Geological Evidences of the Antiquity of Man/Chapter 14
CHRONOLOGICAL RELATIONS OF THE GLACIAL PERIOD AND THE EARLIEST SIGNS OF MAN'S APPEARANCE IN EUROPE,
SIGNS OF EXTINCT GLACIERS IN WALES—GREAT SUBMERGENCE OF WALES DURING THE GLACIAL PERIOD PROVED BY MARINE SHELLS—STILL GREATER DEPRESSION INFERRED FROM STRATIFIED DRIFT—SCARCITY OF ORGANIC REMAINS IN GLACIAL FORMATIONS—SIGNS OF EXTINCT GLACIERS IN ENGLAND—ICE ACTION IN IRELAND—MAPS ILLUSTRATING SUCCESSIVE REVOLUTIONS IN PHYSICAL GEOGRAPHY DURING THE POST-PLIOCENE PERIOD—SOUTHERNMOST EXTENT OF ERRATICS IN ENGLAND—SUCCESSIVE PERIODS OF JUNCTION AND SEPARATION OF ENGLAND, IRELAND, AND THE CONTINENT—TIME REQUIRED FOR THESE CHANGES—PROBABLE CAUSES OF THE UPHEAVAL AND SUBSIDENCE OF THE EARTH'S CRUST—ANTIQUITY OF MAN CONSIDERED IN RELATION TO THE AGE OF THE EXISTING FAUNA AND FLORA.
Extinct Glaciers in Wales.
THE considerable amount of vertical movement in opposite directions, which was suggested in the last chapter, as affording the most probable explanation of the position of some of the stratified and fossiliferous drifts of Scotland, formed since the commencement of the glacial period, will appear less startling, if it can be shown that independent observations lead us to infer that a geographical revolution of still greater magnitude accompanied the successive phases of glaciation through which the Welsh mountains have passed.
That Wales was once an independent centre of the dispersion of erratic blocks, has long been acknowledged. Dr. Buckland published in 1842 his reasons for believing that the Snowdonian mountains in Caernarvonshire were formerly covered with glaciers, which radiated from the central heights through the seven principal valleys of that chain, where striæ and flutings are seen on the polished rocks directed towards as many different points of the compass. He also described the 'moraines' of the ancient glaciers, and the rounded masses of polished rock, called in Switzerland 'roches moutonnées.' His views respecting the old extinct glaciers of North Wales were subsequently confirmed by Mr. Darwin, who attributed the transport of many of the larger erratic blocks to floating ice. Much of the Welsh glacial drift had already been shown by Mr. Trimmer to have had a submarine origin, and Mr. Darwin maintained that when the land rose again to nearly its present height, glaciers filled the valleys, and 'swept them clean of all the rubbish left by the sea.'
Professor Ramsay, in a paper read to the Geological Society in 1851, and in a later work on the glaciation of North Wales, described three successive glacial periods, during the first of which the land was much higher than it now is, and the quantity of ice excessive; secondly, a period of submergence when the land was 2,300 feet lower than at present, and when the higher mountain tops only stood out of the sea as a cluster of low islands, which nevertheless were covered with snow; and lastly, a third period when the marine boulder drift formed in the middle period was ploughed out of the larger valleys by a second set of glaciers, smaller than those of the first period. This last stage of glaciation may have coincided with that of the parallel roads of Glen Roy, spoken of in the last chapter. In Wales it was certainly preceded by submergence, and the rocks had been exposed to glacial polishing and friction before they sank.
Fortunately the evidence of the sojourn of the Welsh mountains beneath the waters of the sea is not deficient, as in Scotland, in that complete demonstration which the presence of marine shells affords. The late Mr. Trimmer discovered such shells on Moel Tryfane, in North Wales, in drift elevated 1,392 feet above the level of the sea. It appears from his observations, and those of the late Edward Forbes, corroborated by others of Professor Ramsay and Mr. Prestwich, that about twelve species of shells, including Fusus bamfius, F. antiquus, Venus striatula (Forbes and Hanley), have been met with at heights of between 1,000 and 1,400 feet, in drift, reposing on a surface of rock which had been previously exposed to glacial friction and striation. The shells, as a whole, are those of the glacial period, and not of the Norwich Crag. Two localities of these shells in Wales, in addition to that first pointed out by Mr. Trimmer, have since been observed by Professor Ramsay, who, however, is of opinion that the amount of submergence can by no means be limited to the extreme height to which the shells happen to have been traced; for drift of the same character as that of Moel Tryfane extends continuously to the height of 2,300 feet.
Rarity of Organic Remains in Glacial Formations.
The general dearth of shells in such formations, below as well as above the level at which Mr. Trimmer first found them, deserves notice. Whether we can explain it or not, it is a negative character which seems to belong very generally to deposits formed in glacial seas. The porous nature of the strata, and the length of time during which they have been permeated by rain-water, may partly account, as we hinted in a former chapter, for the destruction of organic remains. But it is also possible that they were originally scarce, for we read of the waters of the sea being so freshened and chilled by the melting of ice-bergs in some Norwegian and Icelandic fiords, that the fish are driven away, and all the mollusca killed. The moraines of glaciers are always from the first devoid of shells, and if transported by ice-bergs to a distance, and deposited where the ice melts, may continue as barren of every indication of life, as they were when they originated.
Nevertheless, it may be said, on the other hand, that herds of seals and walruses crowd the floating ice of Spitzbergen in lat. 80° north, of which Mr. Lamont has recently given us a lively picture, and huge whales fatten on myriads of pteropods in polar regions. It had been suggested that the bottom of the sea, at the era of extreme submergence in Scotland and Wales, was so deep as to reach the zero of animal life, which, in part of the Mediterranean (the Egean, for example), the late Edward Forbes fixed, after a long series of dredgings, at 300 fathoms. But the shells of the glacial drift of Scotland and Wales, when they do occur, are not those of deep seas; and, moreover, our faith in the uninhabitable state of the ocean at great depths has been rudely shaken, by the recent discovery by Captain M'Clintock and Dr. Wallich, of starfish in water more than a thousand fathoms deep (7,560 feet!), midway between Greenland and Iceland. That these radiata were really dredged up from the bottom, and that they had been living and feeding there, appeared from the fact that their stomachs were full of globigerina, of which foraminiferous creatures, both living and dead, the oozy bed of the ocean at that vast depth was found to be exclusively composed.
Whatever may be the cause, the fact is certain, that over large areas in Scotland, Ireland, and Wales, I might add throughout the northern hemisphere on both sides of the Atlantic, the stratified drift of the glacial period is very commonly devoid of fossils, in spite of the occurrence here and there, at the height of 500, 700, and even 1,400 feet, of marine shells. These, when met with, belong, with few exceptions, to known living species. I am therefore unable to agree with Mr. Kjerulf that the amount of former submergence can be measured by the extreme height at which shells happen to have been found.
Glacial Formations in England.
The mountains of Cumberland and Westmoreland, and the English lake district, afford equally unequivocal vestiges of ice-action not only in the form of polished and grooved surfaces, but also of those rounded bosses before mentioned, as being so abundant in the Alpine valleys of Switzerland, where glaciers exist, or have existed. Mr. Hull has lately published a faithful account of these phenomena, and has given a representation of some of the English 'roches moutonnées,' which precisely resemble hundreds of dome-shaped protuberances in North Wales, Sweden, and North America.
The marks of glaciation on the rocks, and the transportation of erratics from Cumberland to the eastward, have been traced by Professor Phillips over a large part of Yorkshire, extending to a height of 1,500 feet above the sea; and similar northern drift has been observed in Lancashire, Cheshire, Derbyshire, Shropshire, Staffordshire, and Worcestershire. It is rare to find marine shells, except at heights of 200 or 300 feet; but a few instances of their occurrence have been noticed, especially of Turritella communis (a gregarious shell), far in the interior, at elevations of 500 feet, and even of 700 in Derbyshire, and some adjacent counties, as I learn from Mr. Binney and Mr. Prestwich.
Such instances are of no small theoretical interest, as enabling us to account for the scattering of large erratic blocks at equal or much greater elevations, over a large part of the northern and midland counties, such as could only have been conveyed to their present sites by floating ice. Of this nature, among others, is a remarkable angular block of syenitic greenstone, four feet and a half by four feet square, and two feet thick, which Mr. Darwin describes as lying on the summit of Ashley Heath, in Staffordshire, 803 feet above the sea, resting on new red sandstone.
Signs of Ice-action and Submergence in Ireland during the Glacial Period.
In Ireland we encounter the same difficulty as in Scotland, in determining how much of the glaciation of the higher mountains should be referred to land glaciers, and how much to floating ice, during submergence. The signs of glacial action have been traced by Professor Jukes to elevations of 2,500 feet in the Killarney district, and to great heights in other mountainous regions; but marine shells have rarely been met with higher than 600 feet above the sea, and that chiefly in gravel, clay and sand in Wicklow and Wexford. They are so rare in the drift east of the Wicklow mountains, that an exception to the rule, lately observed at Ballymore Eustace, by Professor Jukes, is considered as a fact of no small geological interest. The wide extent of drift of the same character, spread over large areas in Ireland, shows that the whole island was, in some part of the glacial period, an archipelago, as represented in the maps, figs. 39, 40, pp. 276 and 278.
Speaking of the Wexford drift, the late Professor E. Forbes states that Sir H. James found in it, together with many of the usual glacial shells, several species which are characteristic of the crag; among others the reversed variety of Fusus antiquus, called F. contrarius, and the extinct species Nucula Cobboldiæ, and Turritella incrassata. Perhaps a portion of this drift of the south of Ireland may belong to the close of the newer pliocene period, and may be of a some what older date than the shells of the Clyde, alluded to at p. 231. They may also correspond still more nearly in age with the fauna of the uppermost strata of the Norwich Crag, occurring at Chillesford, and alluded to p. 199.
The scarcity of mammalian remains in the Irish drift favours the theory of its marine origin. In the superficial deposits of the whole island, I have only met with three recorded examples of the mammoth, one in the south near Dungarvan, where the bones of Elephas primigenius, two species of bear (Ursus Arctos, and Ursus spelæus?), the rein-deer, horse, &c., were found in a cave; another in the centre of the island near Belturbet, in the county of Cavan.
Perhaps the conversion into land of the bed of the glacial sea, and the immigration into the newly upheaved region of the elephant, rhinoceros, and hippopotamus, which coexisted with the fabricators of the St. Acheul flint hatchets, were events which preceded in time the elevation of the Irish drift, and the union of that island with England. Ireland may have continued for a longer time in the state of an archipelago, and was therefore for a much shorter time inhabited by the large extinct post-pliocene pachyderms.
In one of the reports of the geological survey of Ireland, published in 1859, Professor Jukes, in explanation of sheet 184 of the maps, alludes to beds of sand and gravel, and signs of the polishing and furrowing of the rocks in the counties of Kerry and Killarney, as high as 2,500 feet above the sea, and supposes (perhaps with good reason) that the land was depressed even to that extent. He observes that above that elevation (2,500 feet) the rocks are rough, and not smoothed, as if by ice. Some of the drift was traced as high as 1,500 feet, the highest hills there exceeding 3,400 feet. Mr. Jukes, however, is by no means inclined to insist on submergence to the extent of 2,500 feet, as he is aware that ice, like that now prevailing in Greenland, might explain most, if not all, the appearances of glaciation in the highest regions.
Although the course taken by the Irish erratics in general is such that their transportation seems to have been due to floating ice or coast-ice, yet some granite blocks have travelled from south to north, as recorded by Sir R. Griffiths, namely, those of the Ox Mountains in Sligo; a fact from which Mr. Jamieson infers that those mountains formed at one time a centre of dispersion. In the same part of Ireland, the general direction in which the boulders have travelled is everywhere from north-west to south-east, a course directly at right angles to the prevailing trend of the present mountain ridges.
Maps illustrating successive Revolutions in Physical Geography during the Post-pliocene Period.
The late Mr. Trimmer, before referred to, has endeavoured to assist our speculations as to the successive revolutions in physical geography, through which the British Islands have passed since the commencement of the glacial period, by four 'sketch maps' as he termed them, in the first of which he gave an ideal restoration of the original Continental period, called by him the first elephantine period, or that of the forest of Cromer, before described (p. 214). He was not aware that the prevailing elephant of that era (E. meridionalis) was distinct from the mammoth. At this era he conceived Ireland and England to have been united with each other and with France, but much of the area represented as land in the map, fig. 41, p. 279, was supposed to be under water. His second map, of the great submergence of the glacial period, was not essentially different from our map, fig. 39, p. 276. His third map expressed a period of partial re-elevation, when Ireland was reunited to Scotland and the north of England; but England still separated from France. This restoration appears to me to rest on insufficient data, being constructed to suit the supposed area over which the gigantic Irish deer, or Megaceros, migrated from east to west, also to explain an assumed submergence of the district called the Wealden, in the south-east of England, which had remained land during the grand glacial submergence.
The fourth map is a return to nearly the same continental conditions as the first—Ireland, England, and the Continent being united. This he called the second elephantine period; and it would coincide very closely with that part of the post-pliocene era in which man coexisted with the mammoth, and when, according to Mr. Trimmer's hypothesis, the Thames was a tributary of the Rhine.
These geographical speculations were indulged in ten years after Edward Forbes had published his bold generalisations on the geological changes which accompanied the successive establishment of the Scandinavian, Germanic, and other living floras and faunas in the British Islands, and, like the theories of his predecessor, were the results of much reflection on a vast body of geological facts. It is by repeated efforts of this kind, made by geologists who are prepared for the partial failure of some of their first attempts, that we shall ultimately arrive at a knowledge of the long series of geographical revolutions which have followed each other since the beginning of the post-pliocene period.
The map, fig. 39, p. 276, will give some idea of the great extent of land which would be submerged, were we to infer, as many geologists have done, from the joint evidence of marine shells, erratics, glacial striæ and stratified drift at great heights, that Scotland was, during part of the glacial period, 2,000 feet below its present level, and other parts of the British Isles, 1,300 feet. A subsidence to this amount can be demonstrated in the case of North Wales by marine shells (see above, p. 267). In the lake district of Cumberland and Yorkshire, and in Ireland, we must depend on proofs derived from glacial striæ and the transportation of erratics for so much of the supposed submergence as exceeds 600 feet. As to central England, or the country north of the Thames and Bristol Channel, marine shells of the glacial period sometimes reach as high as 600 and 700 feet, and erratics still higher, as we have seen above (p. 270). But this region is of such moderate elevation above the sea, that it would be almost equally laid under water, were there a sinking of no more than 600 feet.
To make this last proposition clear, I have constructed, from numerous documents, many of them unpublished, the map, fig. 40, given at p. 278, which shows how that small amount of subsidence would reduce the whole of the British Isles to an archipelago of very small islands, with the exception of parts of Scotland, and the north of England and Wales, where four islands of considerable dimensions would still remain.
As to the district south of the Thames and the Bristol Channel, it seems to have remained land during the whole of the glacial period at a time when the northern area was under water.
The map, fig. 40, p. 278, just alluded to, represents simply the effects of a downward movement of a hundred fathoms, or 600 English feet, supposed to have been uniform over the whole of the British Isles. It shows the very different state of the physical geography of the area in question, when contrasted with the results of an opposite movement, or one of upheaval, to an equal amount, of which Sir Henry de la Beche had already given us a picture (from which I have borrowed the map, fig. 41, p. 279), in his excellent treatise called 'Theoretical Researches.'
If we are surprised when looking at the first map, fig. 40 at the vast expanse of sea which so moderate a subsidence as 600 feet would cause, we shall probably be still more astonished to perceive, in fig. 41, that a rise of the same number of feet would unite all the British Isles, including the Hebrides, Orkneys, and Shetlands, with one another and the continent, and lay dry the sea now separating Great Britain from Sweden and Denmark.
The submergence of Scotland is to the extent of 2,000 feet, and of other parts of the British Isles, 1,300.
In the map, the dark shade expresses the land which alone remained above water. The area shaded by diagonal lines is that which cannot be shown to have been under water at the period of floating ice by the evidence of erratics, or by marine shells of northern species. How far the several parts of the submerged area were simultaneously or successively laid under water, in the course of the glacial period, cannot, in the present state of our knowledge, be determined.
It seems that, during former and perhaps repeated oscillations of level undergone by the British Isles, the sea has had time to cut back the cliffs for miles in many places, while in others the detritus derived from wasting cliffs drifted along the shores, together with the sediment brought down by rivers and swept by currents into submarine valleys, has exerted a levelling power, filling up such depressions as may have pre-existed. Owing to this twofold action few marked inequalities of level have been left on the sea-bottom, the 'silver-pits' off the mouth of the Humber offering a rare exception to the general rule, and even there the narrow depression is less than 300 feet in depth.
Beyond the 100 fathom line, the submarine slope surrounding the British coast is so much steeper that a second elevation of equal amount (or of 600 feet) would add but slightly to the area of gained land; in other words, the 100 and 200 fathom lines run very near each other.The naturalist would have been entitled to assume the former union, within the post-pliocene period, of all the British Isles with each other and with the continent, as expressed in the map, fig. 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 identity of the fauna and flora found throughout these lands. Had they been separated ever since
The authorities to whom I am indebted for the information contained in this map are—for
W. Whitaker, Esq.
The darker shade expresses what is now land, the lighter shade the space intervening between the present coast line and the 100 fathom line, which would be converted by such a movement into land.
Southernmost Extent of Erratics in England.
In reference to that portion of the south of England which is marked by diagonal lines in the map at p. 260, 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 Sussex, 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, fifteen miles south of Chichester, in lat. 50° 40' N.
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, twenty-seven feet in circumference. They are not of northern origin, but must have come from the coast of Normandy or Brittany, 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 shells. 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 shells, all recent, but forming an assemblage differing as a whole from that now inhabiting the English Channel. 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. These transported rocks of Sussex are somewhat older than a sea-beach with recent marine shells which at Brighton is covered by chalk rubble, called the 'elephant-bed,' which I cannot describe in this place, but allude to it as one of many geological proofs of the former existence of a seashore 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.
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 (p. 214): 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, fig. 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, fig. 40; and finally to such a general prevalence of sea as is seen in map, fig. 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 shells and erratic blocks, was laid dry, and when the quantity of land equalled that of the first period, and therefore probably exceeded that represented in the map, p. 279. 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, fig. 41, p. 279, 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 a hundred fathoms (amounting in Wales to 1,400 feet, as shown by marine shells, and to 2,300, by stratified drift), it is not improbable that the upward movement was on a corresponding scale.
In passing 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 passage 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.
The first appearance of man, when, together with the mammoth and woolly rhinoceros, or with the Elephas antiquus, Rhinoceros hemitæchus, and Hippopotamus major, he ranged freely from all parts of the continent into the British area, belongs probably to a late portion of 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 shells 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 Carses of the Tay and Forth, before alluded to, pp. 47, 51, 54.
In a memoir by Professor E. Forbes, before cited, he observes, that the land of passage 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 shores of Wicklow and Wexford, others are found in the Isle of Man full of arctic shells, 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 St. George's 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 assumed that the migration of species westward having been the work of time, there was not a 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 St. George's Channel seeming to have arrested them in their westward course.
The depth of the St. George's 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 Post-Pliocene Period.
The time which it would require to bring about such changes of level, according to the average rate assumed at p. 58, 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 shells 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 post-pliocene 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 assume.
The submergence of Wales to the extent of 1,400 feet, as proved by glacial shells, would require 56,000 years, at the rate of 21 feet per century; but taking Professor Ramsay's estimate of 800 feet more, as stated at p. 267, that elevation being required for the deposition 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 no more than 600 feet above the present level, as in map, p. 279, this 600 feet would have taken another 26,000 years; the whole of the grand oscillation, comprising the submergence and re-emergence, having taken, in round numbers, 180,000 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 six feet in a century, and at Spitzbergen, according to Mr. Lamont, a still faster upheaval during the last 400 years. 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 assume 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 ten miles of the earth's crust, the gradual upheaval of the incumbent mass 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 passing from a melted, or as some would say its plastic condition, to a solid state, must be more than ten per cent. 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 passage of voluminous masses, 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 substitution 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 shells, marine and freshwater, of the forest bed, and associated 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 shells 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 classes of vertebrata is concerned.
If we reflect on the long series of events of the post-pliocene and recent periods contemplated in this chapter, it will be remarked that the time assigned 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.
- Philosophical Magazine, ser. 3, vol. xxi. p. 180.
- Ramsay, Quarterly Geological Journal, vol. viii. p. 372, 1852
- Seasons with the Sea-Horses, 1861.
- Edinburgh New Philosophical Journal, vol. xi. pl. i. p. 31, 1860.
- Hull, Edinburgh New Philosophical Journal, July 1860.
- Ancient Glaciers of Caernarvonshire, Philosophical Magazine, series 3, xxi. p. 180.
- Forbes' Memoirs of Survey, &c., vol. i. p. 377.
- E. Brenan and Dr. Carte, Dublin, 1859.
- Joshua Trimmer, Quarterly Geological Journal, vol. ix. plate xiii. 1853.
- Also repeated in De la Beche's Geological Observer.
- Manual of Geology, p. 74.
- De la Beche, Geological Researches, p. 191.
- Geological Quarterly Journal, vol. xiii. p. 50.
- E. Forbes, Fauna and Flora of British Isles; Memoirs of Geological Survey, vol. i. p. 344, 1846.
- Seasons with the Sea-Horses, p. 202.
- Bulletin de la Société Géologique, 2nd series, vol. iv. p. 1312.