modern science enables us to frame out of the landscapes around
Edinburgh. They may be taken as illustrations of what may be drawn, and
sometimes with even greater fulness and vividness, from any district
in these islands. But I cite them especially because of their local
interest in connection with the present meeting of the Association,
and because the rocks that yield them gave inspiration to those
great masters whose claims on our recollection, not least for their
explanation of the origin of scenery, I have tried to recount this
evening. But I am further impelled to dwell on these scenes from an
overmastering personal feeling to which I trust I may be permitted to
give expression. It was these green hills and grey crags that gave me
in boyhood the impulse that has furnished the work and joy of my life.
To them, amid changes of scene and surroundings, my heart ever fondly
turns, and here I desire gratefully to acknowledge that it is to their
influence that I am indebted for any claim I may possess to stand in
the proud position in which your choice has placed me.
FOOTNOTES:
[69] The Address of the President of the British Association for the
Advancement of Science at the Meeting held in Edinburgh, 1892.
[70] The problem of Geological Time was made the subject of a later
address. See _postea_, p. 198.
VI
Geological Time[71]
Among the many questions of great theoretical importance which have
engaged the attention of geologists, none has in late years awakened
more interest or aroused livelier controversy than that which deals
with Time as an element in geological history. The various schools
which have successively arisen--Cataclysmal, Uniformitarian, and
Evolutionist--have had each its own views as to the duration of their
chronology, as well as to the operations of terrestrial energy. But
though holding different opinions, they did not make these differences
matter of special controversy among themselves. About thirty years ago,
however, they were startled by a bold irruption into their camp from
the side of physics. They were then called on to reform their ways,
which were declared to be flatly opposed to the teachings of natural
philosophy. Since that period the discussion then started regarding
the age of the Earth and the value of geological time has continued
with varying animation. Evidence of the most multifarious kind has been
brought forward, and arguments of widely different degrees of validity
have been pressed into service both by geologists and palæontologists
on one side, and by physicists on the other. For the last year or
two there has been a pause in the controversy, though no general
agreement has been arrived at in regard to the matters in dispute.
The present interval of comparative quietude seems favourable for a
dispassionate review of the debate. I propose, therefore, to take, as
perhaps a not inappropriate subject on which to address geologists
upon a somewhat international occasion like this present meeting of
the British Association at Dover, the question of Geological Time. In
offering a brief history of the discussion, I gladly avail myself of
the opportunity of enforcing one of the lessons which the controversy
has impressed upon my own mind, and to point a moral which, as it seems
to me, we geologists may take home to ourselves from a consideration
of the whole question. There is, I think, a practical outcome which
may be made to issue from the dispute in a combination of sympathy and
co-operation among geologists all over the world. A lasting service
will be rendered to our science if by well-concerted effort we can
place geological dynamics and geological chronology on a broader and
firmer basis of actual experiment and measurement than has yet been
laid.
To understand aright the origin and progress of the dispute regarding
the value of time in geological speculation, we must take note of
the attitude maintained towards this subject by some of the early
fathers of the science. Among these pioneers none has left his mark
more deeply graven on the foundations of modern geology than James
Hutton. To him, more than to any other writer of his day, do we owe
the doctrine of the high antiquity of our globe. No one before him had
ever seen so clearly the abundant and impressive proofs of this remote
antiquity recorded in the rocks of the earth's crust. In these rocks
he traced the operation of the same slow and quiet processes which he
observed to be at work at present in gradually transforming the face of
the existing continents. When he stood face to face with the proofs of
decay among the mountains, there seems to have arisen uppermost in his
mind the thought of the immense succession of ages which these proofs
revealed to him. His observant eye enabled him to see 'the operations
of the surface wasting the solid body of the globe, and to read the
unmeasurable course of time that must have flowed during those amazing
operations, which the vulgar do not see, and which the learned seem
to see without wonder.'[72] In contemplating the stupendous results
achieved by such apparently feeble forces, Hutton felt that one great
objection he had to contend with in the reception of his theory, even
by the scientific men of his day, lay in the inability or unwillingness
of the human mind to admit such large demands as he made on the past.
'What more can we require?' he asks in summing up his conclusions;
and he answers the question in these memorable words: 'What more can
we require? Nothing but time. It is not any part of the process that
will be disputed; but after allowing all the parts, the whole will be
denied; and for what?--only because we are not disposed to allow that
quantity of time which the ablution of so much wasted mountain might
require.'[73]
Far as Hutton could follow the succession of events registered in the
rocky crust of the globe, he found himself baffled by the closing
in around him of that dark abysm of time into which neither eye nor
imagination seemed able to penetrate. He well knew that, behind and
beyond the ages recorded in the oldest of the primitive rocks, there
must have stretched a vast earlier time, of which no record met
his view. He did not attempt to speculate beyond the limits of his
evidence. 'I do not pretend,' he said, 'to describe the beginning of
things; I take things such as I find them at present, and from these I
reason with regard to that which must have been.'[74] In vain did he
look, even among the oldest formations, for any sign of the infancy
of the planet. He could only detect a repeated series of similar
revolutions, the oldest of which was assuredly not the first in the
terrestrial history, and he concluded, as 'the result of this physical
inquiry, that we find no vestige of a beginning, no prospect of an
end.'[75]
This conclusion from strictly geological evidence has been impugned
from the side of physics, and, as further developed by Playfair,
has been declared to be contradicted by the principles of natural
philosophy. But if it be considered on the basis of the evidence on
which it was originally propounded, it was absolutely true in Hutton's
time and remains true to-day. That able reasoner never claimed that the
earth has existed from all eternity, or that it will go on existing for
ever. He admitted that it must have had a beginning, but he had been
unable to find any vestige of that beginning in the structure of the
planet itself. And notwithstanding all the multiplied researches of
the century that has passed since the immortal _Theory of the Earth_
was published, no relic of the first condition of our earth has been
found. We have speculated much, indeed, on the subject, and our friends
the physicists have speculated still more. Some of the speculations
do not seem to me more philosophical than many of those of the older
cosmogonists. As far as reliable evidence can be drawn from the rocks
of the globe itself, we do not seem to be nearer the discovery of the
beginning than Hutton was. The most ancient rocks that can be reached
are demonstrably not the first-formed of all. They were preceded by
others which we know must have existed, though no vestige of them may
be accessible.
It may be further asserted that, while it was Hutton who first
impressed upon modern geology the conviction that for the adequate
comprehension of the past history of the earth vast periods of time
must be admitted to have elapsed, our debt of obligation to him is
increased by the genius with which he linked the passage of these
vast periods with the present economy of nature. He first realised
the influence of time as a factor in geological dynamics, and first
taught the efficacy of the quiet and unobtrusive forces of nature.
His predecessors and contemporaries were never tired of invoking the
more vigorous manifestations of terrestrial energy. They saw in the
composition of the land and in the structure of mountains and valleys
memorials of numberless convulsions and cataclysms. In Hutton's
philosophy, however, 'it is the little causes, long continued, which
are considered as bringing about the greatest changes of the earth.'[76]
And yet, unlike many of those who derived their inspiration from
his teaching, but pushed his tenets to extremes which he doubtless
never anticipated, he did not look upon time as a kind of scientific
fetich, the invocation of which would endow with efficacy even the
most trifling phenomena. As if he had foreseen the use that might be
made of his doctrine, he uttered this remarkable warning: 'With regard
to the effect of time, though the continuance of time may do much in
those operations which are extremely slow, where no change, to our
observation, had appeared to take place, yet, where it is not in the
nature of things to produce the change in question, the unlimited
course of time would be no more effectual than the moment by which we
measure events in our observations.'[77]
We thus see that in the philosophy of Hutton, out of which so much
of modern geology has been developed, the vastness of the antiquity
of the globe was deduced from the structure of the terrestrial crust
and the slow rate of action of the forces by which the surface of the
crust is observed to be modified. But no attempt was made by him to
measure that antiquity by any of the chronological standards of human
contrivance. He was content to realise for himself and to impress upon
others that the history of the earth could not be understood, save by
the admission that it occupied prolonged though indeterminate ages in
its accomplishment. And assuredly no part of his teaching has been more
amply sustained by the subsequent progress of research.
Playfair, from whose admirable _Illustrations of the Huttonian Theory_
most geologists have derived all that they know directly of that
theory, went a little further than his friend and master in dealing
with the age of the earth. Not restricting himself, as Hutton did, to
the testimony of the rocks, which showed neither vestige of a beginning
nor prospect of an end, he called in the evidence of the cosmos outside
the limits of our planet, and declared that in the firmament also no
mark could be discovered of the commencement or termination of the
present order, no symptom of infancy or old age, nor any sign by which
the future or past duration of the universe might be estimated.[78]
He thus advanced beyond the strictly geological basis of reasoning,
and committed himself to statements which, like some made also by
Hutton, seem to have been suggested by certain deductions of the French
mathematicians of his day regarding the stability of the planetary
motions. His statements have been disproved by modern physics; distinct
evidence, both from the earth and the cosmos, has been brought
forward of progress from a beginning which can be conceived, through
successive stages to an end which can be foreseen. But the disproof
leaves Hutton's doctrine about the vastness of geological time exactly
where it was. Surely it was no abuse of language to speak of periods as
being vast, which can only be expressed in millions of years.
It is easy to understand how the Uniformitarian school, which sprang
from the teaching of Hutton and Playfair, came to believe that the
whole of eternity was at the disposal of geologists. In popular
estimation, as the ancient science of astronomy was that of infinite
distance, so the modern study of geology was the science of infinite
time. It must be frankly conceded that geologists, believing themselves
unfettered by any limits to their chronology, made ample use of
their imagined liberty. Many of them, following the lead of Lyell,
to whose writings in other respects modern geology owes so deep a
debt of gratitude, became utterly reckless in their demands for time,
demands which even the requirements of their own science, if they
had adequately realised them, did not warrant. The older geologists
had not attempted to express their vast periods in terms of years.
The indefiniteness of their language fitly denoted the absence of
any ascertainable limits to the successive ages with which they had
to deal. And until some evidence should be discovered whereby these
limits might be fixed and measured by human standards, no reproach
could justly be brought against the geological terminology. It was far
more philosophical to be content, in the meanwhile, with indeterminate
expressions, than from data of the weakest or most speculative kind
to attempt to measure geological periods by a chronology of years or
centuries.
In the year 1862 a wholly new light was thrown on the question of the
age of our globe and the duration of geological time by the remarkable
paper on the Secular Cooling of the Earth communicated by Lord Kelvin
(then Sir William Thomson) to the Royal Society of Edinburgh.[79] In
this memoir he first developed his now well-known argument from the
observed rate of increase of temperature downwards from the surface
of the land. He astonished geologists by announcing to them that some
definite limits to the age of our planet might be ascertained, and by
declaring his belief that this age must be more than 20 millions, but
less than 400 millions of years.
Nearly four years later he emphasised his dissent from what he
considered to be the current geological opinions of the day by
repeating the same argument in a more pointedly antagonistic form in a
paper of only a few sentences, entitled, 'The Doctrine of Uniformity in
Geology briefly refuted.'[80]
Again, after a further lapse of about two years, when, as President
of the Geological Society of Glasgow, it became his duty to give an
address, he returned to the same topic and arraigned more boldly and
explicitly than ever the geology of the time. He then declared that
'a great reform in geological speculation seems now to have become
necessary,' and he went so far as to affirm that 'it is quite certain
that a great mistake has been made--that British popular geology
at the present time is in direct opposition to the principles of
natural philosophy.'[81] In pressing once more the original argument
derived from the downward increase of terrestrial temperature, he
now reinforced it by two further arguments, the one based on the
retardation of the earth's angular velocity by tidal friction, the
other on the limitation of the age of the sun.
These three lines of attack remain still those along which the assault
from physics is delivered against the strongholds of geology. Lord
Kelvin has repeatedly returned to the charge since 1868, his latest
contribution to the controversy having been pronounced two years
ago.[82] While his physical arguments remain the same, the limits of
time which he deduces from them have been successively diminished. The
original maximum of 400 millions of years has now been restricted by
him to not much more than 20 millions, while Professor Tait grudgingly
allows something less than 10 millions.[83]
Soon after the appearance of Lord Kelvin's indictment of modern
geology in 1868, the defence of the science was taken up by Huxley,
who happened at the time to be President of the Geological Society
of London. In his own inimitably brilliant way, half seriously, half
playfully, this doughty combatant, with evident relish, tossed the
physical arguments to and fro in the eyes of his geological brethren,
as a barrister may flourish his brief before a sympathetic jury. He
was willing to admit that 'the rapidity of rotation of the earth _may_
be diminishing, that the sun _may_ be waxing dim, or that the earth
itself _may_ be cooling.' But he went on to add his suspicion that
'most of us are Gallios, who care for none of these things, being
of opinion that, true or fictitious, they have made no practical
difference to the earth, during the period of which a record is
preserved in stratified deposits.'[84]
For the indifference which their advocate thus professed on their
behalf most geologists believed that they had ample justification.
The limits within which the physicist would circumscribe the earth's
history were so vague yet so vast, that whether the time allowed were
400 millions or 100 millions of years did not seem to them greatly to
matter. After all, it was not the time that chiefly interested them,
but the grand succession of events which the time had witnessed. That
succession had been established on observations so abundant and so
precise that it could withstand attack from any quarter, and it had
taken as firm and lasting a place among the solid achievements of
science as could be claimed for any physical speculations whatsoever.
Whether the time required for the transaction of this marvellous
earth-history was some millions of years more or some millions of years
less did not seem to the geologists to be a question on which their
science stood in antagonism with the principles of natural philosophy,
but one which the natural philosophers might be left to settle at their
own good pleasure.
For myself, I may be permitted here to say that I have never shared
this feeling of indifference and unconcern. As far back as the year
1868, only a month after Lord Kelvin's first presentation of his
threefold argument in favour of limiting the age of the earth, I gave
in my adhesion to the propriety of restricting the geological demands
for time. I then showed that even the phenomena of denudation, which,
from the time of Hutton downwards, had been most constantly and
confidently appealed to in support of the inconceivably vast antiquity
of our globe, might be accounted for, at the present rate of action,
within such a period as 100 millions of years.[85] To my mind it
has always seemed that whatever tends to give more precision to the
chronology of the geologist, and helps him to a clearer conception
of the antiquity with which he has to deal, ought to be welcomed by
him as a valuable assistance in his inquiries. And I feel sure that
this view of the matter has now become general among those engaged
in geological research. Frank recognition is made of the influence
which Lord Kelvin's persistent attacks have had upon our science.
Geologists have been led by his criticisms to revise their chronology.
They gratefully acknowledge that to him they owe the introduction of
important new lines of investigation, which link the solution of the
problems of geology with those of physics. They realise how much he has
done to dissipate the former vague conceptions as to the duration of
geological history, and even when they emphatically dissent from the
greatly restricted bounds within which he would now limit that history,
and when they declare their inability to perceive that any further
reform of their speculations in this subject is needful, or that their
science has placed herself in opposition to the principles of physics,
they none the less pay their sincere homage to one who has thrown over
geology, as over so many other departments of natural knowledge, the
clear light of a penetrating and original genius.
When Lord Kelvin first developed his strictures on modern geology he
expressed his opposition in the most uncompromising language. In the
short paper to which reference has already been made he announced,
without hesitation or palliation, that he 'briefly refuted' the
doctrine of Uniformitarianism which had been espoused and illustrated
by Lyell and a long list of the ablest geologists of the day. The
severity of his judgment of British geology was not more marked than
was his unqualified reliance on his own methods and results. This
confident assurance of a distinguished physicist, together with a
formidable array of mathematical formulæ, produced its effect on
some geologists and palæontologists who were not Gallios. Thus, even
after Huxley's brilliant defence, Darwin could not conceal the deep
impression which Lord Kelvin's arguments had made on his mind. In one
letter he wrote that the proposed limitation of geological time was
one of his 'sorest troubles.' In another, he pronounced the physicist
himself to be 'an odious spectre.'[86]
The same self-confidence of assertion on the part of some, at least,
of the disputants on the physical side has continued all through the
controversy. Yet when we examine the three great physical arguments in
themselves, we find them to rest on assumptions which, though certified
as 'probable' or 'very sure,' are nevertheless admittedly assumptions.
The conclusions to which these assumptions lead must depend for their
validity on the degree of approximation to the truth in the premises
which are postulated.
Now it is interesting to observe that neither the assumptions nor
the conclusions drawn from them have commanded universal assent even
among physicists themselves. If they were as self-evident as they have
been claimed to be, they should at least receive the loyal support of
all those whose function it is to pursue and extend the applications
of physics. It will be remembered, however, that thirteen years ago
Professor George Darwin, who has so often shown his inherited sympathy
in geological investigation, devoted his presidential address before
the Mathematical Section of this Association to a review of the three
famous physical arguments respecting the age of the earth. He summed
up his judgment of them in the following words: 'In considering these
three arguments I have adduced some reasons against the validity of
the first [tidal friction]; and have endeavoured to show that there
are elements of uncertainty surrounding the second [secular cooling of
the earth]; nevertheless they undoubtedly constitute a contribution of
the first importance to physical geology. Whilst, then, we may protest
against the precision with which Professor Tait seeks to deduce
results from them, we are fully justified in following Sir William
Thomson, who says that "the existing state of things on the earth,
life on the earth--all geological history showing continuity of life,
must be limited within some such period of past time as 100,000,000
years."'[87]