The Antiquity of Man Part 27

(* De la Beche, "Geological Researches" page 191.)

The naturalist would have been ent.i.tled to a.s.sume the former union, within the Pleistocene period, of all the British Isles with each other and with the Continent, as expressed in the map, Figure 41, even if there had been no geological facts in favour of such a junction. For in no other way would he be able to account for the ident.i.ty of the fauna and flora found throughout these lands. Had they been separated ever since the Miocene period, like Madeira, Porto Santo, and the Desertas, const.i.tuting the small Madeiran Archipelago, we might have expected to discover a difference in the species of land-sh.e.l.ls, not only when Ireland was compared to England, but when different islands of the Hebrides were contrasted one with another, and each of them with England. It would not, however, be necessary, in order to effect the complete fusion of the animals and plants which we witness, to a.s.sume that all parts of the area formed continuous land at one and the same moment of time, but merely that the several portions were so joined within the Pleistocene era as to allow the animals and plants to migrate freely in succession from one district to another.

SOUTHERNMOST EXTENT OF ERRATICS IN ENGLAND.

In reference to that portion of the south of England which is marked by diagonal lines in Figure 39, the theory of its having been an area of dry land during the period of great submergence and floating ice does not depend merely on negative evidence, such as the absence of the northern drift or boulder clay on its surface; but we have also, in favour of the same conclusion, the remarkable fact of the presence of erratic blocks on the southern coast of Suss.e.x, implying the existence there of an ancient coast-line at a period when the cold must have been at its height.

These blocks are to be seen in greatest number at Pagham and Selsea, 15 miles south of Chichester, in lat.i.tude 50 degrees 40 minutes north.

They consist of fragments of granite, syenite and greenstone, as well as of Devonian and Silurian rocks, some of them of large size. I measured one of granite at Pagham, 27 feet in circ.u.mference. They are not of northern origin, but must have come from the coast of Normandy or Brittany, or from land which may once have existed to the south-west, in what is now the English Channel.

They were probably drifted into their present site by coast ice, and the yellow clay and gravel in which they are embedded are a littoral formation, as shown by the sh.e.l.ls. Beneath the gravel containing these large erratics, is a blue mud in which skeletons of Elephas antiquus, and other mammalia, have been observed. Still lower occurs a sandy loam, from which Mr. R.G. Austen* has collected thirty-eight species of marine sh.e.l.ls, all Recent, but forming an a.s.semblage differing as a whole from that now inhabiting the English Channel.

(* "Quarterly Journal of the Geological Society" volume 13 1857 page 50.)

The presence among them of Lutraria rugosa and Pecten polymorphus, not known to range farther north in the actual seas than the coast of Portugal, indicates a somewhat warmer temperature at the time when they flourished. Subsequently, there must have been great cold when the Selsea erratics were drifted into their present position, and this cold doubtless coincided in time with a low temperature farther north. [30]

These transported rocks of Suss.e.x are somewhat older than a sea-beach with Recent marine sh.e.l.ls which at Brighton is covered by Chalk rubble, called the "elephant-bed" which I cannot describe in this place, but I allude to it as one of many geological proofs of the former existence of a seash.o.r.e in this region, and of ancient cliffs bounding the channel between France and England, all of older date than the close of the glacial period. [31]

In order to form a connected view of the most simple series of changes in physical geography which can possibly account for the phenomena of the glacial period, and the period of the establishment of the present provinces of animals and plants, the following geographical states of the British and adjoining areas may be enumerated.

First, a continental period, towards the close of which the forest of Cromer flourished: when the land was at least 500 feet above its present level, perhaps much higher, and its extent probably greater than that given in the map, Figure 41.

Secondly, a period of submergence, by which the land north of the Thames and Bristol Channel, and that of Ireland, was gradually reduced to such an archipelago as is pictured in map, Figure 40; and finally to such a general prevalence of sea as is seen in map, Figure 39. This was the period of great submergence and of floating ice, when the Scandinavian flora, which occupied the lower grounds during the first continental period, may have obtained exclusive possession of the only lands not covered with perpetual snow.

Thirdly, a second continental period when the bed of the glacial sea, with its marine sh.e.l.ls and erratic blocks, was laid dry, and when the quant.i.ty of land equalled that of the first period, and therefore probably exceeded that represented in the map, Figure 41. During this period there were glaciers in the higher mountains of Scotland and Wales, and the Welsh glaciers, as we have seen, pushed before them and cleared out the marine drift with which some valleys had been filled during the period of submergence. The parallel roads of Glen Roy are referable to some part of the same era.

As a reason for presuming that the land which in map, Figure 41, is only represented as 600 feet above its present level, was during part of this period much higher, Professor Ramsay has suggested that, as the previous depression far exceeded 100 fathoms (amounting in Wales to 1400 feet, as shown by marine sh.e.l.ls, and to 2300, by stratified drift), it is not improbable that the upward movement was on a corresponding scale.

In pa.s.sing from the period of chief submergence to this second continental condition of things, we may conceive a gradual change first from that of Map 39 to Map 40, then from the latter phase to that of Map 41, and finally to still greater accessions of land. During this last period the pa.s.sage of the Germanic flora into the British area took place, and the Scandinavian plants, together with northern insects, birds, and quadrupeds, retreated into the higher grounds.

Judging from the evidence at present before us, the first appearance of Man, when, together with the mammoth and woolly rhinoceros, or with the Elephas antiquus, Rhinoceros hemitoechus, and Hippopotamus major, he ranged freely from all parts of the Continent into the British area, took place during this second continental period.

Fourthly, the next and last change comprised the breaking up of the land of the British area once more into numerous islands, ending in the present geographical condition of things. There were probably many oscillations of level during this last conversion of continuous land into islands, and such movements in opposite directions would account for the occurrence of marine sh.e.l.ls at moderate heights above the level of the sea, notwithstanding a general lowering of the land. To the close of this era belong the marine deposits of the Clyde and the Ca.r.s.es of the Tay and Forth, before alluded to.

In a memoir by Professor E. Forbes, before cited, he observes, that the land of pa.s.sage by which the plants and animals migrated into Ireland consisted of the upraised marine drift which had previously formed the bottom of the glacial sea. Portions of this drift extend to the eastern sh.o.r.es of Wicklow and Wexford, others are found in the Isle of Man full of arctic sh.e.l.ls, others on the British coast opposite Ireland. The freshwater marl, containing numerous skeletons of the great deer, or Megaceros, overlie in the Isle of Man that marine glacial drift.

Professor Forbes also remarks that the subsequent disjunction of Ireland from England, or the formation of the Irish Channel, which is less than 400 feet in its greatest depth, preceded the opening of the Straits of Dover, or the final separation of England from the Continent. This he inferred from the present distribution of species both in the animal and vegetable kingdoms. Thus, for example, there are twice as many reptiles in Belgium as in England, and the number inhabiting England is twice that found in Ireland. Yet the Irish species are all common to England, and all the English to Belgium. It is therefore a.s.sumed that the migration of species westward having been the work of time, there was not sufficient lapse of ages to complete the fusion of the continental and British reptilian fauna, before France was separated from England and England from Ireland.

For the same reason there are also a great number of birds of short flight, and small quadrupeds, inhabiting England which do not cross to Ireland, the Irish Channel seeming to have arrested them in their westward course.*

(* E. Forbes, Fauna and Flora of British Isles, "Memoir of the Geological Survey" volume 1 1846 page 344.)

The depth of the Irish Channel in the narrower parts is only 360 feet, and the English Channel between Dover and Calais less than 200, and rarely anywhere more than 300 feet; so that vertical movements of slight amount compared to some of those previously considered, with the aid of denuding operations or the waste of sea cliffs, and the scouring out of the channel, might in time effect the insulation of the lands above alluded to.

TIME REQUIRED FOR SUCCESSIVE CHANGES IN PHYSICAL GEOGRAPHY IN THE PLEISTOCENE PERIOD.

The time which it would require to bring about such changes of level, according to the average rate a.s.sumed in Chapter 3, however vast, will not be found to exceed that which would best explain the successive fluctuations in terrestrial temperature, the glaciation of solid rocks, the transportation of erratics above and below the sea-level, the height of arctic sh.e.l.ls above the sea, and last, not least, the migration of the existing species of animals and plants into their actual stations, and the extinction of some conspicuous forms which flourished during the Pleistocene ages. When we duly consider all these changes which have taken place since the beginning of the glacial epoch, or since the forest of Cromer and the Elephas meridionalis flourished, we shall find that the phenomena become more and more intelligible in proportion to the slowness of the rate of elevation and depression which we a.s.sume.

The submergence of Wales to the extent of 1400 feet, as proved by glacial sh.e.l.ls, would require 56,000 years, at the rate of 2 1/2 feet per century; but taking Professor Ramsay's estimate of 800 feet more, that depression being implied by the position of some of the stratified drift, we must demand an additional period of 32,000 years, amounting in all to 88,000; and the same time would be required for the re-elevation of the tract to its present height. But if the land rose in the second continental period as much as 600 feet above its present level, as in Figure 41, this 600 feet, first of rising and then of sinking, would require 48,000 years more; the whole of the grand oscillation, comprising the submergence and re-emergence, having taken about 224,000 years for its completion; and this, even if there were no pause or stationary period, when the downward movement ceased, and before it was converted into an upward one.

I am aware that it may be objected that the average rate here proposed is a purely arbitrary and conjectural one, because, at the North Cape, it is supposed that there has been a rise of about 5 feet in a century, and at Spitsbergen, according to Mr. Lamont, a still faster upheaval during the last 400 years.*

(* "Seasons with the Sea-Horses" page 202.)

But, granting that in these and some exceptional cases (none of them as yet very well established) the rising or sinking has, for a time, been accelerated, I do not believe the average rate of motion to exceed that above proposed. Mr. Darwin, I find, considers that such a mean rate of upheaval would be as high as we could a.s.sume for the west coast of South America, where we have more evidence of sudden changes of level than anywhere else. He has not, however, attempted to estimate the probable rate of secular elevation in that or any other region.

Little progress has yet been made in divining the most probable causes of these great movements of the earth's crust; yet what little we know of the state of the interior leads us to expect that the gradual expansion or contraction of large portions of the solid crust may be the result of fluctuations in temperature, with which the existence of hundreds of active and thousands of extinct volcanoes is probably connected.

It is ascertained that solid rocks, such as granite and sandstone, expand and contract annually, even under such a moderate range of temperature as that of a Canadian winter and summer. If the heat should go on increasing through a thickness, say only of 10 miles of the earth's crust, the gradual upheaval of the inc.u.mbent ma.s.s may amount to many hundreds of feet; and the elevation may be carried still farther, by the complete fusion of part of the inferior rocks.

According to the experiments of Deville, the contraction of granite, in pa.s.sing from a melted, or as some would say its plastic condition, to a solid state, must be more than 10 per cent.*

(* "Bull. Societe Geologique France" 2nd series volume 4 page 1312.)

So that we have at our command a source of depression on a grand scale, at every period when granitic rocks have originated in the interior of the earth's crust. All mineralogists are agreed that the pa.s.sage of voluminous ma.s.ses, from a liquid or pasty to a solid and crystalline state, must be an extremely slow process. It may often happen that, in the same series of superimposed rocks, some are expanding while still solid or while partially melting, while others are at the same time crystallising and contracting; so that the alterations of level at the surface may be the result of complicated and often of conflicting agencies. The more gradually we conceive such changes to take place, the more comprehensible they become in the eyes of the chemist and natural philosopher who speculates on the changes of the earth's interior; and the more fertile are they in the hands of the geologist in accounting for revolutions on the habitable surface.

We may presume, that after the movement has gone on for a long time in one determinate direction, whether of elevation or depression, the change to an opposite movement, implying the subst.i.tution of a heating for a refrigerating operation, or the reverse, would not take place suddenly; but would be marked by a period of inaction, or of slight movement, or such a state of quiescence, as prevails throughout large areas of dry land in the normal condition of the globe.

I see no reason for supposing that any part of the revolutions in physical geography, to which the maps above described have reference, indicate any catastrophes greater than those which the present generation has witnessed. If Man was in existence when the Cromer forest was becoming submerged, he would have felt no more alarm than the Danish settlers on the east coast of Baffin's Bay, when they found the poles, which they had driven into the beach to secure their boats, had subsided below their original level.

Already, perhaps, the melting ice has thrown down till and boulders upon those poles, a counterpart of the boulder clay which overlies the forest-bed on the Norfolk cliffs.

We have seen that all the plants and sh.e.l.ls, marine and freshwater, of the forest bed, and a.s.sociated fluvio-marine strata of Norfolk, are specifically identical with those of the living European flora and fauna; so that if upon such a stratum a deposit of the present period, whether freshwater or marine, should be thrown down, it might lie conformably over it, and contain the same invertebrate fauna and flora.

The strata so superimposed would, in ordinary geological language, be called contemporaneous, not only as belonging to the same epoch, but as appertaining strictly to the same subdivision of one and the same epoch; although they would in fact have been separated by an interval of several hundred thousand years.

If, in the lower of the two formations, some of the mammalia of the genera elephant and rhinoceros were found to be distinct in species from those of the same genera in the upper or "recent" stratum, it might appear as though there had been a sudden coming in of new forms, and a sudden dying out of old ones; for there would not have been time in the interval for any perceptible change in the invertebrate fauna, by which alone we usually measure the lapse of time in the older formations.

When we are contrasting the vertebrate contents of two sets of superimposed strata of the Cretaceous, Oolitic, or any other ancient formation in which the sh.e.l.ls are identical in species, we ought never to lose sight of the possibility of their having been separated by such intervals or by two or three thousand centuries. That number of years may sometimes be of small moment in reference to the rate of fluctuation of species in the lower animals, but very important when the succession of forms in the highest cla.s.ses of vertebrata is concerned.

If we reflect on the long series of events of the Pleistocene and Recent periods contemplated in this chapter, it will be remarked that the time a.s.signed to the first appearance of Man, so far as our geological inquiries have yet gone, is extremely modern in relation to the age of the existing fauna and flora, or even to the time when most of the living species of animals and plants attained their actual geographical distribution. At the same time it will also be seen, that if the advent of Man in Europe occurred before the close of the second continental period, and antecedently to the separation of Ireland from England and of England from the Continent, the event would be sufficiently remote to cause the historical period to appear quite insignificant in duration, when compared to the antiquity of the human race.

CHAPTER 15. -- EXTINCT GLACIERS OF THE ALPS AND THEIR CHRONOLOGICAL RELATION TO THE HUMAN PERIOD.

Extinct Glaciers of Switzerland.

Alpine Erratic Blocks on the Jura.

Not transported by floating Ice.

Extinct Glaciers of the Italian Side of the Alps.

Theory of the Origin of Lake-Basins by the erosive Action of Glaciers considered.

Successive phases in the Development of Glacial Action in the Alps.

Probable Relation of these to the earliest known Date of Man.

Correspondence of the same with successive Changes in the Glacial Condition of the Scandinavian and British Mountains.

Cold Period in Sicily and Syria.

EXTINCT GLACIERS OF SWITZERLAND.