Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/646

Rh 628 superﬁcial layers are more snowy and white, in fact nearly pure snow; the deeper ones have more colour aml consist- ence, and break on the large scale into vast fragments, which at Chamouni are called seracs. The névé moves, as the glacier proper does, and it is ﬁssured by the inequalities of the ground over which it passes. These ﬁssures are less regular than those of the lower glacier. They are often much wider, in fact of stupendous dimensions, and, being often covered with treacherous snowy roofs, constitute one of the chief dangers of glacier travelling. The constitution of the névé may be well studied on the Glacier du Géant, a tributary of the Mer de Glace. The mountain-clefts in which large glaciers lie usually expand in their higher portions (in conformity with the ordinary structure of valleys) into extensive basins in which snow is perpetual, and which therefore contain the névé, the true origin and material of the glacier, which is literally the overﬂow of these snowy reservoirs. The amount of overﬂow, or the dis- charge of the glacier—upon which depends the extent of its prolongation into the lower valleys——depends in its turn on the extent of the néeé or collecting reservoir. Glaciers with small reservoirs of necessity perish soon. Their thickness being small, the wedge of the glacier soon thins out. They are common in confined cirques of the higher mountains. Such are the glaciers of the second order described by De Saussure. Their slope is often very great——from 20° to 40°. The ice of the glacier proper has a very peculiar struc- ture, quite distinct from the stratiﬁcation of the snow on the névé (the relics of its mode of deposit), and one which requires special notice. When we examine the appearance of the ice in the wall of an ordinary crevasse (especially if it be tolerably near the side of the glacier) we are struck with the beautiful vertically laminated structure (first observed by Principal Forbes) which it commonly presents, resembling delicately 1 veined marble (especially thevariety called in Italy cipollino), in shades varying from bluish-green, through green, to white. When we trace the direction of the planes constituting the laminated structure, by observing them on the surface of the glacier (where they are usually well seen after rain__. or in the channels of superﬁcial water-runs), we ﬁnd that where best developed (or not very far from the sides of the glacier) these laminae are nearly parallel to the sides, but rather ' incline from the shore to the centre of the ice stream as we follow the declivity of the glacier. Forbes found that certain superﬁcial discolorations in the form of excessively elongated hyperbolas are due to the recurrence (at intervals of some hundred feet along the course of the glacier) of portions of ice in which the veined structure is more energetically developed than elsewhere, and where, by the decomposition of the softer laminae, portions of sand and dirt become entangled in the superficial ice, and give rise to the phenomena of “dirt bands,” which thus at a distance display (though in a 111anner requiring some attention to discover) the exact course of this singular structure on the surface of the glacier. Fig. 1 displays Fig. 1. 1-*ig_ 3 the superficial form of the dirt bands, and the course of the structural laminae projected horizontally. Fig. 2 shows an ideal transverse section of the glacier, and ﬁg. 3 another vertical section parallel to its length. These three sections in rectangular planes will serve to give a correct idea of the course of this remarkable structure within the ice, but a more popular conception will be formed of it from the imaginary sections of a canal-shaped glacier in (fr LA C‘. I E R ‘ﬁg.-L. The structure of the compound glacier, originally double, becomes gradually single; and the “frontal dip” of the laminae at the loop of the horizontal curves, which in the upper region is nearly vertical, gradually Fig. -1. slopes forwards until at the lower termination it has a very slight dip inwards, or indeed may be reversed and fallout- wards and forwards. The general form of a structural lamina of a glacier rudely resembles that of a spoon. This structure and the accompanying dirt bands have been recognized by different observers in almost all glaciers, including those of Norway a11d of India. The interval between the dirt bands has been shown in the case of the Mer de Glace (a11d therefore probably in other cases) to coincide with annual rate of progression, and in the higher parts of the glacier (towards the névé) to be accompanied by wrinkles or inequalities of the surface which are well marked by the snow lying in them during the period of its partial disappearance. T/ze Zlfotion of Glaciers and its C'cmses.—Tl1ere is sonic- thing about a glacier which almost inevitably conveys to the mind the idea of a stream. This may be traced in the descriptions of unscientiﬁc tourists, of poets, and of some of those who have addressed themselves more seriously to the question of the real nature of these bodies. To the latter class of observers belong Captain Basil Hall and Monseigneur Rendu, bishop of Annecy, who had much more than hinted at the possibility of a true n1ecl1-.mical connexion between the descent of a glacier and that of a mountain torrent, or of a stream of lava. But until the actual conditions of motion were reduced to rule, it was impossible to know how far the analogy was real. The most characteristic and remarkable feature of gla- ciers is their motion downwards from the névé towards the lower valley. The explanation of it is by far the ‘most important application of mechanical physics connected with the subject. The prin- cipal theories to account for the progressive motion of glaciers which were prevalent pre- vious to 1842 may be brieﬂy characterized as De Saussure’s and De Charpentier’s, though each had been maintained long before by the earlier Swiss writers. The ﬁrst may be called the _r/2'az-ilalion theory, the latter the rlilalaliun theory. Both suppose that the motion of the ice takes place by its sliding bodily over its rocky bed,but they differ as to the force which urges it over the obstacles opposed by friction and the irregularities of the surface on which it moves. The following quotation from De Saussure explains his