[A] Read to the American Association for the Advancement of
Science, at New York, June 26, 1900, as the address of the
retiring President.
Custom dictates that in complying with the rule of the association I
shall address you on some subject of a scientific character. But before
doing so I may be permitted to pay my personal tribute to the honored
and cherished leader of whose loss we are so keenly sensible on this
occasion. His kindly personality, the charm which his earnestness and
sincerity gave to his conversation, the range of his accomplishment,
are inviting themes; but it is perhaps more fitting that I touch
this evening on his character as a representative president of this
body. The association holds a peculiar position among our scientific
organizations of national or continental extent. Instead of narrowing
its meetings by limitations of subject matter or membership, it
cultivates the entire field of research and invites the interest and
coöperation of all. It is thus not only the integrating body for
professional investigators, but the bond of union between these and the
great group of cultured men and women--the group from whose ranks the
professional guild is recruited, through whom the scientific spirit
is chiefly propagated, and through whose interest scientific research
receives its financial support. Its aims and form of organization
recognize, what pure science does not always itself recognize, that
pure science is fundamentally the creature and servant of the material
needs of mankind, and it thus stands for what might be called the
human side of science. Edward Orton, throughout his career as teacher
and investigator, was conspicuous for his attention to the human side
of science. His most abstract work was consciously for the benefit
of the community, and he ever sought opportunity to make its results
directly available. In promoting the interests of the people of his
adopted State he incidentally accomplished much for a larger community
by helping it to an appreciation of the essential beneficence of the
scientific study of nature and man. As an individual he was a diligent
and successful laborer in the field which the association cultivates,
and when the association selected him as its standard-bearer it made
choice of one who was peculiarly its representative.
The subject to which I shall invite your attention this evening is by
no means novel, but might better be called perennial or recurrent; for
the problem of our earth’s age seems to bear repeated solution without
loss of vigor or prestige. It has been a marked favorite, moreover,
with presidents and vice-presidents, retiring or otherwise, when called
upon to address assemblies whose fields of scientific interest are
somewhat diverse--for the reason, I imagine, that while the specialist
claims the problem as his peculiar theme of study, he feels that other
denizens of the planet in question may not lack interest in the early
lore of their estate.
The difficulty of the problem inheres in the fact that it not only
transcends direct observation, but demands the extrapolation or
extension of familiar physical laws and processes far beyond the
ordinary range of qualifying conditions. From whatever side it is
approached, the way must be paved by postulates, and the resulting
views are so discrepant that impartial onlookers have come to be
suspicious of these convenient and inviting stepping stones.
That vain expectation may not be aroused, I admit at the outset that
I have not solved the problem and shall submit to you no estimates.
My immediate interest is in the preliminary question of the available
methods of approach, and it leads to the consideration of the ways,
or the classes of ways, in which the measurement of time has been
accomplished or attempted.
Of the artificial devices employed in practical horology there are two
so venerable that their origins are lost in the obscurity of legendary
myth. These are the clepsydra and the taper. In the clepsydra advantage
is taken of the approximately uniform rate at which water escapes
through a small orifice, and time is measured by gaging the loss of
water from a discharging vessel or the gain in a receiving vessel.
The hour-glass is one of its latest forms, in which sand takes the
place of water. The taper depends for its value as a timepiece on the
approximate uniformity of combustion when the area of fuel exposed to
the air is definitely regulated. It survives chiefly in the prayer
stick and safety fuse, but the graduated candle is perhaps still used
to regulate monastic vigils.
The pendulum, a comparatively modern invention, excelling the clepsydra
and taper in precision, has altogether supplanted them as the servant
of civilization. Its accuracy results from the remarkable property
that the period in which it completes an oscillation is almost exactly
the same, whatever the arc through which it swings. It regulates the
movements not only of our clocks, watches and chronometers, but of
barographs, thermographs and a great variety of other machines for
recording events and changes in their proper order and relation in
respect to time.
I must mention also a special apparatus invented by astronomers and
called a chronograph. It consists ordinarily of a revolving drum about
which a paper is wrapped and against which rests a pen. As the drum
turns the pen draws a line on the paper. Through an electric circuit
the pen is brought under the influence of a pendulum in such a way
that at the middle of each swing of the pendulum the pen is deflected,
making a mark at right angles to the straight line. The series of
marks thus drawn constitutes a time scale. The electric arrangements
are so made that the pen will also be disturbed in consequence of
some independent event, such as the firing of a gun or the transit of
a star; and the mark caused by such disturbance, being automatically
platted on the time scale, records the time of the event.
No attempt has been made to characterize these various timepieces with
fullness, because they are already well known to most of those present,
and, in fact, the chief motive for giving them separate mention is that
they may serve as the basis of a classification. In the use of the
clepsydra and taper, time is measured in terms of a continuous movement
or process; in the use of the pendulum time is measured in terms of a
movement which is periodically reversed. The classification embodies
the fundamental distinction between continuous motion and rhythmic
motion.
Passing now from the artificial to the natural measures of time, we
find that they are all rhythmic. It is true that the spinning of the
earth on its axis is in itself a continuous motion, but it would
yield no time measure if the earth were alone in space, and so soon
as the motion is considered in relation to some other celestial body
it becomes rhythmic. As viewed from, or compared with, a fixed star,
the period of its rhythm is the sidereal day; compared with the sun,
it is the solar day, nearly four minutes longer; and compared with
the moon, it is the lunar day, still longer by 49 minutes. As the
sun supplies the energy for most of the physical and all the vital
processes of the earth’s surface, the rhythm of the solar day is
impressed in multitudinous ways on man and his environment, and he
makes it his primary or standard unit of time. He has arbitrarily
divided it into hours, minutes and seconds, and in terms of these units
he says that the length of the sidereal day is a little more than 23
hours, 56 minutes and 4 seconds, and the average length of the lunar
day is a little less than 24 hours and 49 minutes. The lunar day finds
expression in the tides and is of moment to maritime folk, but the
sidereal is known only to astronomers.
Next in the series of our natural time units is the month, or the
rhythmic period of the moon regarded as a luminary. By our savage
ancestors, who credited the moon with powers of great importance
to themselves, much use was made of this unit, but as progress in
knowledge has shown that the influence of the satellite had been vastly
overrated, less and less attention has been paid to the returning
crescent, and it is only in ecclesiastic calendars that the chronology
of civilization now recognizes the natural month. Its shadow survives,
without the substance, in the calendar month; and the week possibly
represents an early attempt to subdivide it.
In passing to our third natural unit, the year, we again encounter
solar influence, and find the rhythm of the earth’s orbit echoed and
reechoed in innumerable physical and vital vibrations. As the attitude
of the earth’s axis inclines one hemisphere toward the sun for part
of the year and the other hemisphere for the remainder, the whole
complex drama of climate is annually enacted, and the sequence of man’s
activities is made to assume an annual rhythm. The year is second only
to the day as a terrestrial unit of duration; and as the day is man’s
standard for the minute division of time, so the year is his standard
for larger divisions, and the decade, the century and the millenium are
its multiples.
But the rhythms of day and night, of summer and winter, are not
the only tides in the affairs of men. At birth we are small, weak
and dependent, we grow larger and stronger, we become mature and
independent, and then by reproducing our kind we complete the cycle,
which begins again with our children. The cycle of human life is the
_generation_, a time unit of somewhat indefinite length and varying
in phase from family to family, but holding a place, nevertheless, in
human chronology.
Still less definite is the rhythm of hereditary rulership, progressing
from vigor through luxury to degeneracy, and closing its cycle in
usurpation; yet it makes an epoch in the life of a nation or empire,
and so the _dynasty_ is one of the units of the historian.
The generation and the dynasty are of waning importance in human
chronology, and they can claim no connection with the problem of
geologic time; but here again I have turned aside for a moment in order
to illustrate a principle of classification. The daily rhythm of waking
and sleeping, of activity and rest, does not originate with man, but is
imposed on him by the rhythm of light and darkness, and that in turn
springs from the turning of the earth in relation to the shining sun.
The yearly rhythm of sowing and harvesting, of the fan and the furnace,
does not originate with man, but is imposed on him by the rhythm of the
seasons, and that in turn springs from certain motions of the earth in
relation to the glowing sun. But the rhythm of the generation and the
rhythm of the dynasty have origin in the nature of man himself. The
rhythms of human chronology may thus be grouped according to source in
two classes, the _imposed_ and the _original_; and the same distinction
holds for other rhythms. The lunar day is an original rhythm of the
earth as seen from the moon; the ground swell is an original rhythm
of the ocean; but the tide is an imposed rhythm of the ocean, being
derived from the lunar day. The swing of the pendulum is an original
rhythm, but the regular excursion of the chronograph pen, being caused
by the swing of the pendulum, is an imposed rhythm.
In giving brief consideration to each of the more important ways by
which the problem of the earth’s age has been approached, I shall
mention first those which follow the action of some continuous process,
and afterward those which depend on the recognition of rhythms.
The earliest computations of geologic time, as well as the majority of
all such computations, have followed the line of the most familiar and
fundamental of geologic processes. All through the ages the rains, the
rivers and the waves have been eating away the land, and the product
of their gnawing has been received by the sea and spread out in layers
of sediment. These layers have been hardened into rocky strata, and
from time to time portions have been upraised and made part of the
land. The record they contain makes the chief part of geologic history,
and the groups into which they are divided correspond to the ages and
periods of that history. In order to make use of these old sediments
as measures of time it is necessary to know either their thickness
or their volume, and also the rate at which they were laid down. As
the actual process of sedimentation is concealed from view, advantage
is taken of the fact that the whole quantity deposited in a year is
exactly equalled by the whole quantity washed from the land in the same
time, and measurements and estimates are made of the amounts brought to
the sea by rivers and torn from the cliffs of the shore by waves. After
an estimate has been obtained of the total annual sedimentation at the
present time, it is necessary to assume either that the average rate in
past ages has been the same or that it has differed in some definite
way.
At this point the course of procedure divides. The computer may
consider the aggregate amount of the sedimentary rocks, irrespective
of their subdivisions, or he may consider the thicknesses of the
various groups as exhibited in different localities. If he views the
rocks collectively, as a total to be divided by the annual increment,
his estimate of the total is founded primarily on direct measurements
made at many places on the continents, but to the result of such
measurements he must add a postulated amount for the rocks concealed
by the ocean, and another postulated amount for the material which has
been eroded from the land and deposited in the sea more than once.
If, on the other hand, he views each group of rocks by itself, and
takes account of its thickness at some locality where it is well
displayed, he must acquire in some way definite conceptions of the
rates at which its component layers of sand, clay and limy mud were
accumulated, or else he must postulate that its average rate of
accretion bore some definite ratio to the present average rate of
sedimentation for the whole ocean. This course is, on the whole, more
difficult than the other, but it has yielded certain preliminary
factors in which considerable confidence is felt. Whatever may have
been the absolute rate of rock building in each locality, it is
believed that a group of strata which exhibits great thickness in many
places must represent more time than a group of similar strata which is
everywhere thin, and that clays and marls, settling in quiet waters,
are likely to represent, foot for foot, greater amounts of time than
the coarser sediments gathered by strong currents; and studying the
formations with regard to both thickness and texture, geologists have
made out what are called _time ratios_--series of numbers expressing
the relative lengths of the different ages, periods and epochs. Such
estimates of ratios, when made by different persons, are found to vary
much less than do the estimates of absolute time, and they will serve
an excellent purpose whenever a satisfactory determination shall have
been made of the duration of any one period.
Reade has varied the sedimentary method by restricting attention to the
limestones, which have the peculiarity that their material is carried
from the land in solution; and it is a point in favor of this procedure
that the dissolved burdens of rivers are more easily measured than
their burdens of clay and sand.
An independent system of time ratios has been founded on the principle
of the evolution of life. Not all formations are equally supplied with
fossils, but some of them contain voluminous records of contemporary
life; and when account is taken of the amount of change from each full
record to the next, the steps of the series are found to be of unequal
magnitude. Though there is no method of precisely measuring the steps,
even in a comparative way, it has yet been found possible to make
approximate estimates, and these in the main lend support to the time
ratios founded on sedimentation. They bring aid also at a point where
the sedimentary data are weak, for the earliest formations are hard to
classify and measure. It is true that these same formations are almost
barren of fossils, but biologic inference does not therefore stop. The
oldest known fauna, the Eocambrian, does not represent the beginnings
of life, but a well-advanced stage, characterized by development along
many divergent lines; and by comparing Eocambrian life with existing
life the paleontologist is able to make an estimate of the relative
progress in evolution before and after the Eocambrian epoch. The only
absolute blank left by the time ratios pertains to an azoic age which
may have intervened between the development of a habitable earth crust
and the actual beginning of life.
Erosion and deposition have been used also, in a variety of ways,
to compute the length of very recent geologic epochs. Thus, from
the accumulation of sand in beaches Andrews estimated the age of
Lake Michigan, and Upham the age of the glacial lake Agassiz; and
from the erosion of the Niagara gorge the age of the river flowing
through it has been estimated. But while these discussions have
yielded conceptions of the nature of geologic time, and have served to
illustrate the extreme complexity of the conditions which affect its
measurement, they have accomplished little toward the determination
of the length of a geologic period; for they have pertained only to a
small fraction of what geologists call a period, and that fraction was
of a somewhat abnormal character.
Wholly independent avenues of approach are opened by the study of
processes pertaining to the earth as a planet, and with these the name
of Kelvin is prominently associated.
As the rotation of the earth causes the tides, and as the tides expend
energy, the tides must act as a brake, checking the speed of rotation.
Therefore the earth has in the past spun faster than now, and its rate
of spinning at any remote point of time may be computed. Assuming that
the whole globe is solid and rigid, and that the geologic record could
not begin until that condition had been attained, there could not have
been great checking of rotation since consolidation. For if there had
been, it would have resulted in the gathering of the oceans about the
poles and the baring of the land near the equator, a condition very
different from what actually obtains. This line of reasoning yields an
obscure outer limit to the age of the earth.
On the assumption that the globe lacks something of perfect rigidity,
G. H. Darwin has traced back the history of the earth and the moon to
an epoch when the two bodies were united, their separation having been
followed by the gradual enlargement of the moon’s orbit and the gradual
retardation of the earth’s rotation; and this line of inquiry has also
yielded an obscure outer limit to the antiquity of the earth as a
habitable globe.
One of the most elaborate of all the computations starts with the
assumption that at an initial epoch, when the outer part of the earth
was consolidated from a liquid condition, the whole body of the planet
had approximately the same temperature; and that as the surface
afterward cooled by outward radiation there was a flow of heat to the
surface by conduction from below. The rate of this flow has diminished
from that epoch to the present time according to a definite law, and
the present rate, being known from observation, affords a measure of
the age of the crust. The strength of this computation lies in its
definiteness and the simplicity of its data; its weakness in the fact
that it postulates a knowledge of certain properties of rock--namely,
its fusibility, conductivity and viscosity--when subjected to pressures
and temperatures far greater than have ever been investigated
experimentally.
A parallel line of discussion pertains to the sun. Great as is the
quantity of heat which that incandescent globe yields to the earth, it
is but a minute fraction of the whole amount with which it continually
parts, for its radiation is equal in all directions, and the earth is
but a speck in the solar sky. On the assumption that this immense loss
of heat is accompanied by a corresponding loss of volume, the sun is
shrinking at a definite rate, and a computation based on this rate has
told how many millions of years ago the sun’s diameter should have been
equal to the present diameter of the earth’s orbit. Manifestly the
earth can not have been ready for habitation before the passage of that
epoch, and so the computation yields a superior limit to the extent of
geologic time.
Before passing to the next division of the subject--the computations
based on rhythms--a few words may be given to the results which have
been obtained from the study of continuous processes. Realizing that
your patience may have been strained by the kaleidoscopic character of
the rapid review which has seemed unavoidable, I shall spare you the
recitation of numerical details and merely state in general terms that
the geologists, or those who have reasoned from the rocks and fossils,
have deduced values for the earth’s age very much larger than have
been obtained by the physicists, or those who have reasoned from earth
cooling, sun cooling and tidal friction. In order to express their
results in millions of years the geologists must employ from three
to five digits, while the physicists need but one or two. When these
enormous discrepancies were first realized it was seen that serious
errors must exist in some of the observational data or else in some of
the theories employed; and geologists undertook with zeal the revision
of their computations, making as earnest an effort for reconciliation
as had been made a generation earlier to adjust the elements of the
Hebrew cosmogony to the facts of geology. But after rediscussing the
measurements and readjusting the assumptions so as to reduce the time
estimates in every reasonable way--and perhaps in some that were not so
reasonable--they were still unable to compress the chapters of geologic
history between the narrow covers of physical limitation; and there
the matter rests for the present.
The rocks which were formed as sediments show many traces of rhythm.
Some are composed of layers, thin as paper, which alternate in color,
so that when broken across they exhibit delicate banding. In the time
of their making there was a periodic change in the character of the mud
that settled from the water. Others are banded on a larger scale; and
there are also bandings of texture where the color is uniform. Many
formations are divided into separate strata, as though the process of
accretion had been periodically interrupted. Series of hard strata are
often separated by films or thin layers of softer material. Strata of
two kinds are sometimes seen to alternate through many repetitions.
Borings in the delta of the Mississippi show soils and remains of trees
at many levels, alternating with river silts. The rock series in which
coal occurs are monotonous repetitions of shale and sandstone. Belgian
geologists have been so impressed by the recurrence of short sequences
of strata that they have based an elaborate system of rock notation
upon it.
Passing to still greater units, the large aggregates of strata
sometimes called systems show in many cases a regular sequence, which
Newberry called a “circle of deposition.” When complete, it comprises
a sandstone or conglomerate, at base, then shale, limestone, shale and
sandstone. This sequence is explained as the result of the gradual
encroachment, or transgression, as it is called, of the sea over the
land and its subsequent recession.
In certain bogs of Scandinavia deep accumulations of peat are traversed
horizontally by layers including tree stumps in such way as to indicate
that the ground has been alternately covered by forest and boggy
moss. The broad glaciers of the Ice age grew alternately smaller
and larger--or else were repeatedly dissipated and reformed--and
their final waning was characterized by a series of halts or partial
readvances, recorded in concentric belts of ice-brought drift. Of these
belts, called moraines of recession, Taylor enumerates seventeen in a
single system.
In explanation of these and other repetitive series incorporated in the
structure of the earth’s crust, a variety of rhythmic causes have been
adduced; and mention will be made of the more important, beginning with
those which have the character of original rhythms.
A river flowing through its delta clogs its channel with sediment,
and from time to time shifts its course to a new line, reaching the
sea by a new mouth. Such changes interrupt and vary sedimentation in
neighboring parts of the sea. Storms of rain make floods, and each
flood may cause a separate stratum of sediment. Storms of wind give
destructive force to the waves that beat the shore, and each storm
may cause the deposit of an individual layer of sediment. Varying
winds may drive currents this way and that, causing alternations in
sedimentation.
To explain the forest beds buried in the Mississippi silts it has
been suggested that the soft deposits of the delta from time to time
settled and spread out under their own weight. Various alternations of
strata, and especially those of the coal measures, have been ascribed
to successive local subsidences of the earth’s crust, caused by the
addition of loads of deposit. It has been suggested also that land
undergoing erosion may rise up from time to time because relieved
of load, and the character of sediment might be changed by such
rising. Subterranean forces, of whatever origin, seemingly slumber
while strains are accumulating, and then become suddenly manifest in
dislocations and eruptions, and such catastrophes affect sedimentation.
A more general rhythm has been ascribed to the tidal retardation of
rotation and the resulting change of the earth’s form. If the body
of the earth has a rather high rigidity, we should expect that it
would for a time resist the tendency to become more nearly spherical,
while the water of the ocean would accommodate itself to the changing
conditions of equilibrium by seeking the higher latitudes. Eventually,
however, the solid earth would yield to the strain and its figure
become adjusted to the slower rotation, and then the mobile water
would return. Thus would be caused periodic transgressions by the sea,
occurring alternately in high and low latitudes.
Another general rhythm has been recently suggested by Chamberlin in
connection with the hypothesis that secular variations of climate are
chiefly due to variations of the quantity of carbon dioxide in the
atmosphere.[B] The system of interdependent factors he works out is too
complex for presentation at this time, and I must content myself with
saying that his explanation of the moraines of recession involves the
interaction of a peculiar atmospheric condition with a condition of
glaciation, each condition tending to aggravate the other, until the
cumulative results brought about a reaction and the climatic pendulum
swung in the opposite direction. With each successive oscillation the
momentum was less, and an equilibrium was finally reached.
[B] An attempt to frame a working hypothesis of the cause of
glacial periods on an atmospheric basis. _Journ. Geol._,
Vol. VII., 1899.
Few of these original rhythms have been used in computations of
geologic time, and it is not believed that they have any positive
value for that purpose. Nevertheless, account must be taken of them,
because they compete with imposed rhythms for the explanation of
many phenomena, and the imposed rhythms, wherever established, yield
estimates of time.
The tidal period, or the half of the lunar day, is the shortest
imposed rhythm appealed to in the explanation of the features of
sedimentation. It is quite conceivable that the bottom of a quiet
bay may receive at each tide a thin deposit of mud which could be
distinguished in the resulting rock as a papery layer or lamina. If one
could in some way identify a rock thus formed, he might learn how many
half-days its making required by counting its laminæ, just as the years
of a tree’s age are learned by counting its rings of growth.
The next imposed rhythm of geologic importance is the year. There are
rivers, like the Nile, having but one notable flood in each year,
and so depositing annual layers of sediment on their alluvial plains
and on the sea beds near their mouths. Where oceanic currents are
annually reversed by monsoons, sedimentation may be regularly varied,
or interrupted, once a year. Streams from a glacier cease to run in
winter, and this annual interruption may give a definite structure to
resulting deposits. It is therefore probable that some of the laminæ
or strata of rocks represent years, but the circumstances are rarely
such that the investigator can bar out the possibility that part of
the markings or separations were caused by original rhythms of unknown
period.
The number of rhythms existing in the solar system is very large, but
there are only two, in addition to the two just mentioned, which seem
competent to write themselves in a legible way in the geologic record.
These are the rhythms of precession and eccentricity.
Because the earth’s orbit is not quite circular and the sun’s position
is a little out of the center, or eccentric, the two hemispheres into
which the earth is divided by the equator do not receive their heat
in the same way. The northern summer, or the period during which
the northern hemisphere is inclined toward the sun, occurs when the
earth is farthest from the sun, and the northern winter occurs when
the earth is nearest to the sun, or in that part of the orbit called
perihelion. These relations are exactly reversed for the southern
hemisphere. The general effect of this is that the southern summer is
hotter than the northern, and the southern winter is colder than the
northern. In the southern part of the planet there is more contrast
between summer and winter than in the northern. The sun sends to each
half the same total quantity of heat in the course of a year, but the
difference in distribution makes the climates different. The physics of
the atmosphere is so intricate a subject that meteorologists are not
fully agreed as to the theoretic consequences of these differences of
solar heating, but it is generally believed that they are important,
involving differences in the force of the winds, in the velocity and
course of ocean currents, in vegetation, and in the extent of glaciers.
Now, the point of interest in the present connection is that the
astronomic relations which occasion these peculiarities are not
constant, but undergo a slow periodic change. The relation of the
seasons to the orbit is gradually shifting, so that each season in
turn coincides with the perihelion; and the climatic peculiarities
of the two hemispheres, so far as they depend on planetary motions,
are periodically reversed. The time in which the cycle of change is
completed, or the period of the rhythm, is not always the same, but
averages 21,000 years. It is commonly called the precessional period.[C]
[C] Strictly speaking, 21,000 years is the period of the
precession of the equinoxes as referred to perihelion; but
the perihelion is itself in motion. As referred to a fixed
star the precession of the equinoxes has an average period
of about 25,700 years.
Assuming that the climates of many parts of the earth are subject
to a secular cycle, with contrasted phases every 10,500 years, we
should expect to find records of the cycle in the sediments. A moist
climate would tend to leach the calcareous matter from the rock,
leaving an earthy soil behind, and in a succeeding drier climate the
soil would be carried away; and thus the adjacent ocean would receive
first calcareous and then earthy sediments. The increase of glaciers
in one hemisphere would not only modify adjacent sediments directly,
but, by adding matter on that side, would make a small difference in
the position of the earth’s center of gravity. The ocean would move
somewhat toward the weighted hemisphere, encroaching on some coasts
and drawing down on others; and even a small change of that sort would
modify the conditions of erosion and deposition to an appreciable
extent in many localities.
Blytt ascribed to this astronomic cause the alternations of bog and
forest in Scandinavia, as well as other sedimentary rhythms observed
in Europe; and it has seemed to me competent to account for certain
alternations of strata in the Cretaceous formations of Colorado. Croll
used it to explain interglacial epochs, and Taylor has recently applied
it to the moraines of recession.
The remaining astronomic rhythm of geologic import is the variation of
eccentricity. At the present time our greatest distance from the sun
exceeds our least distance by its thirtieth part, but the difference
is not usually so small as this. It may increase to the seventh part
of the whole distance, and it may fall to zero. Between these limits
it fluctuates in a somewhat irregular way, in which the property of
periodicity is not conspicuous. The effect of its fluctuation is
inseparable from the precessional effect, and is related to it as a
modifying condition. When the eccentricity is large the precessional
rhythm is emphasized; when it is small the precessional effect is weak.
The variation of eccentricity is connected with the most celebrated
of all attempts to determine a limited portion of geologic time. In
the elaboration of the theory of the Ice age which bears his name,
Croll correlated two important epochs of glaciation with epochs of
high eccentricity computed to have occurred about 100,000 and 210,000
years ago. As the analysis of the glacial history progresses, these
correlations will eventually be established or disproved, and should
they be established it is possible that similar correlations may be
made between events far more remote.
The studies of these several rhythms, while they have led to the
computation of various epochs and stages of geologic time, have not yet
furnished an estimate either of the entire age of the earth or of any
large part of it. Nevertheless, I believe that they may profitably be
followed with that end in view.
The system of rock layers, great and small, constituting the record
of sedimentation, may be compared to the scroll of a chronograph. The
geologic scroll bears many separate lines, one for each district where
rocks are well displayed, but these are not independent, for they are
labeled by fossils, and by means of these labels can be arranged in
proper relation. In each time line are little jogs--changes in kind
of rock or breaks in continuity--and these jogs record contemporary
events. A new mountain was uplifted, perhaps, on the neighboring
continent, or an old uplift received a new impulse. Through what Davis
calls stream piracy a river gained or lost the drainage of a tract of
country. Escaping lava threw a dam across the course of a stream, or
some Krakatoa strewed ashes over the land and gave the rivers a new
material to work on. The jogs may be faint or strong, many or few, and
for long distances the lines may run smooth and straight; but so long
as the jogs are irregular they give no clue to time. Here and there,
however, the even line will betray a regularly recurring indentation or
undulation, reflecting a rhythm and possibly significant of a remote
pendulum whose rate of vibration is known. If it can be traced to such
a pendulum there will result a determination of the rate at which the
chronograph scroll moved when that part of the record was made; and
a moderate number of such determinations, if well distributed, will
convert the whole scroll into a definite time scale.
In other words, if a sufficient number of the rhythms embodied in
strata can be identified with particular imposed rhythms, the rates
of sedimentation under different circumstances and at different times
will become known, and eventually so many parts of geologic time will
have become subject to direct calculation that the intervals can be
rationally bridged over by the aid of time ratios.
For this purpose there is only one of the imposed rhythms of practical
value, namely, the precessional; but that one is, in my judgment, of
high value. The tidal rhythm can not be expected to characterize any
thick formation. The annual is liable to confusion with a variety of
original rhythms, especially those connected with storms. The rhythm
of eccentricity, being theoretically expressed only as an accentuation
of the precessional, can not ordinarily be distinguished from it. But
none of these qualifications apply to the precessional. It is not
liable to confusion with the tidal and annual because its period is
so much longer, being more than 2,000 times that of the annual. It has
an eminently practical and convenient magnitude, in that its physical
manifestation is well above the microscopic plane, and yet not so large
as to prevent the frequent bringing of several examples into a single
view. It is also practically regular in period, rarely deviating from
the average length by more than the tenth part.
From the greater number of original rhythms it is distinguished, just
as from the annual and tidal, by magnitude. The practical geologist
would never confuse the deposit occasioned by a single storm, for
example, with the sediments accumulated during an astronomic cycle
of 20,000 years. But there are other original rhythms, known or
surmised, which might have magnitudes of the same general order, and
to discriminate the precessional from these it is necessary to employ
other characters. Such characters are found in its regularity or
evenness of period, and in its practical perpetuity. The diversion of
the mouth of a great river such as the Hoang Ho or the Mississippi
might recur only after long intervals; but from what we know of the
behavior of smaller streams we may be sure that such events would be
very irregular in time as well as in other ways. The intervals between
volcanic eruptions at a particular vent or in a particular district
may at times amount to thousands of years, but their irregularity is
a characteristic feature. The same is true of the recurrent uplifts
by which mountains grow, so far as we may judge them by the related
phenomena of earthquakes; and the same category would seem to hold also
the theoretically recurrent collapse of the globe under the strains
arising from the slowing of rotation. The carbon-dioxide rhythm,
known as yet only in the field of hypothesis, is hypothetically a
running-down oscillation, like the lessening sway of the cradle when
the push is no longer given.
But the precessional motion pulses steadily on through the ages, like
the swing of a frictionless pendulum. Its throb may or may not be
caught by the geologic process which obtains in a particular province
and in a particular era, but whenever the conditions are favorable and
the connection is made, the record should reflect the persistence and
the regularity of the inciting rhythm.
The search of the rocks for records of the ticks of the precessional
clock is an out-of-door work. Pursued as a closet study it could have
no satisfactory outcome, because the printed descriptions of rock
sequences are not sufficiently complete for the purpose; and the closet
study of geology is peculiarly exposed to the perils of hobby-riding.
A student of the time problem cannot be sure of a persistent, equable
sedimentary rhythm without direct observation of the characters of
the repeated layers. He needs to avail himself of every opportunity
to study the series in its horizontal extent, and he should view the
local problem of original _versus_ imposed rhythm with the aid of
all the light which the field evidence can cast on the conditions of
sedimentation.
Neither do I think of rhythm seeking as a pursuit to absorb the
whole time and energy of an individual and be followed steadily to a
conclusion; but hope rather that it may receive the incidental and
occasional attention of many of my colleagues of the hammer, as other
errands lead them among cliffs of bedded rocks. If my suggestion
should succeed in adding a working hypothesis or point of view to the
equipment of field geologists, I should feel that the search had been
begun in the most promising and advantageous manner. For not only
would the subject of rhythms and their interpretations be advanced by
reactions from multifarious individual experiences, but the stimulus of
another hypothesis would lead to the discovery of unexpected meanings
in stratigraphic detail.
It is one of the fortunate qualities of scientific research that its
incidental and unanticipated results are not infrequently of equal or
even greater value than those directly sought. Indeed, if it were not
so, there would be no utilitarian harvest from the cultivation of the
field of pure science.
In advocating the adoption of a new point of view from which to peer
into the mysterious past, I would not be understood to advise the
abandonment of old stand-points, but rather to emulate the surveyor,
who makes measurement to inaccessible points by means of bearings from
different sides. Every independent bearing on the earth’s beginning is
a check on other bearings, and it is through the study of discrepancies
that we are to discover the refractions by which our lines of sight are
warped and twisted. The three principal lines we have now projected
into the abyss of time miss one another altogether, so that there is no
point of intersection. If any one of them is straight, both the others
are hopelessly crooked. If we would succeed we should not only take new
bearings from each discovered point of vantage, but strive in every
way to discover the sources of error in the bearings we have already
attempted.