"WATER AS A MECHANICAL AGENT. 243 



2. Lamellar or straticulate structure of glacier-ice modified by the flow; the 

 " blue bands," or " veined stricctare." — The ice of a glacier, as first observed 

 by G-uyot (1838), is often vertically laminated parallel to its sides, and 

 sometimes so delicately that the ice appears like a semi-transparent striped 

 marble or agate. The layers are alternations of cellular (or snowy) ice and 

 clear, bluish, solid ice, and an indication of the porosity of a glacier. The 

 melting of the surface sometimes leaves the ledges of the more solid layers 

 projecting. This is well seen either side of the middle portion of the Mer 

 de Glace, and in the Brenva and Aar glaciers. Guyot found the structure 

 in the ice of the summit of the Gries Glacier at a height near 7500 feet. 

 He concluded that the layers were made from the daily driftings, deposi- 

 tions, and hardenings of snow over the iieve region ; that they were origi- 

 nally horizontal or parallel to the surface of the glacier, as in the bedding 

 and straticulation of a shale ; and that the various positions assumed, includ- 

 ing parallelism to the sides of the valley, were probably a consequence of 

 the flow, and of the greater velocity at middle. The structure is attributed 

 by Tyndall to the pressure to which the ice is subjected in making its way 

 between the walls of a valley ; but regions present it that have had no such 

 pressure. 



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 specific 

 recognition of the idea of plasticity in the ice, and of the influence, on the movement, 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' in three years, and 2354' in nine years. 

 Guyot made his early observations in 1838, and drew up a paper containing his conclu- 

 sions ; but failed to publish it because of his giving the field up to his friend Agassiz. (See 

 Memoir, U. S. Nat. Acad., 1886 ; Am. Jour. Sci., 1886 ; and Smiths. Inst., 1889.) 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 6 poles in the line across having 

 moved severally 160', 225', 269', 240', 210', and 120'), were published in the Comptes 

 Bendus. His investigations were continued for several years afterward; and in 1847 

 appeared his first great work, entitled Systhne Glaciaire. Professor Forbes visited 

 Agassiz at his work on the Aar, in 1841, and in the summer of 1842 undertook an inde- 

 pendent 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 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. 



3. Rate of flow. — The rate of descent in the mass of a glacier in the Alps 

 varies from one or two inches a day to over 50 ; it is about half as much in 

 winter as in summer. Ten to twenty inches a day in the warm season is most 



