ADVANCE AND RETREAT OF GLACIERS. 201 CHAPTER XXXIV. PROGRESS AND RETIREMENT OF GLACIERS. APPEARANCE OF THE BED WHEN ABANDONED BY THE ICE.—ROCHES MOUTONNÉES.—PARALLEL FURROWS. In several parts of the Alps the mountaineers, influenced by the superstitious ideas of former days, continue to believe that the base of a glacier advances and recedes alternately every seven years.* The fact is, that if the progress and retreat of the fields of ice take place under the influence of any regular law, this law, which, at any rate, must be disturbed by a host of special local phenomena, has not yet been discovered. Since the date when regular observations first began to be made on the forward motion of the Alpine glaciers, they have been subject to very considerable fluctuations in their movements. Sometimes they have advanced, sometimes they have receded, and sometimes, even, they have remained stationary for several years together; but it appears that, on the whole, they have moved onward. Several of the Swiss glaciersthose of Zmutt, Aletsch, the Rhone, the Aar, and Grindenwald-have increased in length in their rocky beds. It appears to be certain that, in spite of temporary retirements, some fields of ice, even in the last century or two, have extended sufficiently to close mountain passes which were once practicable even for horses. Thus, several passes in the groups of Mont Blanc, Monte Rosa, and the Bernese Oberland, which still remained open in the fifteenth century, and were indeed used for troops, became more and more difficult to cross, and ultimately, during the course of the eighteenth century, have been rendered inaccessible, either for horsemen or pedestrians. The Lötschenpass, near the Gemmi, which was used less than a century back, is now closed up. Several facts of this kind are instanced in the Tyrol. One of the Etzthal glaciers, that of Gurgl, has certainly advanced a mile and a quarter since the year 1717, for that was the date when it commenced to dam up the side valley of Langenthal, in which the stream has accumulated to form a lake. In like manner, in Asia, the glaciers of the Karakorum seem to have uniformly advanced during the course of a century at least. The pass of Jusserpo was formerly accessible to horsemen; it can now only be crossed on foot. The glacier of Baltoro and the ancient pass of the Mustack have become impracticable.§ But this is not all, * William Huber, Les Glaciers. + Venetz, Denkschriften der Schweizerischen Gesellschaft, Part I., 1830. § Godwin-Austen, Journal of the Geographical Society of London, 1846. CHAPTER XXXV. DISTRIBUTION OF GLACIERS OVER THE SURFACE OF THE EARTH. MOUNTAIN Summits which rise above the limit of perpetual snow do not all give rise to rivers of ice; the concurrence of several meteorological and orographical conditions is necessary in order that the snow and névé should be changed into glaciers. In the first place, it is requisite that the snow zone of the mountain tops should be of some considerable breadth, and that vast beds of névé-those reservoirs for the supply of glaciersshould be formed in the mountain amphitheatres and in the upper plateaux. It also requires that the winds which blow against the mountains should be charged with an amount of humidity sufficient to leave. immense beds of snow on the summits and the slopes. Added to all this, the gorges which open into the thickness of the chain must be of a gentle inclination, so that the snow may not slide down immediately into the valleys below in the form of avalanches; and the mountains themselves must be grouped in such a way that their gorges unite to form a common basin, where the snow may be finally elaborated in order to constitute genuine rivers of ice. Lastly, it is indispensable that the various seasons of the year should afford extremes of temperature sufficiently great to allow of the phenomena of thawing and regelation taking place in the masses of névé. It is owing to the great uniformity of climate that so little ice is seen on the sides of the lofty snow-clad peaks of the equatorial Andes. The large number of different conditions which must all be combined as necessary to the formation of glaciers will readily show why these rivers of transformed snows are comparatively very rare in the regions of torrid and temperate zones. They are produced with a high degree of uniformity and grandeur only on the sides of lofty summits; while on mountains of less elevation, as those of the Vosges* and the Riesengebirge, they are formed in very snowy years in the recesses of ravines sheltered from the sun. It is, however, only in the vicinity of the poles that the ice-system manifests all its magnificence, and indeed constitutes the predominant feature of nature. In Europe, the Central Alps form the orographical system in which all the conditions necessary for the formation of glaciers are fulfilled at the greatest number of points. These mountains, too, will ever remain for savants the classical region of glaciers, for among them a De Saussure, a Charpentier, an Agassiz, a Rendu, a Forbes, and a Tyndall have gone on from discovery to discovery, and have ultimately brought to light the * Collomb, Comptes Rendus de l'Académie des Sciences, 1846, vol. xxi. GLACIERS OF THE ALPS. 207 true theory of the motion of ice. There are in the Alps nearly 1100 glaciers, a hundred of which may be looked upon as primary glaciers.* The total surface of the fields of snow, névé, and ice on the Alps is estimated by the brothers Schlagint weit at 1177 square miles, or about one seventh of the whole area of the great mountains from the Pelvoux to the GrossGlockner. The glaciers of Mont Blanc alone, though inferior in extent to those of Monte Rosa, cover a surface of 109 square miles. According to M. Huber, their total mass amounts to nearly 1834 millions of cubic yards, and represents a body of water equal to the whole of the discharge of the Seine during nine years. The glaciers of the Alps descend on the average to a point about 7414 feet above the level of the sea, that is, about 1650 to 2000 feet below the level of perpetual snow; but there are a great number of glaciers, and they are generally the most important, the base of which is below the Fig. 69. Glaciers of the Alps-after A. and H. Schlagintweit. a, a, Lower limit of persistent snows. b, b, Lower limit of secondary Glaciers. mean altitude of about 7000 feet. The Mer-de-Glace, the receptacle of the greatest part of the snow of Mont Blanc, reached in 1862, at the source of the Arveiron, a point which is only 3659 feet above the level of the sea. The Glacier des Bossons, fed by the snow of the same mountain group, descended to a level of 3605 feet; lastly, the lower glacier of Grindenwald, which of all the glaciers in the Alps pushes its way the farthest into the valleys below, has its terminal grotto placed at only 3225 feet above the sea-level. This fact must be attributed to the northern aspect of the glacier, to the narrow straits of rocks through which it has to flow, and its rapid declivity, exceeding 14 degrees. With regard to the Glacier d'Aletsch, which in its dimensions is the most important of all, and rolls down a wide current over a total length of 23,304 yards, it does not descend into the lower gorges, and in 1860 it stopped at an altitude of 5137 feet above the level of the sea. The Glacier d'Aletsch owes *The brothers Schlagintweit. The Bavarian savants ciers both of the Western and even the Central Alps. probably somewhat larger than they have estimated it. † Studer, Bibliothèque de Genève, September, 1866. have omitted in their list many glaThe ice area in these mountains is its enormous development to the great body of névé collected in the high mountain hollows. This glacier has a total area of no less than 130,000,000 square yards. The glaciers of the Tyrol are numerous, since, in the Etzthal and Stubaier groups alone, Sonklar reckons 309 glaciers, 16 of which are of the |