If we turn from these temperate regions to those in which exactly opposite conditions prevail, we find them still in the icy chains of a glacial period. Greenland, for instance, which is a typical case, shows us what happens when a continental mass of land stands at a high elevation in high latitudes with no Gulf Stream, but instead of it cold currents from a Polar ocean, and seas around it frozen or covered with icebergs for nine months out of the year. We have a dome of solid ice piled up to the height of 9000 feet or upwards, and sending millions upon millions of tons of glaciers down to the sea to be floated off as icebergs. The only trace we can see here of the old great glacial period is that these conditions were formerly more intense. Thus the glaciation of some of the mountain sides and islands off the coast of Greenland seem to show that the ice formerly stood 2000 or 3000 feet higher than at present, a result which would be attained if the whole continental mass, which is now slowly subsiding, had then been elevated to that extent.
The southern hemisphere affords a still more striking example of this on a larger scale, for we have there, in all probability, higher land in higher latitudes, surrounded by frozen seas, and washed by cold currents. I pass from this however, as beyond these general facts the special conditions of the Antarctic Circle are not known to us like those of Greenland.
From the above facts we are very safe in drawing the conclusion that during the great quaternary glacial period the conditions which now cause glaciation must have existed in an aggravated degree, and those which now give us temperate climates in regions once glaciated must have disappeared or been reversed. On the other hand, the warm climates which prevailed during the tertiary and other geological epochs, and permitted a temperate flora to flourish as far north as Grinnell Land and Spitzbergen, could only have occurred under conditions exactly the reverse of those which produced the cold. If high land in high latitudes is the principal cause of the present glaciation of Greenland, still higher land must have been so in causing the still greater glaciation of the former period. Scandinavia, Laurentia, the British Islands, the Alps, Apennines, Rocky Mountains, Sierra Nevada, and all great mountain ranges in the northern hemisphere must have stood at greater elevations. There must have been such an accumulation of ice and snow as to chill the air, cause fogs, and prevent the summer heat of the sun from melting off the water which fell in the solid form during winter; and on the other hand, there must have been hot summers and great expanses of ocean to the south to supply the abundant evaporation which became condensed by contact with the chilly mountains and uplands of the north.
One supposition is that the Isthmus of Panama was then submerged, so that the Gulf Stream ran into the Pacific. But this wants geological confirmation, as the Isthmus shows no sign of such recent marine formations as must have been deposited if it had been submerged to a sufficient depth to let the Gulf Stream escape, and the extension of the ice-cap in North America to much lower latitudes than in Europe, points rather to the conclusion that the Gulf Stream must have run very much in its present course.
The only geological evidence bearing on this question is the recent discovery of deep oceanic deposits such as the Globigerena ooze, above tertiary deposits in Barbadoes and Jamaica, leading to the inference that the whole West Indian area was a deep sea in comparatively modern times. This no doubt might affect both the temperature and the velocity of the Gulf Stream to a considerable extent.
But the geological evidence is much more conclusive for the greater elevation of the land during the periods of greater glaciation as well as for its depression during the inter-glacial period. American geologists estimate that a large part of Eastern Canada with adjacent regions must have been at least 2000, and may have been as much as 3000 feet above its present level during the first great glaciation; while the Champlain marine beds show that it was some hundreds of feet below the present, sea-level during part of the inter-glacial period. Scandinavia stood at least 2000 feet higher than at present during the climax of the glacial period as proved by the depths of the fiords, and afterwards 500 or 600 lower as proved by the raised beaches. In Great Britain and Ireland we have conclusive evidence both of higher elevation, and of depression of at least 1300 feet, and probably more than 2000 feet below the present sea-level, as proved by the marine shells on the top of Moel-Tryfen.
But these elevations and depressions are small in amount compared with the mountain building which is known to have occurred in Asia in comparatively recent geological times. Here the Himalayas, stretching for 1500 miles from east to west, and rising to heights of from 20,000 to 29,000 feet above the sea, have been formed in great part during this period. Within the same period the great table-lands of Thibet and Central Asia have been uplifted, and the Asian Mediterranean Sea, of which the Black Sea, the Caspian, the Salt deserts and Lake Balkash are the remnants, has been converted into dry land. Movements of this magnitude, of which there are many other examples, may well account for great changes in isothermal lines and climates.
The complete removal of the conditions which produced the glacial period might go far to account for the preceding tertiary period. We have only to suppose a different configuration of sea and land; nothing but low lands and islands in high latitudes; free access for warm oceanic currents like the Gulf Stream into the limited area of the Polar basin; no great continents or lofty mountain ranges to drain the return trade winds of their moisture; in short, all the conditions of a mild and moist insular climate, as opposed to those of a continental one, to understand how forests of temperate trees might flourish as far north as Greenland and Spitzbergen. And the geological evidence which, as we have seen, shows that great elevation of land in the northern hemisphere did in fact inaugurate the glacial period, favours the conclusion that the reverse conditions actually prevailed during the tertiary and preceding epochs.
The presence of the Nummulitic and other marine Eocene and Miocene formations over such extensive areas, and at such great elevations, is a conclusive proof that a great part of our existing continents were then at the bottom of deep oceans. The Alps were certainly 10,000 feet lower than their present level, and the Himalayas more so; and when this was the case a great part of Europe and Asia must have been sea, in which only a few of the highest peaks and elevated plateaux stood up as islands. The Pacific and Indian Oceans as well as the Atlantic might then have poured their Gulf Streams into the Polar basin, and prevalent southerly and westerly winds, blowing over wide expanses of water, have deposited their vapour in genial showers instead of in solid snow. The effect of such geographical conditions in producing both heat and cold is admirably worked out by Wallace in his Island Life, and few who read it can doubt that Lyell was right in saying that they have been the principal causes of the vicissitudes of climate. And here I may say a word to express my admiration of the innate sagacity with which Lyell, many years ago, and with comparatively few facts to work upon, sketched out the leading lines of geology, which have been confirmed by subsequent research. Details may have been corrected or added, but his main theories have stood the fullest test of the survival of the fittest. His law of the uniformity of natural causes, continued for long intervals of time, holds the field unchallenged. These causes may have operated a little more quickly or slowly in former ages than at present, but they have been of the same order. The waste of continents, instead, of averaging one foot in 4000 years, may have averaged ten or twenty feet during certain periods, and certain portions of the earth's crust may have been elevated or depressed at a quicker rate than is now going on in Scandinavia; but no one any longer believes in paroxysms throwing up mountain chains or sinking continents below the ocean at a single blow.
In like manner later geologists have corrected details in the distribution of land and sea suggested by Lyell to account for the glacial period, but his main law has only received confirmation—viz. that this distribution, and especially high land in high latitudes, has been the principal cause of such periods.