“How old are they?” “How can you learn their names from the rocks?”
These are typical examples of questions most frequently asked concerning
fossils. The second question follows the usual reply to the first, for
prehistoric plants and animals are as old as the rocks in which they are
found. The answer, as to age, must come from the rocks and what we have
learned about them through many years of hard work, thoughtful
observation, and careful study. Names, however, come from a different
source. Nature, apparently, managed for a long time to carry on without
the use of words. Since man began talking he has had no trouble
inventing names for things which interest him.
Early students of rocks and fossils likewise accomplished a great deal
without being able to date events in terms of years although many of
their efforts and interests centered on the problem of discovering a
continuous sequence of events in the fragments of evidence that had been
uncovered. This relatively simple problem has not been fully worked out,
and some of the breaks in the record are recognized as “time gaps” which
may never be converted into history.
The question of time, expressed in years, has been a puzzle which
attracted some attention even in the earliest days of investigation. Its
solution was attempted by several methods long before there was
sufficient information to make them work satisfactorily, which accounts
in part for the extreme variation in results of the calculations. Even
now it is to be expected that changes will have to be made as long as
pertinent studies are continued. Two of the most promising methods of
investigation in late years have been producing figures which are
surprisingly large. More accuracy than ever before is probably present
in modern estimates but, except for comparatively recent time, there is
yet no way of knowing within a range of millions of years when a
creature lived.
Astronomy and physics were used in early calculations but, although
taken seriously by some geologists, it was soon recognized by others
that certain events revealed by earth history could not be explained
with so short a time allowance as these methods indicated. One of the
first estimates provided a total of only twenty-five millions of years
and included a great stretch of time during which the earth, according
to prevailing theory, was more sun-like than rock-like, a time when
planets were being born and the earth could not have been in its present
physical condition, which is the chief concern of the geologist. Since
those earlier conditions could not have supported life as we know it,
our knowledge of cosmic history renders small service in the study of
fossils.
Among the methods suggested by astronomy and the laws of physics is one
which is based on the probable rate at which the earth cooled from its
molten condition to present temperature. It is believed now that the
heat of the earth is not necessarily due to an original molten state and
that a steady rate of cooling cannot be ascertained. Any figures based
on such procedure, therefore, are discredited today.
The amount of salt in the oceans, and the time required for its
concentration there by natural processes, offers another way of
attacking the problem. It is a well known fact that salt is being added
to the seas at a fairly constant rate; sea water, then, must become
saltier from year to year. The salt comes from rocks exposed on land
surfaces and is transported by the rivers which drain these areas. By
analyzing the river waters it is possible to estimate the amount of salt
annually dumped into the oceans and, also by chemical analysis, it is a
comparatively simple matter to figure the total amount now present in
the oceans. Some recent calculations indicate that thirty-five million
tons of salt are being added each year, and this figure divided into the
total amount for all the years places the age of the oceans at three
hundred sixty millions of years.
However, there are certain other factors which complicate the problem.
For instance, it is known that land areas exposed to surface drainage
have not always been of their present size, and the annual production of
salt by the different types of rocks exposed at various times in the
history of the earth has not always been as it is now. The rocks also
must be older than the oceans, but how much older cannot be determined
by means of figures obtained in this way.
Until the beginning of this century there was little anticipation of a
better measuring stick than one in use at the time which placed its
reliance on the total thickness of the sedimentary deposits and the
length of time required to produce this great accumulation of material
which is known as the geological column. Since the total thickness, or
height of the column, was not accurately known, and with recognized time
gaps to bridge, there was little hope of working out a complete
chronology by this device, but it has supplied highly desirable and
reliable information concerning parts of the record.
The system has been somewhat improved since its earliest use, and one of
its latest applications gives us an age, for known sedimentary rocks, of
nearly half a billion years, this being based on a total thickness of
one hundred miles and an average rate of 880 years for the building up
of one foot of sediments. Its greatest weakness is due to the absence of
a reliable factor to take care of long stretches of time in which the
sedimentary rocks are known to have been subjected to destructive
processes. A yardstick of this character cannot be applied to rocks that
have been destroyed, and there are excellent reasons for believing that
these interruptions may account for several times the lapse of years
indicated by the amount of rock remaining in the column which has been
pieced together.
Following the discovery of radium, however, the present century provided
a new field of knowledge which has contributed greatly to the
measurement of geologic time. The penetrating rays produced by radium
and other radioactive substances are due to extremely slow but violent
disintegration of the material. Uranium and thorium are radioactive
elements which occur in the rocks of many parts of the world. There is
little or no loss of material as the so-called disintegration proceeds;
instead there is a complicated series of transformations in which other
elements are produced, radium itself being one of these. Helium and lead
eventually take the place of the less stable elements and the known rate
at which these products accumulate provides the highly desired key to
the age of the rocks.
Part of the gas, helium, may escape, but except in rare instances where
chemical alteration might occur, there probably is no loss of lead.
Fortunately, when this metal is produced by radioactivity it differs
slightly in atomic weight from ordinary lead; otherwise the presence of
the latter would introduce a misleading factor. Since the speed at which
the change goes on cannot be increased or decreased, it is assumed that
throughout past ages it has never been faster or slower. The amount of
such change that has been completed in any body of radioactive minerals
may be measured by techniques employed in physics and chemistry. If it
is found that the amount of helium or lead present requires a hundred
million years for its production at the working speed of the parent
elements, the mineral deposit must be at least that old.
Certain conditions of course complicate the problem seriously: knowing
the age of a piece of rock which happens to contain some radioactive
element is of small service in historical studies unless the rock can be
definitely associated with a flora or fauna, or some outstanding event
disclosed by geological investigations. But there have been a few
instances in which most of the necessary conditions were present, and
more and better opportunities to apply this method will no doubt appear.
Other elements, or their radioactive isotopes, are already being
employed with good results. Some of these, such as carbon 14, are more
sensitive indicators for the accurate dating of events in comparatively
recent time.
When it can be used, this type of measurement is far less subject to
uncertainties than any other. It promises to eliminate all need for
guessing, and comes close to a degree of accuracy which is satisfactory
to the scientist, a person who thoroughly dislikes uncertainties of any
kind. If suitable material can be found in just the right places it
should accomplish what the preceding method cannot do—the accurate
measurement of the great time breaks which interrupt the geological
record in many places. Something along this line already has been
accomplished, for radioactive material has been found in some of the
oldest of the rocks. Regardless of the destruction going on in other
localities, these rocks have continued to register the passing of time,
and a tremendous antiquity for the earth and some of its first
inhabitants has been indicated.
Tests made on radioactive minerals from Gilpin County, Colorado, have
established the age of late Cretaceous or early Cenozoic rocks at sixty
million years, providing a convenient and reasonably accurate date for
the beginning of the Age of Mammals. In Russia, one of the oldest
mineral deposits yet studied in this way and regarded as early
Pre-Cambrian, produced the astonishing figure of 1,850,000,000 years;
what we commonly refer to as geological history may therefore be
regarded as covering a range of approximately two billions of years. The
earth, in some form or other, has in all probability passed through an
earlier history of another billion years or more.
Wherever we may roam, a portion of the prehistoric record is to be found
in the rocks underfoot and not far from the surface. Formations as
already mentioned may be regarded as the pages—often torn and badly
scattered—of nature’s own book, in which the geological periods are
chapters. But instead of numbering these pages and chapters we have
_named_ them, in order to get the parts reassembled in orderly fashion
and restored to a condition which makes the book legible. However, the
names cannot render the service intended except in connection with a
time chart and an outline of earth history.
[Illustration: GEOLOGICAL TIME
Figures to the left denote millions of years that have elapsed up to
recent time]
CENOZOIC
Age of Man
RECENT Man and his Culture
1 PLEISTOCENE Last of Mammoths & Mastodons
Age of Mammals
7 PLIOCENE Horses modernized
20 MIOCENE Grasses and Grazing Animals
Three-toed Horses, Rhinos, Camels
35 OLIGOCENE Specialization of Primitive Ancestors
60 EOCENE Decline of archaic types
Mammals flourishing
MESOZOIC
Age of Reptiles
125 CRETACEOUS Last of Great Reptiles
Specialization of Dinosaurs
160 JURASSIC Bony Fishes thriving
Flowering plants advance
Cycads
Birds and Flying Reptiles
200 TRIASSIC Few small mammals of lower orders
Dinosaurs become prominent
PALEOZOIC
Age of Amphibians
225 PERMIAN Reptiles advancing
Amphibians dominant insects
300 CARBONIFEROUS Dense forests of spore-bearing plants
Age of Fishes
350 DEVONIAN Shark-like Fishes
Land floras established
375 SILURIAN First land animals (scorpions)
Armored Fishes prominent
Age of Invertebrates
425 ORDOVICIAN Corals and Bryozoa
Progress among Mollusks
500 CAMBRIAN Brachiopods gaining
Trilobites dominant
Advance of shelled animals
PROTEROZOIC
EARLIEST LIFE
1000 UPPER PRE-CAMBRIAN Small marine invertebrates
Lowest Forms of Plant and Animal Life
Few Fossils
ARCHEOZOIC
2000 LOWER PRE-CAMBRIAN Some chemical evidence of life
No fossils
Such aids have been devised and revised from time to time. No figures
have been offered as final or absolutely “right” since the beginning of
scientific investigations. Time divisions have been proposed that are
not yet in common use while others have been abandoned or modified.
Sources of information are so numerous that appropriate credit cannot be
given fairly for anything that is up-to-date. The combined chart and
outline here provided is based on time calculations of recent date but
with figures slightly rounded off for the sole purpose of making them
easier to remember. In view of the still existent probability of error
it is felt that the slight alteration of figures may justify itself. It
need not be regarded as misleading if the present purpose be
considered—the stimulation of a natural history interest which is not
vitally concerned with the little difference between a thousand million
years and nine hundred ninety-nine million years.