694 DYNAMICAL GEOLOGY. 



Geant portion of the Mer de Glace. This is well illustrated in Figs. 

 1101, 1102, from Tyndall: the right-hand half of the figure, corre- 

 sponding to the Geant Glacier (the cascade which is alluded to on 

 p. 678), has the transverse bands (carrying dirt and stones) elongated 

 into triangles, while in the other half of the Mer de Glace there are 

 no such bands, as the tributaries making it do not descend in cas- 

 cades. 



The view that the movement of glaciers was essentially like that of rivers or " soft- 

 ened wax " was announced by Bordier in 1773; and afterward more fully, with a spe- 

 cific recognition of the idea of plasticity in the ice, and of the influence, on the move- 

 ment, of friction at bottom and along the sides, by Rendu, in a memoir read before the 

 Academy of Sciences of Savoy, in 1841. Hugi, in 1827, built a hut on the Aar glacier, 

 to determine its rate of motion; and found the movement 330 feet in three years, and 

 2,354 feet in nine years; and afterward Agassiz observed that in fourteen years it had 

 moved 4,712 feet below its first position. Agassiz commenced in 1841 his grand series 

 of observations on the Aar glacier, measuring the rate of movement in a section across 

 the glacier; and, on July 4, 1842. his first results, proving the more rapid flow of the 

 middle portion (his six poles in the line across having moved severally 160, 225, 269, 

 240, 210, and 120 feet), were published in the "Comptes Rendus." His investigations 

 were continued for several years afterward ; and in 1847 appeared his first great work, 

 entitled " Systeme Glaciaire " Prof. Forbes visited Agassiz at his work on the Aar, in 

 1841, and in the summer of 1842 undertook an independent investigation on the Mer de 

 Glace, near Chamouni ; and in October of 1842 his measurements, confirming those of 

 Agassiz, were published. A year afterward, in 1843, appeared his "Travels in the 

 Alps," in which his various careful observations are given in detail, and the theory of 

 glaciers, on the principle that the ice moves like a viscous fluid, is fully elucidated. His 

 later writings on the subject are contained in a volume entitled "Occasional papers on 

 the Theory of Glaciers." Later, Tyndall (from whom these historical notes are taken) 

 made a further series of measurements and observations in the Alps, demonstrating the 

 influence of bends in a glacier, and explaining other glacial phenomena. His views 

 are contained in "The Glaciers of the Alps," 1860, and "The Forms of Water," 1872. 



The rate of descent in the mass of a glacier varies from one or two 

 inches, to over fifty a day ; and the rate is about half less in winter 

 than in summer. Ten to twenty inches a day in the warm season is 

 most common ; twelve inches corresponds to three hundred and sixty- 

 five feet a year, or one mile in about fourteen and a half years. It 

 takes the ice of the Col du Geant one hundred and twenty years to 

 reach the lower end of the Mer de Glace. 



Opposite Montanvert, where there is a bend in the stream, Tyndall found the move- 

 ment per day, at eleven stakes, from the east to the west side, 20, 23, 29, 30, 34, 28, 25, 

 25, 18, 9 inches, the first and last being near the opposite sides. Descending from Tre- 

 laporte to Montanvert, the rate increases from twenty to thirty-four inches a day. At 

 Trelaporte, the three tributary glaciers of the Col du G^ant, Lechaud, and Talefre have 

 become one ; and the ice moves in a channel but half as wide as the sum of the widths 

 of these three tributaries. The rate of movement above this narrowing is hence slow; 

 Tyndall found the movement per day, across the lower part of the Col du Geant, 11, 

 10, 12, 13, 12, 13, 11, 10, 9, 5 inches; across the lower part of the Lechaud glacier, 5, 8, 

 10, 9, 9, 8, 6, 9, 7, 6. 



Forbes deduced, from his measurements, made at two stations on each of the Bois and 

 Bossons Glaciers, the following results. The first station on the Bois Glacier was near 



