3000 feet. Prof. Ramsay has published an account of a downthrow in
Anglesea of 2300 feet; and he informs me that he fully believes there is
one in Merionethshire of 12,000 feet; yet in these cases there is nothing
on the surface to show such prodigious movements; the pile of rocks on the
one or other side having been smoothly swept away. The consideration of
these facts impresses my mind almost in the same manner as does the vain
endeavour to grapple with the idea of eternity.
I am tempted to give one other case, the well-known one of the denudation
of the Weald. Though it must be admitted that the denudation of the Weald
has been a mere trifle, in comparison with that which has removed masses of
our palaeozoic strata, in parts ten thousand feet in thickness, as shown in
Prof. Ramsay's masterly memoir on this subject. Yet it is an admirable
lesson to stand on the North Downs and to look at the distant South Downs;
for, remembering that at no great distance to the west the northern and
southern escarpments meet and close, one can safely picture to oneself the
great dome of rocks which must have covered up the Weald within so limited
a period as since the latter part of the Chalk formation. The distance
from the northern to the southern Downs is about 22 miles, and the
thickness of the several formations is on an average about 1100 feet, as I
am informed by Prof. Ramsay. But if, as some geologists suppose, a range
of older rocks underlies the Weald, on the flanks of which the overlying
sedimentary deposits might have accumulated in thinner masses than
elsewhere, the above estimate would be erroneous; but this source of doubt
probably would not greatly affect the estimate as applied to the western
extremity of the district. If, then, we knew the rate at which the sea
commonly wears away a line of cliff of any given height, we could measure
the time requisite to have denuded the Weald. This, of course, cannot be
done; but we may, in order to form some crude notion on the subject, assume
that the sea would eat into cliffs 500 feet in height at the rate of one
inch in a century. This will at first appear much too small an allowance;
but it is the same as if we were to assume a cliff one yard in height to be
eaten back along a whole line of coast at the rate of one yard in nearly
every twenty-two years. I doubt whether any rock, even as soft as chalk,
would yield at this rate excepting on the most exposed coasts; though no
doubt the degradation of a lofty cliff would be more rapid from the
breakage of the fallen fragments. On the other hand, I do not believe that
any line of coast, ten or twenty miles in length, ever suffers degradation
at the same time along its whole indented length; and we must remember that
almost all strata contain harder layers or nodules, which from long
resisting attrition form a breakwater at the base. Hence, under ordinary
circumstances, I conclude that for a cliff 500 feet in height, a denudation
of one inch per century for the whole length would be an ample allowance.
At this rate, on the above data, the denudation of the Weald must have
required 306,662,400 years; or say three hundred million years.
The action of fresh water on the gently inclined Wealden district, when
upraised, could hardly have been great, but it would somewhat reduce the
above estimate. On the other hand, during oscillations of level, which we
know this area has undergone, the surface may have existed for millions of
years as land, and thus have escaped the action of the sea: when deeply
submerged for perhaps equally long periods, it would, likewise, have
escaped the action of the coast-waves. So that in all probability a far
longer period than 300 million years has elapsed since the latter part of
the Secondary period.
I have made these few remarks because it is highly important for us to gain
some notion, however imperfect, of the lapse of years. During each of
these years, over the whole world, the land and the water has been peopled
by hosts of living forms. What an infinite number of generations, which
the mind cannot grasp, must have succeeded each other in the long roll of
years! Now turn to our richest geological museums, and what a paltry
display we behold!
On the poorness of our Palaeontological collections. -- That our
palaeontological collections are very imperfect, is admitted by every one.
The remark of that admirable palaeontologist, the late Edward Forbes,
should not be forgotten, namely, that numbers of our fossil species are
known and named from single and often broken specimens, or from a few
specimens collected on some one spot. Only a small portion of the surface
of the earth has been geologically explored, and no part with sufficient
care, as the important discoveries made every year in Europe prove. No
organism wholly soft can be preserved. Shells and bones will decay and
disappear when left on the bottom of the sea, where sediment is not
accumulating. I believe we are continually taking a most erroneous view,
when we tacitly admit to ourselves that sediment is being deposited over
nearly the whole bed of the sea, at a rate sufficiently quick to embed and
preserve fossil remains. Throughout an enormously large proportion of the
ocean, the bright blue tint of the water bespeaks its purity. The many
cases on record of a formation conformably covered, after an enormous
interval of time, by another and later formation, without the underlying
bed having suffered in the interval any wear and tear, seem explicable only
on the view of the bottom of the sea not rarely lying for ages in an
unaltered condition. The remains which do become embedded, if in sand or
gravel, will when the beds are upraised generally be dissolved by the
percolation of rain-water. I suspect that but few of the very many animals
which live on the beach between high and low watermark are preserved. For
instance, the several species of the Chthamalinae (a sub-family of sessile
cirripedes) coat the rocks all over the world in infinite numbers: they
are all strictly littoral, with the exception of a single Mediterranean
species, which inhabits deep water and has been found fossil in Sicily,
whereas not one other species has hitherto been found in any tertiary
formation: yet it is now known that the genus Chthamalus existed during
the chalk period. The molluscan genus Chiton offers a partially analogous
case.
With respect to the terrestrial productions which lived during the
Secondary and Palaeozoic periods, it is superfluous to state that our
evidence from fossil remains is fragmentary in an extreme degree. For
instance, not a land shell is known belonging to either of these vast
periods, with one exception discovered by Sir C. Lyell in the carboniferous
strata of North America. In regard to mammiferous remains, a single glance
at the historical table published in the Supplement to Lyell's Manual, will
bring home the truth, how accidental and rare is their preservation, far
better than pages of detail. Nor is their rarity surprising, when we
remember how
large a proportion of the bones of tertiary mammals have been
discovered either in caves or in lacustrine deposits; and that not a cave
or true lacustrine bed is known belonging to the age of our secondary or
palaeozoic formations.
But the imperfection in the geological record mainly results from another
and more important cause than any of the foregoing; namely, from the
several formations being separated from each other by wide intervals of
time. When we see the formations tabulated in written works, or when we
follow them in nature, it is difficult to avoid believing that they are
closely consecutive. But we know, for instance, from Sir R. Murchison's
great work on Russia, what wide gaps there are in that country between the
superimposed formations; so it is in North America, and in many other parts
of the world. The most skilful geologist, if his attention had been
exclusively confined to these large territories, would never have suspected
that during the periods which were blank and barren in his own country,
great piles of sediment, charged with new and peculiar forms of life, had
elsewhere been accumulated. And if in each separate territory, hardly any
idea can be formed of the length of time which has elapsed between the
consecutive formations, we may infer that this could nowhere be
ascertained. The frequent and great changes in the mineralogical
composition of consecutive formations, generally implying great changes in
the geography of the surrounding lands, whence the sediment has been
derived, accords with the belief of vast intervals of time having elapsed
between each formation.
But we can, I think, see why the geological formations of each region are
almost invariably intermittent; that is, have not followed each other in
close sequence. Scarcely any fact struck me more when examining many
hundred miles of the South American coasts, which have been upraised
several hundred feet within the recent period, than the absence of any
recent deposits sufficiently extensive to last for even a short geological
period. Along the whole west coast, which is inhabited by a peculiar
marine fauna, tertiary beds are so scantily developed, that no record of
several successive and peculiar marine faunas will probably be preserved to
a distant age. A little reflection will explain why along the rising coast
of the western side of South America, no extensive formations with recent
or tertiary remains can anywhere be found, though the supply of sediment
must for ages have been great, from the enormous degradation of the
coast-rocks and from muddy streams entering the sea. The explanation, no
doubt, is, that the littoral and sub-littoral deposits are continually worn
away, as soon as they are brought up by the slow and gradual rising of the
land within the grinding action of the coast-waves.
We may, I think, safely conclude that sediment must be accumulated in
extremely thick, solid, or extensive masses, in order to withstand the
incessant action of the waves, when first upraised and during subsequent
oscillations of level. Such thick and extensive accumulations of sediment
may be formed in two ways; either, in profound depths of the sea, in which
case, judging from the researches of E. Forbes, we may conclude that the
bottom will be inhabited by extremely few animals, and the mass when
upraised will give a most imperfect record of the forms of life which then
existed; or, sediment may be accumulated to any thickness and extent over a
shallow bottom, if it continue slowly to subside. In this latter case, as
long as the rate of subsidence and supply of sediment nearly balance each
other, the sea will remain shallow and favourable for life, and thus a
fossiliferous formation thick enough, when upraised, to resist any amount
of degradation, may be formed.
I am convinced that all our ancient formations, which are rich in fossils,
have thus been formed during subsidence. Since publishing my views on this
subject in 1845, I have watched the progress of Geology, and have been
surprised to note how author after author, in treating of this or that
great formation, has come to the conclusion that it was accumulated during
subsidence. I may add, that the only ancient tertiary formation on the
west coast of South America, which has been bulky enough to resist such
degradation as it has as yet suffered, but which will hardly last to a
distant geological age, was certainly deposited during a downward
oscillation of level, and thus gained considerable thickness.
All geological facts tell us plainly that each area has undergone numerous
slow oscillations of level, and apparently these oscillations have affected
wide spaces. Consequently formations rich in fossils and sufficiently
thick and extensive to resist subsequent degradation, may have been formed
over wide spaces during periods of subsidence, but only where the supply of
sediment was sufficient to keep the sea shallow and to embed and preserve
the remains before they had time to decay. On the other hand, as long as
the bed of the sea remained stationary, thick deposits could not have been
accumulated in the shallow parts, which are the most favourable to life.
Still less could this have happened during the alternate periods of
elevation; or, to speak more accurately, the beds which were then
accumulated will have been destroyed by being upraised and brought within
the limits of the coast-action.
Thus the geological record will almost necessarily be rendered
intermittent. I feel much confidence in the truth of these views, for they
are in strict accordance with the general principles inculcated by Sir C.
Lyell; and E. Forbes independently arrived at a similar conclusion.
One remark is here worth a passing notice. During periods of elevation the
area of the land and of the adjoining shoal parts of the sea will be
increased, and new stations will often be formed;--all circumstances most
favourable, as previously explained, for the formation of new varieties and
species; but during such periods there will generally be a blank in the
geological record. On the other hand, during subsidence, the inhabited
area and number of inhabitants will decrease (excepting the productions on
the shores of a continent when first broken up into an archipelago), and
consequently during subsidence, though there will be much extinction, fewer
new varieties or species will be formed; and it is during these very
periods of subsidence, that our great deposits rich in fossils have been
accumulated. Nature may almost be said to have guarded against the
frequent discovery of her transitional or linking forms.
From the foregoing considerations it cannot be doubted that the geological
record, viewed as a whole, is extremely imperfect; but if we confine our
attention to any one formation, it becomes more difficult to understand,
why we do not therein find closely graduated varieties between the allied
species which lived at its commencement and at its close. Some cases are
on record of the same species presenting distinct varieties in the upper br />
and lower parts of the same formation, but, as they are rare, they may be
here passed over. Although each formation has indisputably required a vast
number of years for its deposition, I can see several reasons why each
should not include a graduated series of links between the species which
then lived; but I can by no means pretend to assign due proportional weight
to the following considerations.
Although each formation may mark a very long lapse of years, each perhaps
is short compared with the period requisite to change one species into
another. I am aware that two palaeontologists, whose opinions are worthy
of much deference, namely Bronn and Woodward, have concluded that the
average duration of each formation is twice or thrice as long as the
average duration of specific forms. But insuperable difficulties, as it
seems to me, prevent us coming to any just conclusion on this head. When
we see a species first appearing in the middle of any formation, it would
be rash in the extreme to infer that it had not elsewhere previously
existed. So again when we find a species disappearing before the uppermost
layers have been deposited, it would be equally rash to suppose that it
then became wholly extinct. We forget how small the area of Europe is
compared with the rest of the world; nor have the several stages of the
same formation throughout Europe been correlated with perfect accuracy.
With marine animals of all kinds, we may safely infer a large amount of
migration during climatal and other changes; and when we see a species
first appearing in any formation, the probability is that it only then
first immigrated into that area. It is well known, for instance, that
several species appeared somewhat earlier in the palaeozoic beds of North
America than in those of Europe; time having apparently been required for
their migration from the American to the European seas. In examining the
latest deposits of various quarters of the world, it has everywhere been
noted, that some few still existing species are common in the deposit, but
have become extinct in the immediately surrounding sea; or, conversely,
that some are now abundant in the neighbouring sea, but are rare or absent
in this particular deposit. It is an excellent lesson to reflect on the
ascertained amount of migration of the inhabitants of Europe during the
Glacial period, which forms only a part of one whole geological period; and
likewise to reflect on the great changes of level, on the inordinately
great change of climate, on the prodigious lapse of time, all included
within this same glacial period. Yet it may be doubted whether in any
quarter of the world, sedimentary deposits, including fossil remains, have
gone on accumulating within the same area during the whole of this period.
It is not, for instance, probable that sediment was deposited during the
whole of the glacial period near the mouth of the Mississippi, within that
limit of depth at which marine animals can flourish; for we know what vast
geographical changes occurred in other parts of America during this space
of time. When such beds as were deposited in shallow water near the mouth
of the Mississippi during some part of the glacial period shall have been
upraised, organic remains will probably first appear and disappear at
different levels, owing to the migration of species and to geographical
changes. And in the distant future, a geologist examining these beds,
might be tempted to conclude that the average duration of life of the
embedded fossils had been less than that of the glacial period, instead of
having been really far greater, that is extending from before the glacial
epoch to the present day.
In order to get a perfect gradation between two forms in the upper and
lower parts of the same formation, the deposit must have gone on
accumulating for a very long period, in order to have given sufficient time