126 SOFT AND EASILY DESTROYED DEPOSITS.

are seen in long ranges of inland cliffs, which consist throughout their entire extent of one kind of rock, and differ in this from all those cliffs which are formed along coast-lines by the action of waves, as these constantly exhibit sections of various formations.

It is only the harder strata that can resist subaërial action for any considerable period of time; and the formation of such hard strata probably occurs only very rarely. It is a fact worthy of especial note in this connexion, that very few of the tertiary and post tertiary strata are of a nature to resist the prolonged action of subaërial agencies. Any one who has carefully watched the rapidity with which the exposed surfaces of the mammiferous and red crags are undergoing "weathering" and are being removed, will perceive how very improbable it is that any trace of that deposit will be left to tell its tale to a geologist of the remote future.

The cliffs of the coast of Suffolk are a very instructive instance; these are not only undergoing degradation under the influence of the weather, but are gradually being excavated and broken up by the waves. If subsidence of the land were to occur, successive portions of the deposit would be exposed to the same action; so that it is extremely improbable that any of its fossils would remain in

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127

situ; the whole stratum would probably be carried further south by the action of the tides.

It is certain, moreover, that a very large number of the tertiary deposits of England, with the exception perhaps of the clays of the London basin, would be entirely washed away if the level of the land were raised during any very long period of time and then subsequently depressed gradually, so that the uncovered edges of the strata were successively exposed to the influence of the air and

sea.

A walk across the hills to the left of the road between Sunnindale Station and Farnborough will convince the observer at once of the fact that there is little probability that the denuded Bagshot sands, a marine tertiary formation of great thickness, would be fairly represented in future ages if the waves of an advancing ocean washed amongst these hills. Creeping up slowly by the now fertile alluvial valleys, the mind pictures such a sea washing down and levelling the upper layers of the ancient sea-bed, and depositing in it, perhaps, recent shells and then covering it over with chalk washed from the high lands surrounding the London basin. If previous elevation of our island were to occur, and the valley of the Thames and its tributaries were worn down some eight or ten hundred feet first, so that the now shallow valleys became greatly aug

128

TERTIARY DEPOSITS.

mented, the secondary strata would be denuded in places, and the high sand and clay cliffs would be mingled and levelled by the advancing waves during subsequent depression.

On the other hand it is probable that some at least of the tertiary deposits of the Mediterranean region, which consist of limestone of considerable hardness, would remain even under the least-favourable circumstances; and it is indubitable that the softer clays, marls, and gypsums would be preserved under favourable conditions.

As both subsidence and elevation are always, in all probability at least, extremely gradual, and as very long periods of quiescence intervene, it is far more likely that any given formation, except those of stony hardness, will perish than that it will remain to record the age in which it was deposited. We know that a vast period must have elapsed since the London and plastic clays were deposited; and yet it is extremely improbable that one remnant of the Miocene and Pliocene crag of England which covers them will remain to bear witness of that almost immeasurable period of time. Even the harder rocks of the same era on the continent of Europe may be washed and pulverized until no trace remains; and, as we have seen, the great basins of Eocene deposit are by no means certain to stand the test of time.

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With regard to the deposits which are at present in process of formation, a very large number will in all probability contain no fossils. The process of fossilization is a chemical one, and depends on a large number of conditions. The formation of any concrete, especially such as contains much calcareous matter, is favourable to the preservation of fossils; but the mud and silt at the bottom of our large rivers and estuaries, which is perhaps the most likely resting-place for organic remains, is in the majority of cases quite unfit for their preservation.

The deposition of concrete material is undoubtedly a chemical phenomenon. The recent deposits of the Mediterranean are apparently of this character, and the bed of the Atlantic is doubtless highly adapted in places for the preservation of organic remains. During the process of upheaval, however, a very large portion of these strata will be inevitably subjected to the action of the waves along the coast-line; and future ages will at least, considerably reduce the thickness of these deposits. long before they are again submerged and covered by another layer of thick rock. They cannot become safe records of the present period until all these conditions have been successfully fulfilled. The thin edges of the present Mediterranean deposit alone are likely to contain the remains of

130 PROBABILITIES AGAINST STRATA PERSISTING.

existing terrestrial vertebrates; and if the upheaval of the continent of Europe occur gradually in the remote future, they will undoubtedly be entirely removed by the action of the waves. The deltamud of the great rivers is an extremely unfavourable deposit to stand the test of time, and, like our crag deposits, would probably crumble away under the influence of the air and coast-line action. If this be so, we may well believe that the record of the tertiary and recent periods on any future continent in the European region will be represented only by a few imperfect deposits at most. The surface of every stratum is liable, nay, almost certain, to be removed to a greater or less extent after or during upheaval; it will again undergo destruction during subsidence; and many will be entirely removed, as, for instance, the crag and recent deposits of England.

Lastly, long periods elapse in every region in which the deposits contain no fossils, from the mud being of a nature quite unfavourable for their preservation; so that we may rest assured that the record of the organic world which will remain to chronicle the recent and tertiary periods will be of the most meagre description.

If we once attain the true conception of the probabilities of the case, if we once bring our minds to remember that every thick deposit must