandstones and shales from Georgia northward. Separated from them
by the deep Atlantic oozes are other sedimentary deposits now
accumulating along the west coast of Europe. If now all these offshore
formations were raised to open air, how could they be correlated?
Surely not by lithological likeness, for in this respect they would be
quite diverse. All would be similar, however, in the fossils which
they contain. Some fossil species would be identical in all these
formations and others would be closely allied. Making all due
allowance for differences in species due to local differences in
climate and other physical causes, it would still be plain that plants
and animals so similar lived at the same period of time, and that the
formations in which their remains were imbedded were contemporaneous
in a broad way. The presence of the bones of whales and other marine
mammals would prove that the strata were laid after the appearance of
mammals upon earth, and imbedded relics of man would give a still
closer approximation to their age. In the same way we correlate the
earlier geological formations.
For example, in 1902 there were collected the first fossils ever found
on the antarctic continent. Among the dozen specimens obtained were
some fossil ammonites (a family of chambered shells) of genera which
are found on other continents in certain formations classified as the
Cretaceous system, and which occur neither above these formations nor
below them. On the basis of these few fossils we may be confident that
the strata in which they were found in the antarctic region were laid
in the same period of geologic time as were the Cretaceous rocks of
the United States and Canada.
=The record as a time scale.= By means of the law of included
organisms and the law of superposition the formations of different
countries and continents are correlated and arranged in their natural
order. When the geological record is thus obtained it may be used as a
universal time scale for geological history. Geological time is
separated into divisions corresponding to the times during which the
successive formations were laid. The largest assemblages of formations
are known as groups, while the corresponding divisions of time are
known as eras. Groups are subdivided into systems, and systems into
series. Series are divided into stages and substages,--subdivisions
which do not concern us in this brief treatise. The corresponding
divisions of time are given in the following table.
_Strata_ _Time_
Group Era
System Period
Series Epoch
The geologist is now prepared to read the physical history--the
geographical development--of any country or of any continent by means
of its formations, when he has given each formation its true place in
the geological record as a time scale.
The following chart exhibits the main divisions of the record, the
name given to each being given also to the corresponding time
division. Thus we speak of the _Cambrian system_, meaning a certain
succession of formations which are classified together because of
broad resemblances in their included organisms; and of the _Cambrian
period_, meaning the time during which these rocks were deposited.
_Group and Era_ _System and Period_ _Series and Epoch_
{ Recent
{ Quaternary . . . . { Pleistocene
{
Cenozoic . . . . { { Pliocene
{ Tertiary . . . . { Miocene
{ Eocene
{ Cretaceous
Mesozoic . . . . { Jurassic
{ Triassic
{ Permian
{ Carboniferous . . { Pennsylvanian
{ { Mississippian
Paleozoic . . . . { Devonian
{ Silurian
{ Ordovician
{ Cambrian
Algonkian
Archean
Fossils and what they teach
The geological formations contain a record still more important than
that of the geographical development of the continents; the fossils
imbedded in the rocks of each formation tell of the kinds of animals
and plants which inhabited the earth at that time, and from these
fossils we are therefore able to construct the history of life upon
the earth.
=Fossils.= These remains of organisms are found in the strata in all
degrees of perfection, from trails and tracks and fragmentary
impressions, to perfectly preserved shells, wood, bones, and complete
skeletons. As a rule, it is only the hard parts of animals and plants
which have left any traces in the rocks. Sometimes the original hard
substance is preserved, but more often it has been replaced by some
less soluble material. Petrifaction, as this process of slow
replacement is called, is often carried on in the most exquisite
detail. When wood, for example, is undergoing petrifaction, the woody
tissue may be replaced, particle by particle, by silica in solution
through the action of underground waters, even the microscopic
structures of the wood being perfectly reproduced. In shells
originally made of _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 ever-changing._ 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 zoölogical 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
subkingdom 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.