The converse of this proposition does not seem to be so universal. We have the high authority of Humboldt for the fact, that no glaciers occur in the Andes of tropical America, from the equator to 19° north latitude. Nor is it, I think, possible that the existence of glaciers should have escaped his notice, did they occur of such dimensions as would be indicated by the solitary and doubtful instance mentioned by M. Boussingault, to which Humboldt refers[37], which is stated to have been seen at the same elevation as the town of Quito, or more than 5500 feet below the level of perpetual snow in that region of the Andes. The cause of the non-existence of masses of moving ice, in connection with the perpetual snow of the American tropics, must apparently be sought in the extreme uniformity of the seasons, and in the small quantity of snow which falls at any time of the year.
In every region of the earth, so far as is known to me, where the mean temperatures of summer and winter are very different, or where the climate is what is called excessive, perpetual snow produces glaciers. These rivers of ice, as they have most appropriately been called, vary very much in size and appearance. In the lofty chains of the cis- and trans-Sutlej Himalaya, and of the Kouenlun, whose peaks rise to a very great height, and collect in winter enormous depths of snow, they are of great length. In the central parts of Tibet, which are often lower, and even in their loftiest parts are less snowy than the bounding chains, the glaciers are of inferior dimensions, often of that kind which I have called incipient, where the snow-bed is at once cut off abruptly in an ice cliff, which can hardly be said to be in motion, or rather whose motion must be almost entirely from above downwards.
The general appearance of an Indian glacier seems in every respect to accord with those of Switzerland and of other parts of the temperate zone. It is only of late years, indeed, that they have been generally recognized in the Himalaya; but it must not be forgotten that it is only recently that the researches of modern investigators, and in particular the delightful work of Forbes, have familiarized the untravelled world with their appearance, and more especially with the fact and cause of their motion. It has also, singularly enough, long been the custom to look upon the Himalaya as a tropical range of mountains, in which it was, as a matter of course, regarded as impossible that glaciers could exist[38].
The upper end or origin of a glacier seems commonly to be in an enormous snow-bed, occupying the whole space included by an amphitheatre of snowy peaks. The snow-slips and accumulations by which the snow-bed is added to during winter, must to a great extent remain concealed from human eyes; and in summer, when these icy fields are accessible, they are generally, I believe I may say always, covered by a thick layer of snow, which assists at the same time that it conceals the process by which the snow is converted into ice.
I have never measured the dimensions of any of the great glaciers of the Himalaya, nor is it easy to ascertain the length of any of them even approximately, as they are seldom traversed by roads, and are usually bent so that only a small part of their course can be seen. Many of them must considerably exceed ten miles in length; I have seen several which were more than half a mile broad; and the depth of the icy mass frequently amounts to hundreds of feet.
The appearance of the surface of a glacier seems to depend almost entirely on the inclination of its bed. Where the slope is gentle the surface is nearly uniformly smooth, or at most only slightly fissured. I have not had occasion to observe any fissures of more than a foot or two in width, so that, though often very deep, they are crossed without difficulty. In describing the icy surface as smooth, it is necessary to mention that such is only the case in the upper part of the glacier, where the moraines are small or only lateral. Whenever the surface supports rocky fragments in great quantity, it is extremely unequal till such time as the whole superficies becomes covered with stones, when the melting being uniform, the surface again becomes tolerably even.
On steeply inclined planes the glacier is traversed in every direction by enormous fissures, between which the surface is very irregular, rising into sharp icy pinnacles of the most fantastic shape and appearance. More than once I have seen extremely steeply sloping glaciers, which were terminated abruptly by a lofty precipice, at the bottom of which huge piles of boulders and occasional icy fragments sufficiently indicated the forward motion of the ice; at other times, the slope of the valley in which the glacier lies again becoming gentle, the ice ceases to be fissured and rugged, and is capable of being walked on without difficulty.
Moraines, which, on the larger glaciers and among mountains of easily decaying rocks, are of astonishing dimensions, form the margins of each glacier, and also occur longitudinally on different parts of their surface, increasing in number as the glacier advances, till at last the different series whose origin can long be traced to the different ramifications of the glacier, become blended into one. The nature, origin, and aspect of the moraines, the mode of melting of the ice beneath them, and the isolated pinnacles of ice which support large solitary boulders, agree so entirely with descriptions of glaciers in other parts of the world, that it is unnecessary to dwell upon them. The large glaciers are often a good deal lower in their central parts than where they are covered by a bulky moraine; and a curious ravine-like hollow, between the moraine and the bare ice, which makes the former appear as if entirely disconnected from the glacier, is of very common occurrence. There is, however, also very often an ancient moraine, not now resting on ice, which runs parallel to the glacier, and seems to indicate its former greater extent.
FORMER GREATER EXTENSION OF GLACIERS.
In every part of the Tibetan mountains, and in very many parts of the Indian Himalaya, I have thought that I could recognize unmistakeable proofs of all the valleys having been formerly occupied by glaciers at much lower levels than at present. At first sight it seems rather improbable, that in sub-tropical latitudes the present extension of perpetual snow should at any former period have been exceeded; but it would not be difficult to show that the mean temperature, and particularly the mean summer temperature, is very much higher in the Western Himalaya and Tibet than it might fairly be expected to be in such a latitude. In fact, in the more humid climate of Eastern Bengal, though at least four degrees nearer to the equator, the mean summer temperature at equal elevations in the mountains is probably considerably lower than in the mountains of North-west India, and the snow-level is certainly lower. It is fair, therefore, to conclude, looking back to a period when the sea washed the base of the Himalaya in the upper part of the Punjab, that at that period a very different state of atmospheric circumstances prevailed from that which we find at the present time.