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.