Alternate Cold and Warm Periods.—Warm Inter-glacial Periods a Test of Theories.—Reason why their Occurrence has not been hitherto recognised.—Instances of Warm Inter-glacial Periods.—Dranse, DÜrnten, Hoxne, Chapelhall, Craiglockhart, Leith Walk, Redhall Quarry, Beith, Crofthead, Kilmaurs, Sweden, Ohio, Cromer, Mundesley, &c., &c.—Cave and River Deposits.—Occurrence of Arctic and Warm Animals in some Beds accounted for.—Mr. Boyd Dawkins’s Objections.—Occurrence of Southern Shells in Glacial Deposits.—Evidence of Warm Inter-glacial Periods from Mineral Borings.—Striated Pavements.—Reason why Inter-glacial Land-surfaces are so rare.

Alternate Cold and Warm Periods.—If the theory developed in the foregoing chapters in reference to the cause of secular changes of climate be correct, it follows that that long age known as the glacial epoch did not, as has hitherto been generally supposed, consist of one long unbroken period of cold and ice. Neither did it consist, as some have concluded, of two long periods of ice with an intervening mild period, but it must have consisted of a long succession of cold and warm periods; the warm periods of the one hemisphere corresponding in time with the cold periods of the other and vice versÂ. It follows also from theory that as the cold periods became more and more severe, the warm intervening periods would become more and more warm and equable. As the ice began to accumulate during the cold periods in subarctic and temperate regions in places where it previously did not exist, so in like manner during the corresponding warm periods it would begin to disappear in arctic regions where it had held enduring sway throughout the now closing cycle. As the cold periods in the southern hemisphere became more and more severe, the ice would continue to advance northwards in the temperate regions; but at that very same time the intervening warm periods in the northern hemisphere would become warmer and warmer and more equable, and the ice of the arctic regions would continue to disappear farther and farther to the north, till by the time that the ice had reached a maximum during the cold antarctic periods, Greenland and the arctic regions would, during the warm intervening periods, be probably free of ice and enjoying a mild and equable climate. Or we may say that as the one hemisphere became cold the other became warm, and when the cold reached a maximum in the one hemisphere, the warmth would reach a maximum in the other. The time when the ice had reached its greatest extension on the one hemisphere would be the time when it had disappeared from the other.

Inter-glacial Periods a Test of Theories.—Here we have the grand crucial test of the truth of the foregoing theory of the cause of the glacial epoch. That the glacial epoch should have consisted of a succession of cold and warm periods is utterly inconsistent with all previous theories which have been advanced to account for it. What, then, is the evidence of geology on this subject? If the glacial epoch can be proved from geological evidence to have consisted of such a succession of cold and warm periods, then I have little doubt but the theory will soon be generally accepted. But at the very outset an objection meets us, viz., why call an epoch, which consisted as much of warm periods as of cold, a glacial epoch, or an “Ice Age,” as Mr. James Geikie tersely expresses it? Why not as well call it a warm epoch as a cold one, seeing that, according to theory, it was just as much a warm as a cold epoch? The answer to this objection will be fully discussed in the chapter on the Reason of the Imperfection of Geological Records. But in the meantime, I may remark that it will be shown that the epoch known as the glacial has been justly called the glacial epoch or “Ice Age,” because the geological evidences of the cold periods remain in a remarkably perfect state, whilst the evidences of the warm periods have to a great extent disappeared. The reason of this difference in the two cases will be discussed in the chapter to which I have referred. Besides, the condition of things during the cold periods was so extraordinary, so exceptional, so totally different from those now prevailing, that even supposing the geological records of the warm periods had been as well preserved as those of the cold, nevertheless we should have termed the epoch in question a glacial epoch. There is yet another reason, however, for our limited knowledge of warm inter-glacial periods. Till very lately, little or no attention was paid by geologists to this part of the subject in the way of keeping records of cases of inter-glacial deposits which, from time to time, have been observed. Few geologists ever dreamt of such a thing as warm periods during the age of ice, so that when intercalated beds of sand and gravel, beds of peat, roots, branches, trunks, leaves, and fruits of trees were found in the boulder clay, no physical importance was attached to them, and consequently no description or record of them ever kept. In fact, all such examples were regarded as purely accidental and exceptional, and were considered not worthy of any special attention. A case which came under my own observation will illustrate my meaning. An intelligent geologist, some years ago, read a paper before one of our local geological societies, giving an account of a fossiliferous bed of clay found intercalated between two distinct beds of till. In this intercalated bed were found rootlets and stems of trees, nuts, and other remains, showing that it had evidently been an old inter-glacial land surface. In the transactions of the society a description of the two beds of till was given, but no mention whatever was made of the intercalated bed containing the organic remains, although this was the only point of any real importance.

Since the theory that the glacial epoch resulted from a high state of eccentricity of the earth’s orbit began to receive some little acceptance, geologists have paid a good deal of attention to cases of intercalated beds in the till containing organic remains, and the result is that we have already a great body of evidence of a geological nature in favour of warm inter-glacial periods, and I have little doubt that in the course of a few years the former occurrence of warm inter-glacial periods will be universally admitted.

I shall now proceed to give a very brief outline of the evidence bearing on the subject. But the cases to which I shall have to refer are much too numerous to allow me to enter into details.

Inter-glacial Beds of Switzerland.—The first geologist, so far as I am aware, who directed attention to evidence of a break in the cold of the glacial epoch was M. Morlot. It is now twenty years ago since he announced the existence of a warm period during the glacial epoch from geological evidence connected with the glacial drift of the Alps.[104]

The rivers of Switzerland, he found, show on their banks three well-marked terraces of regularly stratified and well-rounded shingle, identical with the modern deposits of the rivers. They stand at 50, 100, and 150 feet above the present level of the rivers. These terraces were evidently formed by the present system of rivers when these flowed at a higher level, and extend up the Alps to a height of from 3,000 to 4,000 feet above the level of the sea. There is a terrace bordering the Rhine at Camischollas, above Disentis, 4,400 feet above the level of the sea, proving that during the period of its formation the Alps were free of ice up to the height of 4,400 feet above the sea-level. It is well known that a glacial period must have succeeded the formation of these drifts, for they are in many places covered with erratics. At Geneva, for example, an erratic drift nearly 50 feet thick is seen to rest on the drift of the middle terrace, which rises 100 feet above the level of the lake. But it is also evident that a glacial period must have preceded the formation of the drift beds, for they are found to lie in many places upon the unstratified boulder clay or till. M. Morlot observed in the neighbourhood of Clareus, from 7 to 9 feet of drift resting upon a bed of true till 40 feet thick; the latter was composed of a compact blue clay, containing worn and scratched alpine boulders and without any trace of stratification. In the gorge of Dranse, near Thoron, M. Morlot found the whole three formations in a direct superimposed series. At the bottom was a mass of compact till or boulder clay, 12 feet thick, containing boulders of alpine limestone. Over this mass came regularly stratified beds 150 feet thick, made up of rounded pebbles in horizontal beds. Above this again lay a second formation of unstratified boulder clay, with erratic blocks and striated pebbles, which constituted the left lateral moraine of the great glacier of the Rhone, when it advanced for the second time to the Lake of Geneva. A condition of things somewhat similar was observed by M. Ischer in the neighbourhood of Berne.

These facts, M. Morlot justly considers, prove the existence of two glacial periods separated by an intermediate one, during which the ice, which had not only covered Switzerland, but the greater part of Europe, disappeared even in the principal valleys of the Alps to a height of more than 4,400 feet above the present level of the sea. This warm period, after continuing for long ages, was succeeded by a second glacial period, during which the country was again covered with ice as before. M. Morlot even suggests the possibility of these alternations of cold and warm periods depending upon a cosmical cause. “Wild as it may have appeared,” he says, “when first started, the idea of general and periodical eras of refrigeration for our planet, connected perhaps with some cosmic agency, may eventually prove correct.”[105]

Shortly afterwards, evidence of a far more remarkable character was found in the glacial drift of Switzerland, namely, the famous lignite beds of DÜrnten. In the vicinity of Utznach and DÜrnten, on the Lake of Zurich, and near MÖrschwyl, on the Lake of Constance, there are beds of coal or lignite, nearly 12 feet thick, lying directly on the boulder clay. Overlying these beds is another mass of drift and clay 30 feet in thickness, with rounded blocks, and on the top of this upper drift lie long angular erratics, which evidently have been transported on the back of glaciers.[106] Professor Vogt attributes their transport to floating ice; but he evidently does so to avoid the hypothesis of a warm period during the glacial epoch.

Here we have proof not merely of the disappearance of the ice during the glacial epoch, but of its absence during a period of sufficient length to allow of the growth of 10 or 12 feet of coal. Professor Heer thinks that this coal-bed, when in the condition of peat, must have been 60 feet thick; and assuming that one foot of peat would be formed in a century, he concludes that 6,000 years must have been required for the growth of the coal plants. According to Liebig, 9,600 years would be required. This, as we have already seen, is about the average duration of a warm period.

In these beds have been found the bones of the elephant (E. Merkii), stag, cave-bear, and other animals. Numerous insects have also been met with, which further prove the warm, mild condition of climate which must have prevailed at the time of the formation of the lignite.

At Hoxne, near Diss, in Suffolk, a black peaty mass several feet thick, containing fragments of wood of the oak, yew, and fir, was found, overlying the boulder clay.[107] Professor Vogt believes that this peat bed is of the same age as the lignite beds of Switzerland.

In the glacial drift of North America, particularly about Lake Champlain and the valley of the St. Lawrence, there is similar evidence of two glacial periods with an intervening non-glacial or warm period.[108]

Glacial and Inter-glacial Periods of the Southern Hemisphere—(South Africa).—Mr. G. W. Stow, in a paper on the “Geology of South Africa,”[109] describes a recent glaciation extending over a large portion of Natal, British Kaffraria, the Kaga and Krome mountains, which he attributes to the action of land-ice. He sums up the phenomena as follows:—“The rounding off of the hills in the interiors of the ancient basins; the numerous dome-shaped (roches moutonnÉe) rocks; the enormous erratic boulders in positions where water could not have carried them; the frequency of unstratified clays—clays with imbedded angular boulders; drift and lofty mounds of boulders; large tracts of country thickly spread over with unstratified clays and superimposed fragments of rock; the Oliphant’s-Hoek clay, and the vast piles of Enon conglomerate.” In addition to these results of ice-action, he records the discovery by himself of distinct ice-scratches or groovings on the surface of the rocks at Reit-Poort in the Tarka, and subsequently[110] the discovery by Mr. G. Gilfillan of a large boulder at Pniel with striÆ distinctly marked upon it, and also that the same observer found that almost every boulder in the gravel at “Moonlight Rush” had unmistakable striÆ on one or more sides.

In South Africa there is evidence not only of a glacial condition during the Pliocene period, but also of a warmer climate than now prevails in that region. “The evidence,” says Mr. Stow, “of the Pliocene shells of the superficial limestone of the Zwartkops heights, and elsewhere, leads us to believe that the climate of South Africa must have been of a far more tropical character than at present.

“Take, for instance, the characteristic Venericardia of that limestone. This has migrated along the coast some 29° or 30° and is now found within a few degrees of the equator, near Zanzibar, gradually driven, as I presume it must have been, further and further north by a gradual lowering of the temperature of the more southern parts of this coast since the limestone was deposited.”

“During the formation of the shell-banks in the Zwartkops estuary, younger than the Pliocene limestone, the immense number of certain species of shells, which have as yet been found living only in latitudes nearer the equator, point to a somewhat similar though a more modified change of temperature.”

Inter-glacial Beds of Scotland.—Upwards of a dozen years ago, Professor Geikie arrived, from his own observations of the glacial drift of Scotland, at a similar conclusion to that of M. Morlot regarding the intercalation of warm periods during the glacial epoch; and the facts on which Professor Geikie’s conclusions were based are briefly as follows. In a cliff of boulder clay on the banks of the Slitrig Water, near the town of Hawick, he observed a bed of stones or shingle. Over the lower stratum of stones lay a few inches of well-stratified sand, silt, and clay, some of the layers being black and peaty, with enclosed vegetable fibres in a crumbling state.[111] There were some 30 or 40 feet of boulder clay above these stratified beds, and 15 or 20 feet under them. The stones in the shingle band were identical with those of the boulder clay, but they showed no striations, and were more rounded and water-worn, and resembled in every respect the stones now lying in the bed of the Slitrig. The section of the cliff stood as under:—

		1. Vegetable soil.
		2. Boulder clay, thirty to forty feet.
Stratified beds	{	3. Yellowish gravelly sand. 4. Peaty silt and clay. 5. Fine ferruginous sand. 6. Coarse shingle, two to three feet.
		7. Coarse, stiff boulder clay, fifteen to twenty feet.

A few more cases of intercalation of stratified materials in the true till were also found in the same valley.

In a cliff of stiff brown boulder clay, about 20 feet high, on the banks of the Carmichael Water, Lanarkshire, Professor Geikie observed a stratified bed of clay about 3 or 4 inches in thickness. About a mile higher up the stream, he found a series of beds of gravel, sand, and clay in the true till. “A thin seam of peaty matter,” he says, “was observed to run for a few inches along the bottom of a bed of clay and then disappear, while in a band of fine laminated clay with thin sandy partings occasional fragments of mouldering wood were found.”[112]

At Chapelhall, near Airdrie, a sand-bed has been extensively mined under about 114 feet of till. This bed of finely stratified sand is about 20 feet thick. In it were found lenticular beds of fine pale-coloured clay containing layers of peat and decaying twigs and branches. Professor Geikie found the vegetable fibres, though much decayed, still distinct, and the substance when put into the fire burned with a dull lambent flame. Underlying these stratified beds, and forming the floor of the mine, is a deposit of the true till about 24 feet in thickness. In another pit adjoining, the till forming the floor is 30 feet thick, but it is sometimes absent altogether, so as to leave the sand beds resting directly on the sandstone and shale of the coal-measures. At some distance from this sand-pit an old buried river channel was met with in one of the pit workings. This channel was found to contain a coating of boulder clay, on which the laminated sands and clays reposed, showing, as Professor Geikie has pointed out, that this old channel had been filled with boulder clay, and then re-excavated to allow of the deposition of the stratified deposits. Over all lay a thick mantle of boulder clay which buried the whole.

A case somewhat similar was found by Professor Nicol in a cutting on the Edinburgh and Leith Railway. In many places the till had been worn into hollows as if part of it had been removed by the action of running water.[113] One of these hollows, about 5 or 6 feet wide by 3 or 4 feet deep, closely resembled the channel of a small stream. It was also filled with gravel and sand, in all respects like that found in such a stream at the present day. It was seen to exhibit the same characters on both sides of the cutting, but Professor Nicol was unable to determine how far it may have extended beyond; but he had no doubt whatever that it had been formed by a stream of water. Over this old watercourse was a thick deposit of true till.

In reference to the foregoing cases, Professor Geikie makes the following pertinent remarks:—“Here it is evident that the scooping out of this channel belongs to the era of the boulder clay. It must have been effected during a pause in the deposition of the clay, when a run of water could find its way along the inequalities of the surface of the clay. This pause must have been of sufficient duration to enable the runnel to excavate a capacious channel for itself, and leave in it a quantity of sand and shingle. We can scarcely doubt that when this process was going on the ground must have been a land surface, and could not have been under the sea. And lastly, we see from the upper boulder clay that the old conditions returned, the watercourse was choked up, and another mass of chaotic boulder clay was tumbled down upon the face of the country. This indicates that the boulder clay is not the result of one great catastrophe, but of slow and silent, yet mighty, forces acting sometimes with long pauses throughout a vast cycle of time.”[114]

At Craiglockhart Hill, about a mile south of Edinburgh, an extensive bed of fine sand of from one to three feet in thickness was found between two distinct masses of true boulder clay or till. The sand was extensively used for building purposes during the erection of the city poorhouse a few years ago. In this sand-bed I found a great many tree roots in the position in which they had grown. During the time of the excavations I visited the place almost daily, and had every opportunity of satisfying myself that this sand-bed, prior to the time of the formation of the upper boulder clay, must have been a land surface on which the roots had grown. In no case did I find them penetrating into the upper boulder clay, and in several places I found stones of the upper clay resting directly on the broken ends of the roots. These roots were examined by Professor Balfour, but they were so decayed that he was unable to determine their character.

In digging a foundation for a building in Leith Walk, Edinburgh, a few years ago, two distinct beds of sand were passed through, the upper, about 10 feet in thickness, rested upon what appeared to be a denuded surface of the lower bed. In this lower bed, which evidently had been a land surface, numbers of tree roots were found. I had the pleasure of examining them along with my friend Mr. C. W. Peach, who first directed my attention to them. In no instance were the roots found in the upper bed. That these roots did not belong to trees which had grown on the present surface and penetrated to that depth, was further evident from the fact that in one or two cases we found the roots broken off at the place where they had been joined to the trunk, and there the upper sand-bed over them was more than 10 feet in thickness. If we assume that the roots belonged to trees which had grown on the present surface, then we must also assume, what no one would be willing to admit, that the trunks of the trees had grown downwards into the earth to a depth of upwards of ten feet. I have shown these roots to several botanists, but none of them could determine to what trees they belonged. The surface of the ground at the spot in question is 45 feet above sea-level. Mr. Peach and I have found similar roots in the under sand-bed at several other places in the same neighbourhood. That they belong to an inter-glacial period appears probable for the following reasons:—(1.) This upper sand-bed is overlaid by a tough clay, which in all respects appears to be the same as the Portobello clay, which we know belongs to the glacial series. In company with Mr. Bennie, I found the clay in some places to be contorted in a similar manner to the Portobello clays. (2.) In a sand-pit about one or two hundred yards to the west of where the roots were found, the sand-bed was found contorted in the most extraordinary manner to a depth of about 15 feet. In fact, for a space of more than 30 feet, the bedding had been completely turned up on end without the fine layers being in the least degree broken or disarranged, showing that they had been upturned by some enormous powers acting on a large mass of the sand.

One of the best examples of true till to be met with in the neighbourhood of Edinburgh is at Redhall Quarry, about three miles to the south-west of the city. In recently opening up a new quarry near the old one a bed of peat was found intercalated in the thick mass of till overlying the rock. The clay overlying and underlying the peat-bed was carefully examined by Mr. John Henderson,[115] and found to be true till.

In a quarry at Overtown, near Beith, Ayrshire, a sedimentary bed of clay, intercalated between two boulder clays, was some years ago observed by Mr. Robert Craig, of the Glasgow Geological Society. This bed filled an elliptical basin about 130 yards long, and about 30 yards broad. Its thickness averaged from one to two feet. This sedimentary bed rested on the till on the north-east end of the basin, and was itself overlaid on the south-west end by the upper bed of till. The clay bed was found to be full of roots and stems of the common hazel. That these roots had grown in the position in which they were found was evident from the fact that they were in many places found to pass into the “cutters” or fissures of the limestone, and were here found in a flattened form, having in growing accommodated themselves to the size and shape of the fissures. Nuts of the hazel were plentifully found.[116]

At Hillhead, some distance from Overtown, there is a similar intercalated bed full of hazel remains, and a species of freshwater Ostracoda was detected by Mr. David Robertson.

In a railway cutting a short distance from Beith, Mr. Craig pointed out to my colleague, Mr. Jack, and myself, a thin layer of peaty matter, extending for a considerable distance between an upper and lower mass of till; and at one place we found a piece of oak about four feet in length and about seven or eight inches in thickness. This oak boulder was well polished and striated.

Not far from this place is the famous Crofthead inter-glacial bed, so well known from the description given by Mr. James Geikie and others that I need not here describe it. I had the pleasure of visiting the section twice while it was well exposed, once, in company with Mr. James Geikie, and I do not entertain the shadow of a doubt as to its true inter-glacial character.

In the silt, evidently the mud of an inter-glacial lake, were found the upper portion of the skull of the great extinct ox (Bos primigenius), horns of the Irish elk or deer, and bones of the horse. In the detailed list of the lesser organic remains found in the intercalated peat-bed by Mr. J. A. Mahony,[117] are the following, viz., three species of DesmidaceÆ, thirty-one species of DiatomaceÆ, eleven species of mosses, nine species of phanerogamous plants, and several species of annelids, crustacea, and insects. This list clearly shows that the inter-glacial period, represented by these remains, was not only mild and warm, but of considerable duration. Mr. David Robertson found in the clay under the peat several species of Ostracoda.

The well-known Kilmaurs bed of peaty matter in which the remains of the mammoth and reindeer were found, has now by the researches of the Geological Survey been proved to be of inter-glacial age.[118]

In Ireland, as shown by Professors Hull and Harkness, the inter-glacial beds, called by them the “manure gravels,” contain numerous fragments of shells indicating a more genial climate than prevailed when the boulder clays lying above and below them were formed.[119]

In Sweden inter-glacial beds of freshwater origin, containing plants, have been met with by Herr Nathorst and also by Herr HolmstrÖm.[120]

In North America Mr. Whittlesey describes inter-glacial beds of blue clay enclosing pieces of wood, intercalated with beds of hard pan (till). Professor Newberry found at Germantown, Ohio, an immense bed of peat, from 12 to 20 feet in thickness, underlying, in some places 30 feet, and in other places as much as 80 feet, of till, and overlying drift beds. The uppermost layers of the peat contain undecomposed sphagnous mosses, grasses, and sedges, but in the other portions of the bed abundant fragments of coniferous wood, identified as red cedar (Juniperus virginiana), have been found. Ash, hickory, sycamore, together with grape-vines and beech-leaves, were also met with, and with these the remains of the mastodon and great extinct beaver.[121]

Inter-glacial Beds of England.—Scotland has been so much denuded by the ice sheet with which it was covered during the period of maximum glaciation that little can be learned in this part of the island regarding the early history of the glacial epoch. But in England, and more especially in the south-eastern portion of it, matters are somewhat different. We have, in the Norwich Crag and Chillesford beds, a formation pretty well developed, which is now generally regarded as lying at the base of the Glacial Series. That this formation is of a glacial character is evident from the fact of its containing shells of a northern type, such as Leda lanceolata, Cardium GroÈnlandicum, Lucina borealis, Cyprina Islandica, PanopÆa Norvegica, and Mya truncata. But the glacial character of the formation is more strikingly brought out, as Sir Charles Lyell remarks, by the predominance of such species as Rhynchonella psittacea, Tellina calcarea, Astarte borealis, Scalaria GroÈnlandica, and Fusus carinatus.

The “Forest Beds.”—Immediately following this in the order of time comes the famous “Forest Bed” of Cromer. This buried forest has been traced for more than forty miles along the coast from Cromer to near Kessengland, and consists of stumps of trees standing erect, attached to their roots, penetrating the original soil in which they grew. Here and in the overlying fluvio-marine beds we have the first evidence of at least a temperate, if not a warm, inter-glacial period. This is evident from the character of the flora and fauna belonging to these beds. Among the trees we have, for example, the Scotch and spruce fir, the yew, the oak, birch, the alder, and the common sloe. There have also been found the white and yellow water-lilies, the pond-weed, and others. Amongst the mammalia have been met with the Elephas meridionalis, also found in the Lower Pliocene beds of the Val d’Arno, near Florence; Elephas antiquus, Hippopotamus major, Rhinoceros Etruscus, the two latter Val d’Arno species, the roebuck, the horse, the stag, the Irish elk, the Cervus Polignacus, found also at Mont Perrier, France, C. verticornis, and C. carnutorum, the latter also found in Pliocene strata of St. Prest, France. In the fluvio-marine series have been found the Cyclas omnica and the Paludina marginata, a species of mollusc still found in the South of France, but no longer inhabiting the British Isles.

Above the forest bed and fluvio-marine series comes the well-known unstratified Norwich boulder till, containing immense blocks 6 or 8 feet in diameter, many of which must have come from Scandinavia, and above the unstratified till are a series of contorted beds of sand and gravel. This series may be considered to represent a period of intense glaciation. Above this again comes the middle drift of Mr. Searles Wood, junior, yielding shells which indicate, as is now generally admitted, a comparatively mild condition of climate. Upon this middle drift lies the upper boulder clay, which is well developed in South Norfolk and Suffolk, and which is of unmistakable glacial origin. Newer than all these are the Mundesley freshwater beds, which lie in a hollow denuded out of the foregoing series. In this formation a black peaty deposit containing seeds of plants, insects, shells, and scales and bones of fishes, has been found, all indicating a mild and temperate condition of climate. Among the shells there is, as in the forest bed, the Paludina marginata. And that an arctic condition of things in England followed is believed by Mr. Fisher and others, on the evidence of the “Trail” described by the former observer.

Cave and River Deposits.—Evidence of the existence of warm periods during the glacial epoch is derived from a class of facts which have long been regarded by geologists as very puzzling, namely, the occurrence of mollusca and mammalia of a southern type associated in England and on the continent with those of an extremely arctic character. For example, Cyrena fluminalis is a shell which does not live at present in any European river, but inhabits the Nile and parts of Asia, especially Cashmere. Unio littoralis, extinct in Britain, is still abundant in the Loire; Paludina marginata does not exist in this country. These shells of a southern type have been found in post-tertiary deposits at Gray’s Thurrock, in Essex; in the valley of the Ouse, near Bedford; and at Hoxne, in Suffolk, associated with a Hippopotamus closely allied to that now inhabiting the Nile, and Elephas antiquus, an animal remarkable for its southern range. Amongst other forms of a southern type which have been met with in the cave and river deposits, are the spotted hyÆna from Africa, an animal, says Mr. Dawkins, identical, except in size, with the cave hyÆna, the African elephant (E. Africanus), and the Elephas meridionalis, the great beaver (Trogontherium), the cave hyÆna (HyÆna spelÆa), the cave lion (Felis leo, var. spelÆa), the lynx (Felis lynx), the sabre-toothed tiger (Machairodus latidens), the rhinoceros (Rhinoceros megarhinus and R. leptorhinus). But the most extraordinary thing is that along with these, associated in the same beds, have been found the remains of such animals of an arctic type as the glutton (Gulo luscus), the ermine (Mustela erminea), the reindeer (Cervus tarandus), the musk-ox or musk-sheep (Ovibos moschatus), the aurochs (Bison priscus), the woolly rhinoceros (Rhinoceros tichorhinus), the mammoth (Elephas primigenius), and others of a like character. According to Mr. Boyd Dawkins, these southern animals extended as far north as Yorkshire in England, and the northern animals as far south as the latitude of the Alps and Pyrenees.[122]

The Explanation of the Difficulty.—As an explanation of these puzzling phenomena, I suggested, in the Philosophical Magazine for November, 1868, that these southern animals lived in our island during the warm periods of the glacial epoch, while the northern animals lived during the cold periods. This view I am happy to find has lately been supported by Sir John Lubbock; further, Mr. James Geikie, in his “Great Ice Age,” and also in the Geological Magazine, has entered so fully into the subject and brought forward such a body of evidence in support of it, that, in all probability, it will, ere long, be generally accepted. The only objection which has been advanced, so far as I am aware, deserving of serious consideration, is that by Mr. Boyd Dawkins, who holds that if these migrations had been secular instead of seasonal, as is supposed by Sir Charles Lyell and himself, the arctic and southern animals would now be found in separate deposits. It is perfectly true that if there had been only one cold and one warm period, each of geologically immense duration, the remains might, of course, be expected to have been found in separate beds; but when we consider that the glacial epoch consisted of a long succession of alternate cold and warm periods, of not more than ten or twelve thousand years each, we can hardly expect that in the river deposits belonging to this long cycle we should be able to distinguish the deposits of the cold periods from those of the warm.

Shell Beds.—Evidence of warm inter-glacial periods may be justly inferred from the presence of shells of a southern type which have been found in glacial beds, of which some illustrations follow.

In the southern parts of Norway, from the present sea-level up to 500 feet, are found glacial shell beds, similar to those of Scotland. In these beds Trochus magus, Tapes decussata, and Pholas candida have been found, shells which are distributed between the Mediterranean and the shores of England, but no longer live round the coasts of Norway.

At Capellbacken, near Udevalla, in Sweden, there is an extensive bed of shells 20 to 30 feet in thickness. This formation has been described by Mr. Gwyn Jeffreys.[123] It consists of several distinct layers, apparently representing many epochs and conditions. Its shells are of a highly arctic character, and several of the species have not been found living south of the arctic circle. But the remarkable circumstance is that it contains CyprÆa lurida, a Mediterranean shell, which Mr. Jeffreys, after some hesitation, believed to belong to the bed. Again, at Lilleherstehagen, a short distance from Capellbacken, another extensive deposit is exposed. “Here the upper layer,” says Mr. Jeffreys, “gives a singular result. Mixed with the universal Trophon clathratus (which is a high northern species, and found living only within the arctic circle) are many shells of a southern type, such are Ostrea edulis, Tapes pullastra, Corbula gibba, and Aporrhais pes-pelicani.”

At Kempsey, near Worcester, a shell bed is described by Sir R. Murchison in his “Silurian System” (p. 533), in which Bulla ampulla and a species of Oliva, shells of a southern type, have been found.

A case somewhat similar to the above is recorded by the Rev. Mr. Crosskey as having been met with in Scotland at the Kyles of Bute. “Among the Clyde beds, I have found,” he says, “a layer containing shells, in which those of a more southern type appear to exist in greater profusion and perfection than even in our present seas. It is an open question,” he continues, “whether our climate was not slightly warmer than it is now between the glacial epoch and the present day.”[124]

In a glacial bed near Greenock, Mr. A. Bell found the fry of living Mediterranean forms, viz., Conus Mediterraneus and Cardita trapezia.

Although deposits containing shells of a temperate or of a southern type in glacial beds have not been often recorded, it by no means follows that such deposits are actually of rare occurrence. That glacial beds should contain deposits indicating a temperate or a warm condition of climate is a thing so contrary to all preconceived opinions regarding the sequence of events during the glacial epoch, that most geologists, were they to meet with a shell of a southern type in one of those beds, would instantly come to the conclusion that its occurrence there was purely accidental, and would pay no special attention to the matter.

Evidence derived from “Borings.”—With the view of ascertaining if additional light would be cast on the sequence of events, during the formation of the boulder clay, by an examination of the journals of bores made through a great depth of surface deposits, I collected, during the summer of 1867, about two hundred and fifty such records, put down in all parts of the mining districts of Scotland. An examination of these bores shows most conclusively that the opinion that the boulder clay, or lower till, is one great undivided formation, is wholly erroneous.

These two hundred and fifty bores represent a total thickness of 21,348 feet, giving 86 feet as the mean thickness of the deposits passed through. Twenty of these have one boulder clay, with beds of stratified sand or gravel beneath the clay; twenty-five have two boulder clays, with stratified beds of sand and gravel between; ten have three boulder clays; one has four boulder clays; two have five boulder clays; and no one has fewer than six separate masses of boulder clay, with stratified beds of sand and gravel between; sixteen have two or three separate boulder clays, differing altogether in colour and hardness, without any stratified beds between. We have, therefore, out of two hundred and fifty bores, seventy-five of them representing a condition of things wholly different from that exhibited to the geologist in ordinary sections.

The full details of the character of the deposits passed through by these bores, and their bearing on the history of the glacial epoch, have been given by Mr. James Bennie, in an interesting paper read before the Glasgow Geological Society,[125] to which I would refer all those interested in the subject of surface geology.

The evidence afforded by these bores of the existence of warm inter-glacial periods will, however, fall to be considered in a subsequent chapter.[126]

Another important and unexpected result obtained from these bores to which we shall have occasion to refer, was the evidence which they afforded of a Continental Period.

Striated Pavements.—It has been sometimes observed that in horizontal sections of the boulder clay, the stones and boulders are all striated in one uniform direction, and this has been effected over the original markings on the boulders. It has been inferred from this that a pause of long duration must have taken place in the formation of the boulder clay, during which the ice disappeared and the clay became hardened into a solid mass. After which the old condition of things returned, glaciers again appeared, passed over the surface of the hardened clay with its imbedded boulders, and ground it down in the same way as they had formerly done the solid rocks underneath the clay.

An instance of striated pavements in the boulder clay was observed by Mr. Robert Chambers in a cliff between Portobello and Fisherrow. At several places a narrow train of blocks was observed crossing the line of the beach, somewhat like a quay or mole, but not more than a foot above the general level. All the blocks had flat sides uppermost, and all the flat sides were striated in the same direction as that of the rocky surface throughout the country. A similar instance was also observed between Leith and Portobello. “There is, in short,” says Mr. Chambers, “a surface of the boulder clay, deep down in the entire bed, which, to appearance, has been in precisely the same circumstances as the fast rock surface below had previously been. It has had in its turn to sustain the weight and abrading force of the glacial agent, in whatever form it was applied; and the additional deposits of the boulder clay left over this surface may be presumed to have been formed by the agent on that occasion.”[127]

Several cases of a similar character were observed by Mr. James Smith, of Jordanhill, on the beach at Row, and on the shore of the Gareloch.[128] Between Dunbar and Cockburnspath, Professor Geikie found along the beach, for a space of 30 or 40 square yards, numbers of large blocks of limestone with flattened upper sides, imbedded in a stiff red clay, and all striated in one direction. On the shores of the Solway he found another example.[129]

The cases of striated pavements recorded are, however, not very numerous. But this by no means shows that they are of rare occurrence in the boulder clay. These pavements, of course, are to be found only in the interior of the mass, and even there they can only be seen along a horizontal section. But sections of this kind are rarely to be met with, for river channels, quarries, railway cuttings, and other excavations of a similar character which usually lay open the boulder clay, exhibit vertical sections only. It is therefore only along the sea-shore, as Professor Geikie remarks, where the surface of the clay has been worn away by the action of the waves, that opportunities have hitherto been presented to the geologist for observing them.

There can be little doubt that during the warm periods of the glacial epoch our island would be clothed with a luxuriant flora. At the end of a cold period, when the ice had disappeared, the whole face of the country would be covered over to a considerable depth with a confused mass of stones and boulder clay. A surface thus wholly destitute of every seed and germ would probably remain for years without vegetation. But through course of time life would begin to appear, and during the thousands of years of perpetual summer which would follow, the soil, uncongenial as it no doubt must have been, would be forced to sustain a luxuriant vegetation. But although this was the case, we need not wonder that now scarcely a single vestige of it remains; for when the ice sheet again crept over the island everything animate and inanimate would be ground down to powder. We are certain that prior to the glacial epoch our island must have been covered with life and vegetation. But not a single vestige of these are now to be found; no, not even of the very soil on which the vegetation grew. The solid rock itself upon which the soil lay has been ground down to mud by the ice sheet, and, to a large extent, as Professor Geikie remarks, swept away into the adjoining seas.[130] It is now even more difficult to find a trace of the ancient soil under the boulder clay than it is to find remains of the soil of the warm periods in that clay. As regards Scotland, cases of old land surfaces under the boulder clay are as seldom recorded as cases of old land surfaces in it. In so far as geology is concerned, there is as much evidence to show that our island was clothed with vegetation during the glacial epoch as there is that it was so clothed prior to that epoch.

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