second volcanic focus, are curious proofs of how long a palpable cause may be rejected as insufficient to explain a phenomenon, and a totally imaginary one superadded.* I may add, that lava streams sometimes push their extremities up hill; glaciers do the same.‡ In addition to the considerations already stated, which illustrate the viscous theory of glaciers, I am glad to avail myself of two which have reached me from independent and impartial sources. The first is by Mr. Darwin, who in a small book on "Volcanic Islands," published about the time that I was engaged in making the preceding observations on Etna and Vesuvius, pointed out in a very clear manner the explanation which the veined structure of glaciers lends to that of volcanic rocks belonging to the Trachytic and Obsidian Series, where the lamination, instead of being obscure and rare, as it generally is in the Augitic lavas, owing perhaps to their greater fluidity, and more viscid and homogeneous texture, is the general rule. "The most probable explanation," says Mr. Darwin, "of the laminated structure of these felspathic rocks appears to be that they have been stretched whilst flowing slowly onwards in a pasty condition, in precisely the same inanner as Professor Forbes believes that the ice of moving glaciers is stretched and fissured. In both cases the zones may be compared to the finest agates; in both they extend in the direction in which the mass has flowed, and those exposed on the surface are generally vertical." § The other illustration is contained in a communication with which I have been favoured by Mr. Gordon, Professor of Civil * See the view of the termination of a lava stream in Auldjo's Sketches of Vesuvius, facing p. 92. The reader may also compare the view of a grotto in the lava, in the same work, with that of the source of the Arveiron in my Travels, p. 387. † Hamilton, Campi Phlegræi, folio, vol. i. p. 40, note. [As in the Allalein glacier, described in my Travels, p. 352, showing the remarkable development of the frontal dip under these circumstances. See also the Section in ab in the Ninth Topographical Sketch in the same work.) 8 Darwin on Volcanic Islands, 1844. The whole passage, pp. 65-72, illustrates this analogy. 1845.] EXAMPLES OF THE VELOCITY OF LAVA STREAMS. 93 Engineering in Glasgow, and which has been printed in the Philosophical Magazine for March 1845, to which therefore I may refer. I need only state at present that it demonstrates, from observations on the flow of Stockholm pitch with a speed wholly insensible, and which requires some months for its accomplishment even in small masses, that a motion, of the nature of fluid motion, takes place at temperatures at which the pitch remains so hard as to be fragile throughout, and presents angular fragments with a conchoidal fracture. Mr. Gordon adds, that the resistance of the pitch to its own forward motion produces bands of differential velocity, and having the frontal dip. EDINBURGH, FEBRUARY 26, 1845. NOTE ON THE VELOCITY OF LAVA, REFERRED TO IN PAGE 85. The following are a few facts which I have collected on the velocity of lava. That of Vesuvius in 1805 appears to be the most fluid on record. Von Buch, who was in company with MM. de Humboldt and Gay-Lussac, describes it as shooting suddenly before their eyes from top to bottom of the cone in one single instant, which must correspond to a velocity of many hundred feet in a few seconds without interpreting it literally. Melogrami, quoted by Breislak,† says it described. three miles in four minutes, or about seventy-five feet per second at a mean. The same lava, when it reached the level road at Torre del Greco, moved at the rate of only eighteen inches per minute, or three-tenths of an inch per second. The lava of 1794 (Vesuvius) reached the sea, a distance of 12,961 feet, in six hours, or passed over one-third of a mile per hour, or eight inches per second ;§ whilst the lava of Etna, in 1651, described sixteen miles in twenty-four hours, or above a foot per second the whole way. That of 1669 (Etna), which destroyed Bibliothèque Britannique, vol. xxx. The vertical height of the cone proper is 700 or 800 feet; the length of the slope may therefore be 1300 feet. + Institutions Géologiques, iii. 142. Nicholson's Journal, vol. xii. Breislak, Campanie, i. 203. Catania, described the first thirteen miles of its course in twenty days, or at the rate of 162 feet per hour, but required twentythree days for the last two miles, giving a velocity of twenty-two feet per hour; and we learn from Dolomieu, that this same stream moved, during part of its course, at the rate of 1500 feet an hour, and in others took several days to cover a few yards.† The lava of 1753 (Vesuvius), starting with a velocity of 2500 feet per hour, soon diminished to sixty feet, as did that of 1754 to the same; § and of 1766 to thirty feet per hour. The lava of 1831 (Vesuvius) moved over 3600 feet in twentysix hours, and finally advanced steadily at the rate of ten feet an hour. The lava of Etna of November 1843, is said to have moved over three paces per second at the distance of a mile from the crater. The stream of 1761 (Vesuvius), before it stopped flowing, advanced but three yards a-day ;** and that of 1766, which continued moving for about nine months, moved over but a small space in that time. Had the attention of authors been equally directed to the slow as to the rapid advancement of lava, there is no doubt that we should find many instances besides these recorded by Dolomieu and Scrope, of continuous movements of three feet, and even one foot a-day, or less. * Ferrara, Descr. del Etna, p. 105. This appears from the dates, though at variance with one assertion of the author. † Dolomieu, Isles Ponces, p. 286, Note. ? Ibid, p. 130. Della Torre, Histoire, etc., p. 196. Auldjo, Sketches of Vesuvius, p. 79, with a sketch of the front of the stream whilst advancing at this rate. ** Della Torre, p. 182. 1845.] DE SAUSSURE'S THEORY OF GLACIER MOTION. 95 XII. ILLUSTRATIONS OF THE VISCOUS THEORY OF GLACIER MOTION. - PART II. AN ATTEMPT TO ESTABLISH BY OBSERVATION THE PLASTICITY OF GLACIER ICE.* § 3. De Saussure's Theory. § 4. Modifications of De Saussure's Theory. § 5. Experiments at Chamouni on the Plasticity of ice. § 3. DE SAUSSure's Theory. When Gruner proposed the explanation of glacier motion by the sliding of the ice over its bed, and De Saussure illustrated and confirmed it by considerations drawn from the lubricating action of the earth's heat melting the ice in contact with the soil, there is no reason to suppose that either of them thought it necessary to take into account the varying form of the channel through which the glacier had to pass, and the consequently invincible barrier presented to the passage of a rigid cake of ice through a strait or narrow aperture when it occurred. This is the more remarkable, because he [De Saussure] conceives that the inequalities of the bed or bottom may be overcome by the hydrostatic pressure of the water, which he supposes may be imprisoned between the rock and the ice, so as absolutely to heave the latter over the resisting obstacles. I believe that in no part of De Saussure's writings will there be found any, the slightest reference to the possibility of the glacier, when fairly formed, moulding itself to the inequalities of the surfaces over which gravity urges it; nor is there any trace * Philosophical Transactions, 1846, p. 137. Received July 28, 1845.-Read January 15, 1846. To do Gruner justice, he appears to have been aware of the effects of the earth's heat and the lubricating action of the water thawed from the glacier: "Lorsque les côtés de l'amas [de glace] qui touchent la montagne, fondent en entier, toute la masse entraînée par son poids glisse sur son fond et s'avance dans la vallée," French translation, p. 333... "il est vraisemblable que leur surface, inférieure [i. e. des glaciers] se liquéfie autant, et peut-être plus que la supérieure,” ib. p. 289. of the correlative fact of an unequal motion of the sides and centre of the ice, which may in some sense be considered as the geometrical statement of the preceding physical fact. The fact of plasticity was suspected by Basil Hall, and more distinctly announced by Rendu, as shown in the first part of this paper; but it could not be proved until the geometrical fact of the swifter motion of the centre of the glacier relatively to the sides was established in 1842.* The contrary opinion at that time generally entertained would have been conclusive against the hypothesis of plasticity called forth by the gravity of the mass. So far then as appears from his writings, De Saussure considered the ice of glaciers to constitute a mass possessing rigidity in the highest degree, such rigidity in short as common experience assigns to ice tranquilly frozen in small masses, which is sensibly inflexible. It is in this sense in which I have spoken of De Saussure's sliding theory, as one which "supposes the mass of the glacier to be a rigid one sliding over its trough or bed in the manner of solid bodies," and I adhere to the definition as excluding the introduction of the smallest flexibility or plasticity, to which the term rigidity is correctly opposed. I consider too, that De Saussure's theory supposes the mass of the glacier to slide over its trough or bed in the manner of solid bodies, that is, not as a heap of rubbish or absolute fragments, such as a glacier sometimes precipitates over a rock, but which evidently did not enter into De Saussure's explanation, nor, in fact, required any theory. As to the crevasses which form so prominent a feature of many glaciers (although many are in parts almost devoid of them), I do not recollect that De Saussure alludes to them as facilitating in any way the movement of the glacier, but simply as results of its motion and of the rigid character of ice. And I believe that this view (whether it was held by De Saussure * Edinburgh Philosophical Journal, October 1842, and Travels in the Alps of Savoy, p. 134. Travels, p. 362. |