which on this occasion deserves to be held in especial honour, that
of Sir James Hall, of Dunglass. Having accompanied Hutton in some of
his excursions, and having discussed with him the problems presented
by the rocks of Scotland, Hall was familiar with the views of his
master, and was able to supply him with fresh illustrations of them
from different parts of the country. Gifted with remarkable originality
and ingenuity, he soon perceived that some of the questions involved
in the theory of the earth could probably be solved by direct physical
experiment. Hutton, however, mistrusted any attempt 'to judge of the
great operations of Nature by merely kindling a fire and looking into
the bottom of a little crucible.' Out of deference to this prejudice,
Hall delayed to carry out his intention during Hutton's lifetime. But
afterwards he instituted a remarkable series of researches which are
memorable in the history of science as the first methodical endeavour
to test the value of geological speculation by an appeal to actual
experiment. The Neptunists, in ridiculing the Huttonian doctrine that
basalt and similar rocks had once been molten, asserted that, had such
been their origin, these masses would now be found in the condition of
glass or slag. Hall, however, triumphantly vindicated his friend's view
by proving that basalt could be fused and thereafter by slow cooling
could be made to resume a stony texture. Again, Hutton had asserted
that under the vast pressures which must be effective deep within the
earth's crust, chemical reactions must be powerfully influenced, and
that under such conditions even limestone may conceivably be melted
without losing its carbonic acid. Various specious arguments had been
adduced against this proposition, but by an ingeniously devised series
of experiments Hall succeeded in converting limestone under great
pressure into a kind of marble, and even in fusing it, and found that
it then acted vigorously on other rocks. These admirable researches,
which laid the foundations of experimental geology, constitute not the
least memorable of the services rendered by the Huttonian school to the
progress of science.
Clear as was the insight and sagacious the inferences of these great
masters in regard to the history of the globe, their vision was
necessarily limited by the comparatively narrow range of ascertained
fact which up to their time had been established. They taught men to
recognise that the present world is built of the ruins of an earlier
one, and they explained with admirable perspicacity the operation
of the processes whereby the degradation and renovation of land
are brought about. But they never dreamed that a long and orderly
series of such successive destructions and renewals had taken place,
and had left their records in the crust of the earth. They never
imagined that from these records it would be possible to establish a
determinate chronology that could be read everywhere, and applied to
the elucidation of the remotest quarter of the globe. It was by the
memorable observations and generalisations of William Smith that this
vast extension of our knowledge of the past history of the earth became
possible. While the Scottish philosophers were building up their theory
here, Smith was quietly ascertaining by extended journeys that the
stratified rocks of the west of England occur in a definite sequence,
and that each well-marked group of them can be discriminated from the
others and identified across the country by means of its enclosed
organic remains. It is nearly a hundred years since he made known
his views, so that by a curious coincidence we may fitly celebrate
on this occasion the centenary of William Smith as well as that of
James Hutton. No single discovery has ever had a more momentous and
far-reaching influence on the progress of a science than that law of
organic succession which Smith established. At first it served merely
to determine the order of the stratified rocks of England. But it soon
proved to possess a world-wide value, for it was found to furnish the
key to the structure of the whole stratified crust of the earth. It
showed that within that crust lie the chronicles of a long history
of plant and animal life upon this planet, it supplied the means
of arranging the materials for this history in true chronological
sequence, and it thus opened out a magnificent vista through a vast
series of ages, each marked by its own distinctive types of organic
life, which, in proportion to their antiquity, departed more and more
from the aspect of the living world.
Thus a hundred years ago, by the brilliant theory of Hutton and the
fruitful generalisation of Smith, the study of the earth received in
our country the impetus which has given birth to the modern science of
geology.
To review the marvellous progress which this science has made during
the first century of its existence would require not one but many hours
for adequate treatment. The march of discovery has advanced along a
multitude of different paths, and the domains of Nature which have
been included within the growing territories of human knowledge have
been many and ample. Nevertheless, there are certain departments of
investigation to which we may profitably restrict our attention on the
present occasion, and wherein we may see how the leading principles
that were proclaimed in this city a hundred years ago have germinated
and borne fruit all over the world.
From the earliest times the natural features of the earth's surface
have arrested the attention of mankind. The rugged mountain, the cleft
ravine, the scarped cliff, the solitary boulder, have stimulated
curiosity and prompted many a speculation as to their origin. The
shells embedded by millions in the solid rocks of hills far removed
from the sea have still further pressed home these 'obstinate
questionings.' But for many long centuries the advance of inquiry
into such matters was arrested by the paramount influence of orthodox
theology. It was not merely that the Church opposed itself to the
simple and obvious interpretation of these natural phenomena. So
implicit had faith become in the accepted views of the earth's age
and of the history of creation, that even laymen of intelligence
and learning set themselves, unbidden and in perfect good faith, to
explain away the difficulties which Nature so persistently raised up,
and to reconcile her teachings with those of the theologians. In the
various theories thus originating, the amount of knowledge of natural
law usually stood in inverse ratio to the share played in them by an
uncontrolled imagination. The speculations, for example, of Burnet,
Whiston, Whitehurst, and others in this country, cannot be read now
without a smile. In no sense were they scientific researches; they can
only be looked upon as exercitations of learned ignorance. Springing
mainly out of a laudable desire to promote what was believed to be the
cause of true religion, they helped to retard inquiry, and exercised in
that respect a baneful influence on intellectual progress.
It is the special glory of the Edinburgh school of geology to have cast
aside all this fanciful trifling. Hutton boldly proclaimed that it was
no part of his philosophy to account for the beginning of things. His
concern lay only with the evidence furnished by the earth itself as
to its origin. With the intuition of true genius he early perceived
that the only solid basis from which to explore what has taken place
in bygone time is a knowledge of what is taking place to-day. He thus
founded his system upon a careful study of the processes whereby
geological changes are now brought about. He felt assured that
Nature must be consistent and uniform in her working, and that only
in proportion as her operations at the present time are watched and
understood will the ancient history of the earth become intelligible.
Thus, in his hands, the investigation of the Present became the key to
the interpretation of the Past. The establishment of this great truth
was the first step towards the inauguration of a true science of the
earth. The doctrine of the uniformity of causation in Nature became the
fruitful principle on which the structure of modern geology could be
built up.
Fresh life was now breathed into the study of the earth. A new spirit
seemed to animate the advance along every pathway of inquiry. Facts
that had long been familiar came to possess a wider and deeper meaning
when their connection with each other was recognised as parts of one
great harmonious system of continuous change. In no department of
Nature, for example, was this broader vision more remarkably displayed
than in that wherein the circulation of water between land and sea
plays the most conspicuous part. From the earliest times men had
watched the coming of clouds, the fall of rain, the flow of rivers,
and had recognised that on this nicely adjusted machinery the beauty
and fertility of the land depend. But they now learnt that this beauty
and fertility involve a continual decay of the terrestrial surface;
that the soil is a measure of this decay, and would cease to afford us
maintenance were it not continually removed and renewed; that through
the ceaseless transport of soil by rivers to the sea the face of the
land is slowly lowered in level and carved into mountain and valley,
and that the materials thus borne outwards to the floor of the ocean
are not lost, but accumulate there to form rocks, which in the end will
be upraised into new lands. Decay and renovation, in well-balanced
proportions, were thus shown to be the system on which the existence of
the earth as a habitable globe had been established. It was impossible
to conceive that the economy of the planet could be maintained on any
other basis. Without the circulation of water the life of plants and
animals would be impossible, and with that circulation the decay of the
surface of the land and the renovation of its disintegrated materials
are necessarily involved.
As it is now, so must it have been in past time. Hutton and Playfair
pointed to the stratified rocks of the earth's crust as demonstrations
that the same processes which are at work to-day have been in operation
from a remote antiquity. By thus placing their theory on a basis of
actual observation, and providing in the study of existing operations
a guide to the interpretation of those in past times, they rescued
the investigation of the history of the earth from the speculations
of theologians and cosmologists, and established a place for it among
the recognised inductive sciences. To the guiding influence of their
philosophical system the prodigious strides made by modern geology are
in large measure to be attributed. And here in their own city, after
the lapse of a hundred years, let us offer to their memory the grateful
homage of all who have profited by their labours.
But while we recognise with admiration the far-reaching influence
of the doctrine of uniformity of causation in the investigation of
the history of the earth, we must upon reflection admit that the
doctrine has been pushed to an extreme perhaps not contemplated by
its original founders. To take the existing conditions of Nature as a
platform of actual knowledge from which to start in an inquiry into
former conditions was logical and prudent. Obviously, however, human
experience, in the few centuries during which attention has been turned
to such subjects, has been too brief to warrant any dogmatic assumption
that the various natural processes must have been carried on in the
past with the same energy and at the same rate as they are carried on
now. Variations in energy might have been legitimately conceded as
possible, though not to be allowed without reasonable proof in their
favour. It was right to refuse to admit the operation of speculative
causes of change when the phenomena were capable of natural and
adequate explanation by reference to causes that can be watched and
investigated. But it was an error to take for granted that no other
kind of process or influence, nor any variation in the rate of activity
save those of which man has had actual cognisance, has played a part
in the terrestrial economy. The uniformitarian writers laid themselves
open to the charge of maintaining a kind of perpetual motion in the
machinery of Nature. They could find in the records of the earth's
history no evidence of a beginning, no prospect of an end. They saw
that many successive renovations and destructions had been effected on
the earth's surface, and that this long line of vicissitudes formed a
series of which the earliest were lost in antiquity, while the latest
were still in progress towards an apparently illimitable future.
The discoveries of William Smith, had they been adequately
understood, would have been seen to offer a corrective to this
rigidly uniformitarian conception, for they revealed that the crust
of the earth contains the long record of an unmistakable order of
progression in organic types. They proved that plants and animals
have varied widely in successive periods of the earth's history, the
present condition of organic life being only the latest phase of a
long preceding series, each stage of which recedes further from the
existing aspect of things as we trace it backward into the past.
And though no relic had yet been found, or indeed was ever likely to
be found, of the first living things that appeared upon the earth's
surface, the manifest simplification of types in the older formations
pointed irresistibly to some beginning from which the long procession
had taken its start. If then it could thus be demonstrated that there
had been upon the globe an orderly march of living forms, from the
lowliest grades in early times to man himself to-day, and thus that in
one department of her domain, extending through the greater portion of
the records of the earth's history, Nature had not been uniform but
had followed a vast and noble plan of evolution, surely it might have
been expected that those who discovered and made known this plan would
seek to ascertain whether some analogous physical progression from a
definite beginning might not be discernible in the framework of the
globe itself.
But the early masters of the science laboured under two great
disadvantages. In the first place, they found the oldest records of the
earth's history so broken up and defaced as to be no longer legible.
And in the second place, they lived under the spell of that strong
reaction against speculation which followed the bitter controversy
between the Neptunists and Plutonists in the earlier decades of the
century. They considered themselves bound to search for facts, not
to build up theories; and as in the crust of the earth they could
find no facts which threw any light upon the primeval constitution
and subsequent development of our planet, they shut their ears to
any theoretical interpretations that might be offered from other
departments of science. It was enough for them to maintain, as
Hutton had done, that in the visible structure of the earth itself
no trace can be found of the beginning of things, and that the
oldest terrestrial records reveal no physical conditions essentially
different from those in which we still live. They doubtless listened
with interest to the speculations of Kant, Laplace, and Herschel, on
the probable evolution of nebulæ, suns, and planets, but it was with
the languid interest attaching to ideas that lay outside of their own
domain of research. They recognised no practical connection between
such speculations and the data furnished by the earth itself as to its
own history and progress.
This curious lethargy with respect to theory, on the part of men
who were popularly regarded as among the most speculative followers
of science, would probably not have been speedily dispelled by any
discovery made within their own field of observation. Even now, after
many years of the most diligent research, the first chapters of our
planet's history remain undiscovered or undecipherable. On the great
terrestrial palimpsest the earliest inscriptions seem to have been
hopelessly effaced by those of later ages. But the question of the
primeval condition and subsequent history of the planet might be
considered from the side of astronomy and physics. And it was by
investigations of this nature that the geological torpor was eventually
dissipated. To our illustrious former President, Lord Kelvin, who
occupied this chair when the Association last met in Edinburgh, is
mainly due the rousing of attention to this subject. By the most
convincing arguments he showed how impossible it was to believe in the
extreme doctrine of uniformitarianism. And though, owing to uncertainty
in regard to some of the data, wide limits of time were postulated by
him, he insisted that within these limits the whole evolution of the
earth and its inhabitants must have been comprised. While, therefore,
the geological doctrine that the present order of Nature must be our
guide to the interpretation of the past remained as true and fruitful
as ever, it had now to be widened by the reception of evidence
furnished by a study of the earth as a planetary body. The secular
loss of heat, which demonstrably takes place both from the earth and
the sun, made it quite certain that the present could not have been
the original condition of the system. This diminution of temperature
with all its consequences is not a mere matter of speculation, but
a physical fact of the present time as much as any of the familiar
physical agencies that affect the surface of the globe. It points with
unmistakable directness to that beginning of things of which Hutton and
his followers could find no sign.
Another modification or enlargement of the uniformitarian doctrine
was brought about by continued investigation of the terrestrial crust
and consequent increase of knowledge respecting the history of the
earth. Though Hutton and Playfair believed in periodical catastrophes,
and indeed required these to recur in order to renew and preserve the
habitable condition of our planet, their successors gradually came to
view with repugnance any appeal to abnormal, and especially to violent
manifestations of terrestrial vigour, and even persuaded themselves
that such slow and comparatively feeble action as is now witnessed by
man could alone be recognised in the evidence from which geological
history must be compiled. Well do I remember in my own boyhood what
a cardinal article of faith this prepossession had become. We were
taught by our great and honoured master, Lyell, to believe implicitly
in gentle and uniform operations, extended over indefinite periods of
time, though possibly some, with the zeal of partisans, carried this
belief to an extreme which Lyell himself did not approve. The most
stupendous marks of terrestrial disturbance, such as the structure of
great mountain chains, were deemed to be more satisfactorily accounted
for by slow movements prolonged through indefinite ages than by any
sudden convulsion.
What the more extreme members of the uniformitarian school failed to
perceive was the absence of all evidence that terrestrial catastrophes
even on a colossal scale might not be a part of the present economy
of this globe. Such occurrences might never seriously affect the
whole earth at one time, and might return at such wide intervals that
no example of them has yet been chronicled by man. But that they
have occurred again and again, and even within comparatively recent
geological times, hardly admits of serious doubt. How far at different
epochs and in various degrees they may have included the operation of
cosmical influences lying wholly outside the planet, and how far they
have resulted from movements within the body of the planet itself, must
remain for further inquiry. Yet the admission that they have played
a part in geological history may be freely made without impairing
the real value of the Huttonian doctrine, that in the interpretation
of this history our main guide must be a knowledge of the existing
processes of terrestrial change.
As the most recent and best known of these great transformations, the
Ice-Age stands out conspicuously before us. If any one sixty years ago
had ventured to affirm that, at no very distant era in the past, the
snows and glaciers of the Arctic regions stretched southwards to the
Bristol Channel and the basin of the Thames, he would have been treated
as a mere visionary theorist. Many of the facts to which he would have
appealed in support of his statement were already well known, but they
had received various other interpretations. By some observers, notably
by Hutton's friend, Sir James Hall, they were believed to indicate
violent debacles of water that swept over the face of the land. By
others they were attributed to the strong tides and currents of the
sea when the land stood at a lower level. The uniformitarian school of
Lyell had no difficulty in elevating or depressing land to any required
extent. Indeed, when we consider how averse these philosophers were to
admit any kind or degree of natural operation other than those of which
there was some human experience, we may well wonder at the boldness
with which, on sometimes the slenderest evidence, they made land and
sea change places, on the one hand submerging mountain-ranges, and on
the other placing great barriers of land where a deep ocean rolls.
They took such liberties with geography because only well-established
processes of change were invoked in the operations. Knowing that
during the passage of an earthquake a territory bordering the sea may
be upraised or sunk a few feet, they drew the sweeping inference that
any amount of upheaval or depression of any part of the earth's surface
might be claimed in explanation of geological problems. The progress of
inquiry, started here by Agassiz, while it has somewhat curtailed this
geographical license, has now made known in great detail the strange
story of the Ice-Age.
There cannot be any doubt that after man had become a denizen of the
earth, a great physical change came over the northern hemisphere.
The climate, which had previously been so mild that evergreen trees
flourished within ten or twelve degrees of the north pole, now became
so severe that vast sheets of snow and ice covered the north of Europe
and crept southward beyond the south coast of Ireland, almost as far
as the southern shores of England, and across the Baltic into France
and Germany. This Arctic transformation was not an episode that
lasted merely a few seasons, and left the land to resume thereafter
its ancient aspect. With various successive fluctuations it must have
endured for many thousands of years. When it began to disappear it
probably faded away as slowly and imperceptibly as it had advanced,
and when it finally vanished it left Europe and North America
profoundly changed in the character alike of their scenery and of their
inhabitants. The rugged, rocky contours of earlier times were ground
smooth and polished by the march of the ice across them, while the
lower grounds were buried under wide and thick sheets of clay, gravel,
and sand, left behind by the melting ice. The varied and abundant
flora which had spread so far within the Arctic circle was driven away
into more southern and less ungenial climes. But most memorable of all
was the extirpation of the prominent large animals which, before the
advent of the ice, had roamed over Europe. The lions, hyaenas, wild
horses, hippopotami and other creatures either became entirely extinct
or were driven into the Mediterranean basin and into Africa. In their
place came northern forms--the reindeer, glutton, musk ox, woolly
rhinoceros, and mammoth.
Such a marvellous transformation in climate, in scenery, in vegetation,
and in inhabitants, within what was after all but a brief portion of
geological time, though it may have involved no sudden or violent
convulsion, is surely entitled to rank as a catastrophe in the history
of the globe. It was possibly brought about mainly if not entirely by
the operation of forces external to the earth. No similar calamity
having befallen the continents within the time during which man
has been recording his experience, the Ice-Age might be cited as a
contradiction to the doctrine of uniformity. And yet it manifestly
arrived as part of the established order of Nature. Whether or not
we grant that other ice-ages preceded the last great one, we must
admit that the conditions under which it arose, so far as we know
them, might conceivably have occurred before and may occur again. The
various agencies called into play by the extensive refrigeration of the
northern hemisphere were not different from those with which we are
familiar. Snow fell and glaciers crept as they do to-day. Ice scored
and polished rocks exactly as it still does among the Alps and in
Norway. There was nothing abnormal in the phenomena save the scale on
which they were manifested. And thus, taking a broad view of the whole
subject, although we cannot yet explain the origin of the catastrophe,
we see in its progress the operation of natural processes which we know
to be integral parts of the machinery whereby the surface of the earth
is still continually transformed.