It is evident that in this age of forests the German Ocean must have been dry land, and the continent of Europe must have extended beyond the Orkneys and Hebrides, probably to the hundred fathom line. Such movements of elevation and depression, so far as we know anything of them, are extremely slow. There has been no change in the fords of rivers in Britain since Roman times, and the spit connecting St. Michael's Mount with Cornwall was dry at ebb and covered at flood as at the present day, when the British carted their tin across it to trade with the Phœnicians 2500 years ago. Mr. Read goes into elaborate calculations based on the time required for these geological changes, and arrives at the conclusion that they point to a date of not less than 50,000 or 60,000 years ago for the commencement of the post-glacial period. These calculations are disputed, but it seems certain that several multiples of the historical standard of say 10,000 years, must be required to measure the period since the glacial age finally disappeared, and the earth, with its existing fauna, climate, and geographical conditions, came fairly into view. This is confirmed by the great changes which have taken place in the distribution of land and water since the quaternary period. Huxley, in an article on the Aryan question, points out that in recent times four great separate bodies of water—the Black Sea, the Caspian, the Sea of Aral, and Lake Balkash—occupied the southern end of the vast plains which extend from the Arctic Sea to the highlands of the Balkan peninsula, of Asia Minor, of Persia and Afghanistan, and of the high plateaux of Central Asia, as far as the Altai. But he says, "This state of things is comparatively modern. At no very distant period the land of Asia Minor was continuous with that of Europe, across the present site of the Bosphorus, forming a barrier several hundred feet high, which dammed up the waters of the Black Sea. A vast extent of Eastern Europe and of west-central Asia thus became one vast Ponto-Aralian Mediterranean, into which the largest rivers of Europe and Asia, the Danube, Volga, Oxus, and Jaxartes, discharged their waters, and which sent its overflow northwards through the present basin of the Obi." The time necessary for such changes goes far to confirm Mellard Read's estimate for the long duration of the recent or post-glacial period.
In fact, all the evidence from the Old World goes to confirm the long duration of the post-glacial period, and the immensely greater antiquity of the glacial period taken as a whole. It is only from the New World that any serious arguments are forthcoming to abridge those periods, or rather the post-glacial period, for that alone is affected by the facts adduced. It is said that recent measurements of the recession of the Falls of Niagara show, that instead of requiring 35,000 years, as estimated by Lyell, to cut back the gorge of seven miles from Kingston to the Falls, 10,000 years at the outside would have been amply sufficient; and that this is confirmed by the gorges of other rivers, such as that of the Mississippi at St. Paul's. The evidence is not conclusive, for it depends on the rate of erosion going on for the last twenty or thirty years, which may obviously give a different result from the true average, and in fact older estimates, based on longer periods, gave the rate adopted by Lyell. But if we admit the accuracy of the modern estimates, it does not affect the total duration of the glacial period, but simply that of a late phase of the post-glacial, when the ice-cap which covered North America to a depth often of 2000 or 3000 feet, had melted away and shrunk back 400 miles from its original southern boundary, so as to admit of the waters of the great lakes finding an outlet to the north-east instead of by the old drainage to the south. Nothing is more likely than that, as the great Laurentian ice-cap of America was deeper and extended further than the Scandinavian ice-cap of Europe, it may have taken longer to melt the larger accumulation of ice, and thus postponed the establishment of post-glacial conditions and river-drainage to a later period than in the warmer and more insular climate of Europe. It is a matter of everyday observation, that the larger a snowball is the longer it takes to melt, and that when the mass is large it requires a long time to make it disappear even after mild weather has set in.
The only other argument for a short glacial period is drawn from the rate of advance of the glaciers in Greenland, which is shown to be much more rapid than that of the glaciers of Switzerland, from which former calculations had been made. But obviously the rate at which the fronts of glaciers advance when forced by a mass of continental ice down fiords on a steep descending gradient, into a deep sea, where the front is floated off in icebergs, affords no clue as to that of an ice-cap spread, with a front of 1000 miles, over half a continent, retarded by friction, and surmounting mountain chains 3000 feet high. Nor does the rate of advance afford the slightest clue to the time during which the ice-cap may have remained stationary, alternately advanced and retreated, and finally disappeared.[9]
We have now to adjust our time-telescope to a wider range, and see what the Quaternary or glacial period teaches us as to the antiquity of man. The first remark is, that if the post-glacial period is much longer than that for which we have historical records, the glacial exceeds the post-glacial in a far higher proportion. The second, that throughout the whole of this glacial period, from its commencement to its close, we have conclusive evidence of the existence of man, and that not only in a few limited localities, but widely spread over nearly all the habitable regions of the earth.
The first point has been so conclusively established by all geologists of all countries, from the time of Lyell down to the present day, that it is unnecessary to enter on any detailed arguments, and the leading facts may be taken as established. It may be sufficient, therefore, if I give a short summary of those facts, and quote a few of the instances which show the enormous lapse of time which must have elapsed between the close of the tertiary and the commencement of the modern epoch.
The glacial period was not one and simple, but comprised several phases. During the Pliocene the climate was gradually becoming colder, and either towards its close or at the commencement of the Quaternary, this culminated in a first and most intense glaciation. Ice-caps radiating from Scandinavia crept outwards, filling up the North Sea, crossing valleys and mountains, and covering with their boulders and moraines a wide circle, embracing Britain down to the Thames valley, Germany to the Hartz mountains, and Russia almost as far east as the Urals. In North America a still more massive ice-cap overflowed mountain ranges 3000 feet high, and covered the whole eastern half of the continent with an unbroken mantle of ice as far south as New York and Washington.
At the same time every great mountain chain and high plateau sent out enormous glaciers, which, in the case of the Alps, filled up the valley of the Rhone and the Lake of Geneva, buried the whole of the lower country of Switzerland under 3000 feet of ice, and left the boulders of its terminal moraine, carried from the Mont Blanc range, at that height on the opposite range of the Jura. Nor is this a solitary instance. We find everywhere traces of enormous glaciers in the Pyrenees and Carpathians, the Atlas and Lebanon, the Taurus and Caucasus, the highlands of Scotland, Ireland, and Wales; in the Rocky Mountains and Sierra Nevada; the Andes and Cordilleras of South America; in South Africa and in New Zealand. These may not have all been simultaneous, but they certainly all belong to the same period of the great glaciation, and show that it must have been affected by some general cause, and not been entirely due to mere local accidents.
How this first great glacial period came on, or how long it lasted, we do not know, unless Croll's astronomical theory, which will be considered later, affords a clue. But we know generally that it must have lasted for an immense time from the amount of work done and the changes which took place. The ice, which covered so great a portion of the northern hemisphere, was not a polar ice-cap, but spread outwards in all directions from great masses of elevated land, as is proved conclusively from the direction of the striæ which were engraved by it on the subjacent rocks. This land must have been more elevated than at present, so as to rise, like Greenland, far into the region of perpetual snow, where all rain falls and accumulates in the solid form; and also to supply the enormous mass of débris which the ice-caps and glaciers left behind them. It is not too much to say that a million of square miles in Europe, and more in North America, were covered by the débris of rocks ground down by these glaciers, and often to great depths. Most of the débris of the first glaciation have been removed by denudation, or ploughed out by the second great advance of the ice, leaving only the larger and harder boulders to testify to their extent; but enough remains to show that the first series of boulder-clays and drifts must have been on a scale larger than those of the second and subsequent glaciations, which now form the superficial stratum of so much of the earth's surface, and often attain a depth of several hundred feet. Wright, in his Ice Age in North America, estimates that "not less than 1,000,000 square miles of territory in North America is still covered with an average depth of fifty feet of glacial débris."
However, this first period of elevation and of intense glaciation passed away, and was succeeded by one of depression and of milder climate. Whether or no the depression was due, as some think, to the weight of the enormous mass of ice weighing down the yielding crust of the earth, and whether or no the milder climate was partly occasioned by this depression letting in the sea, the fact is certain that the two coincided, and were general and not merely local phenomena. Marine shells at the top of what are now high hills, and which during the preceding glaciation were probably higher, attest the fact that a large amount of land must have sunk below the sea towards the close of this first glacial period. It is equally clear that a long inter-glacial period ensued, during which many changes took place in the geographical conditions and in the fauna and flora, requiring a very long time. Thus Britain, which had been reduced to an Arctic Archipelago, in which only a few of the highest mountain peaks emerged as frozen islands, became united to the continent, and the abode of a fauna consisting in great part of African animals. At one time boreal shells were deposited, at the bottom of an Arctic ocean, on what is now the top of Moel-Tryfen in Wales, a hill 1300 feet above the present sea-level; while at another the hippopotamus found its way, in some great river flowing from the south, as far north as Yorkshire, and the remains of African animals such as the hyena accumulated in our caves. In Southern France we had at one time a vegetation of the Arctic willow and reindeer moss, at another that of the fig-tree and canary-laurel. When we consider that little if any change has occurred either in geographical conditions or in fauna or flora, within the historical period of some 10,000 years, it is difficult to assign the time which would be sufficient to bring about such changes by any known natural causes. And yet it comprises only a portion of the glacial period, for after this inter-glacial period had lasted for an indefinite time the climate again became cold, and culminated in a second glaciation, which, if not equal to the first, was still of extreme severity, and brought back ice-caps and glaciers almost to their former limits, passing away slowly and with several vicissitudes and alternate retreats and advances.
It is not always easy to determine the position of each individual phase of the two glacial and the inter-glacial periods, for they must often be intermixed, and the results of the last glaciation and of subsequent denudation have to a great extent obscured those of the earlier periods. But taking a general view of the glacial period as a whole, there are a few leading facts which testify conclusively to its immense antiquity. First, there is the amount of elevation and depression. We have seen that marine Arctic shells have been found on the top of Moel-Tryfen, 1300 feet above the present sea-level. Nor is this an isolated instance, for marine drifts apparently of the same character have been traced on the mountains of Scotland, Wales, and Ireland to a height of between 2000 and 3000 feet. In Norway, also, old sea beaches are found up to a height of 800 feet. Nor are these great movements confined to the Old World or to limited localities. According to Professor Le Conte, at the last meeting of the Geological Congress at Washington, a great continental movement, commencing in the later tertiary and terminating in the beginning of the quaternary, caused changes of level amounting to 2500 or 3000 feet on both sides of the continent of North America.