side, to challenge the validity of the conclusions so confidently put
forward in limitation of the age of the earth. He has boldly impugned
each of the three physical arguments. That which is based on tidal
retardation, following Mr. Maxwell Close and Professor Darwin, he
dismisses as fallacious. In regard to the argument from the secular
cooling of the earth, he contends that it is perfectly allowable to
assume a much higher conductivity for the interior of the globe, and
that this assumption would vastly increase our estimate of the age of
the planet. As to the conclusions drawn from the history of the sun, he
maintains that, on the one hand, the sun may have been repeatedly fed
by infalling meteorites, and that on the other hand, the earth, during
former ages, may have had its heat retained by a dense atmospheric
envelope. He thinks that 'almost anything is possible as to the present
internal state of the earth,' and he concludes in these words: 'To sum
up, we can find no published record of any lower maximum age of life
on the earth, as calculated by physicists, than 400 millions of years.
From the three physical arguments, Lord Kelvin's higher limits are
1,000, 400, and 500 million years. I have shown that we have reasons
for believing that the age, from all these, may be very considerably
underestimated. It is to be observed that if we exclude everything
but the arguments from mere physics, the _probable_ age of life on
the earth is much less than any of the above estimates; but if the
palæontologists have good reasons for demanding much greater times, I
see nothing from the physicist's point of view which denies them four
times the greatest of these estimates.'[88]
This remarkable admission from a recognised authority on the physical
side re-echoes and emphasises the warning pronounced by Professor
Darwin in the address already quoted--'at present our knowledge of
a definite limit to geological time has so little precision that we
should do wrong to summarily reject any theories which appear to demand
longer periods of time than those which now appear allowable.'[89]
This 'wrong,' which Professor Darwin so seriously deprecated, has
been committed not once, but again and again, in the history of this
discussion. Lord Kelvin has never taken any notice of the strong body
of evidence adduced by geologists and palæontologists in favour of a
much longer antiquity than he is now disposed to allow for the age of
the earth. His own three physical arguments have been successively
re-stated, with such corrections and modifications as he has found
to be necessary, and no doubt further alterations are in store for
them.[90] He has cut off slice after slice from the allowance of time
which at first he was prepared to grant for the evolution of geological
history, his latest pronouncement being that 'it was more than twenty
and less than forty million years, and probably much nearer twenty
than forty.'[91] But in none of his papers is there an admission that
geology and palæontology, though they have again and again raised their
voices in protest, have anything to say in the matter that is worthy of
consideration.
It is difficult satisfactorily to carry on a discussion in which your
opponent entirely ignores your arguments, while you have given the
fullest attention to his. In the present instance, geologists have
most carefully listened to all that has been brought forward from the
physical side. Impressed by the force of the physical reasoning, they
no longer believe that they can make any demands they may please on
past time. They have been willing to accept Lord Kelvin's original
estimate of 100 millions of years as the period within which the
history of life upon the planet must be comprised; while some of them
have even sought in various ways to reduce that sum nearer to his
lower limit. Yet there is undoubtedly a prevalent misgiving, whether
in thus seeking to reconcile their requirements with the demands of
the physicist they are not tying themselves down within limits of time
which on any theory of evolution would have been insufficient for the
development of the animal and vegetable kingdoms.
It is unnecessary to recapitulate before this Section of the British
Association, even in briefest outline, the reasoning of geologists and
palæontologists which leads them to conclude that the history recorded
in the crust of the earth must have required for its transaction a much
vaster period of time than that to which the physicists would restrict
it.[92] Let me merely remark that the reasoning is essentially based on
observations of the present rate of geological and biological changes
upon the earth's surface. It is not, of course, maintained that this
rate has never varied in the past. But it is the only rate with which
we are familiar, which we can watch and in some degree measure, and
which, therefore, we can take as a guide towards the comprehension and
interpretation of the past history of our planet.
It may be, and has often been, said that the present scale of
geological and biological processes cannot be accepted as a reliable
measure for the past. Starting from the postulate, which no one will
dispute, that the total sum of terrestrial energy was once greater
than it is now and has been steadily declining, the physicists have
boldly asserted that all kinds of geological action must have been
more vigorous and rapid during bygone ages than they are to-day;
that volcanoes were more gigantic, earthquakes more frequent and
destructive, mountain-upthrows more stupendous, tides and waves more
powerful, and commotions of the atmosphere more violent, with more
ruinous tempests and heavier rainfall. Assertions of this kind are
temptingly plausible and are easily made. But it is not enough that
they should be made; they ought to be supported by some kind of
evidence to show that they are founded on actual fact and not on mere
theoretical possibility. Such evidence, if it existed, could surely
be produced. The chronicle of the earth's history, from a very early
period down to the present time, has been legibly written within the
sedimentary formations of the terrestrial crust. Let the appeal be made
to that register. Does it lend any support to the affirmation that
the geological processes are now feebler and slower than they used to
be? If it does, the physicists, we might suppose, would gladly bring
forward its evidence as irrefragable confirmation of the soundness of
their contention. But the geologists have found no such confirmation.
On the contrary, they have been unable to discover any indication that
the rate of geological causation has ever, on the whole, greatly varied
during the time which has elapsed since the deposition of the oldest
stratified rocks. They do not assert that there has been no variation,
that there have been no periods of greater activity, both hypogene and
epigene. But they maintain that the demonstration of the existence of
such periods has yet to be made. They most confidently affirm that
whatever may have happened in the earliest ages, throughout the whole
vast succession of sedimentary strata nothing has yet been detected
which necessarily demands that more violent and rapid action which the
physicists suppose to have been the order of nature during the past.
So far as the potent effects of prolonged denudation permit us to
judge, the latest mountain-upheavals were at least as stupendous as any
of older date whereof the basal relics can yet be detected. They seem,
indeed, to have been still more gigantic than these. It may be doubted,
for example, whether among the vestiges that remain of Mesozoic or
Palæozoic mountain-chains, any instance can be found so colossal as
those of Tertiary times, such as the Alps. No volcanic eruptions of the
older geological periods can compare in extent or volume with those
of Tertiary and recent date. The plication and dislocation of the
terrestrial crust are proportionately as conspicuously displayed among
the younger as among the older formations, though the latter, from
their greater antiquity, have suffered during a longer time from the
renewed disturbances of successive periods.
As regards evidence of greater violence in the surrounding envelopes
of atmosphere and ocean, we seek for it in vain among the stratified
rocks. One of the very oldest formations in Europe, the Torridon
Sandstone of North-West Scotland, presents us with a picture of
long-continued sedimentation, such as may be seen in progress now round
the shores of many a mountain-girdled lake. In that venerable deposit,
the enclosed pebbles are not mere angular blocks and chips, swept by
a sudden flood or destructive tide from off the surface of the land,
and huddled together in confused heaps over the floor of the sea. They
have been rounded and polished by the quiet operation of running water,
as stones are rounded and polished now in the channels of brooks or on
the shores of lake and sea. They have been laid gently down above each
other, layer over layer, with fine sand sifted in between them, and
this deposition has taken place along shores which, though the waters
that washed them have long since disappeared, can still be followed
for mile after mile across the mountains and glens of the North-West
Highlands. So tranquil were these waters that their gentle currents
and oscillations sufficed to ripple the sandy floor, to arrange the
sediment in laminæ of current-bedding, and to separate the grains of
sand according to their relative densities. We may even now trace
the results of these operations in thin darker layers and streaks of
magnetic iron, zircon, and other heavy minerals, which have been sorted
out from the lighter quartz-grains, as layers of iron-sand may be seen
sifted together by the tide along the upper margins of many of our
sandy beaches at the present day.
In the same ancient formation there occur also various intercalations
of fine muddy sediment, so regular in their thin alternations, and so
like those of younger formations, that we cannot but hope and expect
that they may eventually yield remains of organisms which, if found,
would be the earliest traces of life in Europe.
It is thus abundantly manifest that even in the most ancient of the
sedimentary registers of the earth's history, not only is there no
evidence of colossal floods, tides and denudation, but there is
incontrovertible proof of continuous orderly deposition, such as
may be witnessed to-day in any quarter of the globe. The same tale,
with endless additional details, is told all through the stratified
formations, down to those which are in the course of accumulation at
the present day.
Not less important than the stratigraphical is the palæontological
evidence in favour of the general quietude of the geological processes
in the past. The conclusions drawn from the nature and arrangement of
the sediments are corroborated and much extended by the structure and
manner of entombment of the enclosed organic remains. From the time
of the very earliest fossiliferous formations there is nothing to
show that either plants or animals have had to contend with physical
conditions of environment different, on the whole, from those in which
their successors now live. The oldest trees, so far as regards their
outer form and internal structure, betoken an atmosphere neither
more tempestuous nor obviously more impure than that of to-day. The
earliest corals, sponges, crustaceans, mollusks, and arachnids were
not more stoutly constructed than those of later times, and they are
found grouped together among the rocks as they lived and died, with no
apparent indication that any violent commotion of the elements tried
their strength when living or swept away their remains when dead.
But, undoubtedly, most impressive of all the palæontological data is
the testimony borne by the grand succession of organic remains among
the stratified rocks as to the vast duration of time required for their
evolution. Professor Poulton has treated this branch of the subject
with great fulness and ability (p. 216). We do not know the present
average rates of organic variation, but all the available evidence goes
to indicate their extreme slowness. They may conceivably have been
more rapid in the past, or they may have been liable to fluctuations
according to vicissitudes of environment.[93] But those who assert
that the rate of biological evolution ever differed materially from
what it may now be inferred to be, ought surely to bring forward
something more than mere assertion in their support. In the meantime,
the most philosophical course is undoubtedly followed by those
biologists who in this matter rest their belief on their own experience
among recent and fossil organisms.
So cogent do these geological and palæontological arguments appear, to
those at least who have taken the trouble to master them, that they
are worthy of being employed, not in defence merely, but in attack.
It seems to me that they may be used with effect in assailing the
stronghold of speculation and assumption in which our physical friends
have ensconced themselves and from which, with their feet, as they
believe, planted well within the interior of the globe and their heads
in the heart of the sun, they view with complete unconcern the efforts
made by those who endeavour to gather the truth from the surface and
crust of the earth. That portion of the records of terrestrial history
which lies open to our investigation has been diligently studied in all
parts of the world. A vast body of facts has been gathered together
from this extended and combined research. The chronicle registered in
the earth's crust, though not complete, is legible and consistent.
From the latest to the earliest of its chapters the story is capable of
clear and harmonious interpretation by a comparison of its pages with
the present condition of things. We know infinitely more of the history
of this earth than we do of the history of the sun. Are we then to be
told that this knowledge so patiently accumulated from innumerable
observations and so laboriously co-ordinated and classified, is to be
held of none account in comparison with the conclusions of physical
science in regard to the history of the central luminary of our
system? These conclusions are founded on assumptions which may or may
not correspond with the truth. They have already undergone revision,
and they may be still further modified as our slender knowledge of
the sun, and of the details of its history, is increased by future
investigation. In the meantime, we decline to accept them as a final
pronouncement of science on the subject. We place over against them
the evidence of geology and palæontology, and affirm that unless the
deductions we draw from that evidence can be disproved, we are entitled
to maintain them as entirely borne out by the testimony of the rocks.
Until, therefore, it can be shown that geologists and palæontologists
have misinterpreted their records, they are surely well within their
logical rights in claiming as much time for the history of this earth
as the vast body of evidence accumulated by them demands. So far as I
have been able to form an opinion, one hundred millions of years would
suffice for that portion of the history which is registered in the
stratified rocks of the crust. But if the palæontologists find such a
period too narrow for their requirements, I can see no reason on the
geological side why they should not be at liberty to enlarge it as far
as they may find to be needful for the evolution of organised existence
on the globe. As I have already remarked, it is not the length of
time which interests us so much as the determination of the relative
chronology of the events which were transacted within that time. As to
the general succession of these events, there can be no dispute. We
have traced its stages from the bottom of the oldest rocks up to the
surface of the present continents and the floor of the present seas.
We know that these stages have followed each other in orderly advance,
and that geological time, whatever limits may be assigned to it, has
sufficed for the passage of the long stately procession.
We may, therefore, well leave the dispute about the age of the earth
to the decision of the future. In so doing, however, I should be
glad if we could carry away from it something of greater service to
science than the consciousness of having striven our best in a barren
controversy, wherein concession has all to be on one side and the
selection of arguments entirely on the other. During these years of
prolonged debate I have often been painfully conscious that in this
subject, as in so many others throughout the geological domain, the
want of accurate numerical data is a serious hindrance to the progress
of our science. Heartily do I acknowledge that much has been done in
the way of measurement and experiment for the purpose of providing a
foundation for estimates and deductions. But infinitely more remains to
be accomplished. The field of investigation is almost boundless, for
there is hardly a department of geological dynamics over which it does
not extend. The range of experimental geology must be widely enlarged,
until every process susceptible of illustration or measurement by
artificial means has been investigated. Field-observation needs to be
supplemented where possible by instrumental determination, so as to be
made more precise and accurate, and more capable of furnishing reliable
numerical statistics for practical as well as theoretical deductions.
The subject is too vast for adequate treatment here. But let me
illustrate my meaning by selecting a few instances where the adoption
of these more rigid methods of inquiry might powerfully assist us
in dealing with the rates of geological processes and the value
of geological time. Take, for example, the wide range of lines of
investigation embraced under the head of Denudation. So voluminous a
series of observations has been made in this subject, and so ample is
the literature devoted to it, that no department of geology, it might
be thought, has been more abundantly and successfully explored. Yet
if we look through the pile of memoirs, articles and books, we cannot
but be struck with the predominant vagueness of their statements,
and with the general absence of such numerical data determined by
accurate, systematic, and prolonged measurement as would alone furnish
a satisfactory basis for computations of the rate at which denudation
takes place. Some instrumental observations of the greatest value have
indeed been made, but, for the most part, observations of this kind
have been too meagre and desultory.
A little consideration will show that in all branches of the
investigation of denudation opportunities present themselves on every
side of testing, by accurate instrumental observation and measurement,
the rate at which some of the most universal processes in the
geological mechanism of our globe are carried on.
It has long been a commonplace of geology that the amount of the
material removed in suspension and solution by Rivers furnishes a clue
to the rate of denudation of the regions drained by the rivers. But
how unequal in value, and generally how insufficient in precision, are
the observations on this topic! A few rivers have been more or less
systematically examined, some widely varying results have been obtained
from the observations, and while enough has been gained to show the
interest and importance of the method of research, no adequate supply
of materials has been gathered for the purposes of wide, accurate
deduction and generalisation. What we need is a carefully organised
series of observations carried out on a uniform plan, over a sufficient
number of years, not for one river only, but for all the important
rivers of a country, and indeed for all the greater rivers of each
continent. We ought to know as accurately as possible the extent of
the drainage-area of each river, the relations of river-discharge
to rainfall and to other meteorological as well as topographical
conditions; the variation in the proportions of mechanical and chemical
impurities in the river-water according to geological formations, form
of the ground, season of the year and climate. The whole geological
_régime_ of each river should be thoroughly studied. The admirable
report of Messrs. Humphreys and Abbot on the 'Physics and Hydraulics
of the Mississippi,' published in 1861, might well serve as a model for
imitation, though these observers necessarily occupied themselves with
some questions which are not specially geological, and did not enter
into others on which, as geologists, we should now gladly have further
information.
Again, the action of Glaciers has still less been subjected to
prolonged and systematic observation. The few data already obtained
are so vague that we may be said to be still entirely ignorant of the
rate at which glaciers are wearing down their channels and contributing
to the denudation of the land. The whole of this inquiry is eminently
suitable for combined research. Each stream or glacier, or each
well-marked section of one, might become the special inquiry of a
single observer, who would soon develop a paternal interest in his
valley and vie with his colleagues of other valleys in the fulness and
accuracy of his records.
Nor is our information respecting the operations of the Sea much more
precise. Even in an island like Great Britain, where the waves and
tides effect so much change within the space of a human life-time, the
estimates of the rate of advance or retreat of the shore-line are based
for the most part on no accurate determinations. It is satisfactory to
be able to announce that the Council of this Association has formed a
Committee for the purpose of obtaining full and precise information
regarding alterations of our coasts, and that, with the sanction of the
Lords of the Admiralty, the co-operation of the Coast-guard throughout
the three kingdoms has been secured. We may therefore hope to be
eventually in possession of trustworthy statistics on this interesting
subject.[94]
The Denudation of the Surface of the Land by the combined agency of the
subaërial forces of decay is a problem which has been much studied,
but in regard to whose varying rates of advance not much has been
definitely ascertained. The meteorological conditions under which it
takes place differ materially according to latitude and climate, and
doubtless its progress is equally variable. An obvious and useful
source of information in regard to atmospheric denudation is to be
found in the decay of the material of buildings of which the time
of erection is known, and in dated tombstones. Twenty years ago I
called attention to the rate at which marble gives way in such a moist
climate as ours, and cited the effects of subaërial waste as these can
be measured on the monuments of our graveyards and cemeteries.[95] I
would urge upon town-geologists, and those in the country who have no
opportunities of venturing far afield, that they may do good service by
careful scrutiny of ancient buildings and monuments. In the churchyards
they will find much to occupy and interest them, not, however, like
Old Mortality, in repairing the tombstones, but in tracing the ravages
of the weather upon them, and in obtaining definite measures of the
rate of their decay.
The conditions under which subaërial disintegration is effected in arid
climates, and the rate of its advance, are still less known, seeing
that most of our information is derived from the chance observations
of passing travellers. Yet this branch of the subject is not without
importance in relation to the denudation not only of the existing
terrestrial surface but of the lands of former periods, for there is
evidence of more than one arid epoch in geological history. Here,
again, a diligent examination of ancient buildings and monuments
might afford some, at least, of the required data. In such a country
as Egypt, for instance, it might eventually be possible to determine
from a large series of observations what has been the average rate of
surface-disintegration of the various kinds of stone employed in human
constructions that have been freely exposed to the air for several
thousand years.