680 DYNAMICAL GEOLOGY. 



more or less heaped, and slower in movement ; and then it moves 

 through the narrows below, with a consequently increased rate of 

 flow. 



(5.) The rate of movement of the glacier as a whole depends on the 

 following conditions : — 



(a.) The amount and rate of supply of moisture precipitated as 

 snow. 



(b.) The slope of the upper surface of the glacier : which slope is 

 determined, in ordinary cases, partly by the supply of snow to the 

 glacier, over its upper portions, and partly by the slope and form of 

 the land beneath ; but the latter slope is not a prerequisite to move- 

 ment, as explained on page 536, just as it is not for the movement of 

 water or pitch. 



(c.) The presence or absence of obstructions, in the valley or region 

 along which it moves. 



All these points have been demonstrated by observation and experi- 

 ment. The greater rapidity of the middle portion is shown by the 

 fact that the transverse ridges made at an ice-cascade, like that of the 

 Rhone, and the lines of earth and sand in the chasms, become after- 

 ward arched in front, as shown in Fig. 1103, in which the crevasses c 

 are at first transverse, but curve below the cascade. The arch is 

 sometimes very much elongated, almost to a triangular form, as in the 

 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 full}-, with a spe- 

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

 ment^ 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 " Sj T steme 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 iu detail, and the theory of 



