Page:Encyclopædia Britannica, Ninth Edition, v. 11.djvu/871

Rh HIMALAYA 831 watershed the snow line is not lower than 18,500 feet, and on the summit of the table-land it reaches a height of about 20,000 feet. On all the higher passes into Tibet the vegetation reaches to about 17,500 feet, and in August they may all be crossed in ordinary years, even up to 13,400 feet, without finding any snow upon them ; and it is as impassible to find snow in the summer on the great plain of Gugd in western Tibet, at 15,500 feet above the sea, as on the plains of India. There was at one time much discussion as to the true cause of the level of the snow line being higher in Tibet than on the outer ranges of the Himalaya. But the reason is obvious, namely, the greater quantity of snow that falls on the exterior ranges. The snow line marks the limit beyond which some snow always remains unmelted, that is, the point where on the average of years the summer heat will destroy the winter s snow. The main element in determining such a point will evidently be the quantity of winter snow that falls, the summer heat being not very different over the whole area. That the fall of snow on the outer ranges is greater than it is nearer the watershed at the same elevation is beyond doubt, and the smaller quantity of snow is more easily and earlier melted. It was long affirmed that there were no true glaciers in the Himalaya, and ingenious arguments were adduced to account for their absence. But, in fact, they abound along all the higher ranges precisely in those circumstances under which they are found elsewhere, and all the phenomena observed elsewhere in connexion with them are reproduced in these mountains. The level to which the Himalayan glaciers descend is very various, being greatly dependent on local, conditions, principally the extent and elevation of the snow basins which feed them, and the slope and position of the mountain on which they are formed. A glacier formed on the southern face of the Himalaya may readily be in a position in which it might, with a length of 10 miles, descend from 18,000 feet to 11,500 feet. But in Tibet, or in the highest valleys, it might as readily happen that in 10 miles no lower level than 16,000 feet could be reached. With a larger snow basin on the south the greater waste in reaching the lower level might easily be compensated, while no conceivable supply would enable the Tibetan glacier to reach 11,500 feet, to do which it must extend perhaps 100 miles. These considerations will sufficiently explain the fact that the glaciers on the outer slopes of the Himalaya descend much lower than is commonly the case in Tibet, or in the most elevated valleys near the watershed. The glaciers of Sikim and the eastern mountains are believed not to reach a lower level than 13,500 or 14,000 feet. In Kumaon many of them descend to between 11,500 and 12,500 feet, In the higher valleys and Tibet 15,000 and 16,000 feet is the ordinary level at which they end, but there are exceptions which descend far lower. In Europe the glaciers descend between 3000 and 5000 feet below the snow line, and in the Himalaya and Tibet about the same holds good. The summer temperatures of the points where the glaciers end on the Himalaya also correspond fairly with those of the corresponding positions in European glaciers, viz., for July a little below 60 Fahr., August 58, and September 55. Measurements of the movement of Himalayan glaciers have been made which also give results according closely with those obtained under analogous conditions in the Alps, viz., rates from 9| to 14 inches in twenty-four hours. The motion of one glacier from the middle of May to the middle of October averaged 8 inches in the twenty-four hours. The average yearly motion of the glacier of the Aar is said to be 7| inches in twenty-four hours. The dimensions of the glaciers on the outer Himalaya, where as before remarked, the valleys descend rapidly to lower levels, are fairly comparable with those of Alpine glaciers, though frequently much exceeding them in length 8 or 10 miles not being unusual. In the elevated valleys of northern Tibet, where the destructive action of the summer heat is far less, the development of the glaciers is enormous. At one locality in north-western Ladak there is a continuous mass of snow and ice extending across a snowy ridge, measuring no less than 64 miles between the extremities of the two glaciers at its opposite ends. Another single glacier has been surveyed 36 miles long. In connexion with almost all the Himalayan glaciers of which precise accounts are forthcoming are found ancient moraines indicating some previous condition in which their extent was much larger than now. In the east these moraines are very remarkable, extending 8 or 10 miles. In the west they seem not to go beyond 2 or 3 miles, and they have been observed on the summit of the table-land as well as on the Himalayan slope. The explanation sug gested to account for the former great extension of glaciers in Norway would seem applicable here. Any modification of the coast-line which should submerge the area now occupied by the North-Indian plain, or any considerable part of it, would be accompanied by a much wetter and more equable climate on the Himalaya ; more snow would fall on the highest ranges, and less summer heat would be O O brought to bear on the destruction of the glaciers, which would receive larger supplies and descend lower. Such an explanation is not inconsistent with what is known of the geological formation of the mountains, and appears to be otherwise supported by the evidence of a greater former extension of the lakes of Tibet, and of the former existence of rivers flowing from the Himalaya between the Jumna and the Sutlej, the dry beds of which are now to be traced, in which water is never seen. Till now no geological evidence has been adduced to indicate in this region any thing corresponding to the glacial epochs of northern Europe, to which these former extensions of the glaciers could be attributed. Speaking broadly, the general type of the flora of the lower, hotter, and wetter regions, which extend along the great plain at the foot of the Himalaya, and include the valleys of the larger rivers which penetrate far into the mountains, does not differ from that of the contiguous Malayan peninsula and islands, though the tropical and insular character gradually becomes less marked as we pass from east to west, where, with a greater elevation and dis tance from the sea and higher latitude, the rainfall and humidity diminish and the winter cold increases. The vegetation of the western part of the plain and of the hot test zone of the western mountains thus becomes closely allied to, or almost identical with, that of the drier parts of the Indian peninsula, more especially of its hilly portions ; and, while a general tropical character is preserved, forms are observed which indicate the addition of an Afghan as well as of an African element, of which last the gay lily Gloriosa superba is an example, pointing to some previous connexion with Africa. The European flora, which is diffused from the Mediter ranean along the high lands of Asia, extends to the Hima laya ; many European species reach the central parts of the chain, though few reach its eastern end, while genera common to Europe and the Himalaya are abundant throughout and at all elevations. From the opposite quarter an influx of Japanese and Chinese forms, such as the rhododendrons, the tea plant, Aucuba, Helwingia, Skimmia, Adamia, Goughia, and others, has taken place, these being more numerous in the east and gradually dis appearing in the west. On the higher and therefore cooler and less rainy ranges of the Himalaya the conditions of Formci exten sion beyond present limits. Botanj Genera charac teristic of flora Relatio to neig bourin&amp;lt; areas.