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Show ~26 ISLAND LIFE. (t•AHT I. Date of last Glacial EpoGh and its becw'i11g on the J.[easU/rement qf Geological Time.-Directly we go back from this stable period we come upon changes both in the forms and in the distribution of species; and when we pass beyond the last glacial epoch into the Pliocene period we find ourselves in a comparatively new world, surrounded by a considerable number of species altogether different from any which now exist, together with many others which, though still living, now inhabit distant regions. It seems not improbable that what is termed the Pliocene period, was really the coming on of the glacial epoch, and this is the opiuion of Professor Jules Marcou.1 According to our views, a considerable amount of geographical change must have occurred at the change from the Miocene to the Pliocene, favouring the refrigeration of the northern hemisphere, and leading, in the way already pointe<l out, to the glacial epoch whenever a high degree of excentricity prevailed. As many reasons combine to make us fix the height of the glacial epoch at the period of high excentricity which occurred 200,000 yenrs back, and as the Pliocene period was probably not of long duration, we must suppose the next grent phase of very high excentricity (850,000 years ago) to fall within the Miocene epoch. Dr. Croll believes that this must have produced a glacial period, but we have shown strong reasons for believing that, in concnrrence with favourable geographical conditions, it led to uninterrupted warm climates in the temperate and northern zones. This, however, did not prevent the occurrence of local glaciation wherever other conditions led to its initiation, and the most powerful of such conditions is a great extent of high land. Now we know that the Alps acquired a considerable p~rt of their elevation during the latter part of the Miocene period, since M.iocene rocks occur at an elevation of over 6,000 feet, while Eocene beds occur at nearly 10,000 feet. But since that time there has been a vast amount of denudatio11, so that these rocks may have been first raised much higher than we now find them, and thus a considerable portion of the Alps may have been once more elevated than now. This would. certainly lead to un enormous accumulation of snow, which 1 Explication d'une seconde edition de la Carte Geolog1'que de la Te1Te (1875), p. G4. CHAI'. X.] MEASUREMENT OF GEOLOGICAL TIME. 227 would be increased when the excentricity reached a maximum, as already fully explained, and may then have caused glaciers to descend into the adjacent sea, carrying those enormous masses of rock which are buried in the Upper Miocene of the Superga in Northern Italy. An earlier epoch of great altitude in the Alps coinciding with the very high excentricity 2,500,000 years ago, may have caused the local glaciation of the Middle Eocene period when the enormous erratics of the Flysch conglomerate were deposited in the inland seas of Northern Switzerland, the Carpathians, and the Apennines. This is quite in harmony with the indications of an uninterrupted warm climate and rich vegetation during the very same period in the adjacent low countries, just as we find at the present day in New Zealand a delightful clima,te and a rich vegetation of Metrosideros, fuchsias and tree-ferns on the very borders of huge glaciers, descending to within 700 feet of the sea-level. It is not pretended that these estimates of geological time have any more value than probable guesses; but it is certainly a curious coincidence that two remarkable periods of high excentricity should have occurred, at such periods and at such intervals apart, as very well accord with the comparative remoteness of the two deposits in which undoubted signs of ice-action have been found, and that both these are localised in the vicinity of mountains which are known to have acquired a considerable elevation at about the same period of time. In the tenth edition of the PTinciples of Geology, Sir Charles Lyell, taking the amount of change in the species of mollusca as a guide, estimated the time elapsed since the commencement of the Miocene as one-third that of the whole Tertiary epoch, and the latter at one-fourth that of geological time since the Cambrian period. Professor Dana, on the other hand, estimates the Tertiary as only one-fifteenth of the M csozoic and. Palreozoic combined. On the estimate above given, founded on the dates of phases of high excentricity, we shall arrive at about four million years for the Tertiary epoch, and sixteen million years for the time elapsed since the Cambrian, according to Lyell, or sixty millions according to Dana. The estimate arrived at from the mte of denudation and deposition (twenty-eight million years) Q 2 |
