lOO SPENCER ON GLACIAL MOTION. 



covered that ice can be melted by pressiire (Thomsou). He attributes the motion to plas- 

 tic flow under gravity, rupture, partial regelation, and a sliding motion (which is slight). 



From observations in the Alps, and especially in Norway, my conclusions are that 

 the motion, in the main, is the result of gravity on a semi-fluid body, wherein there is 

 viscosity as well as plasticity, as defined by Prof. Heim ; the motion, of course, being 

 greatly modified by heat. My conclusions are based upon : — (1) The flow of the glacier, 

 not merely in conformity to the channel, but about loose stones, which cause the lower 

 surfaces of the glacier to be grooved (see fig. 1 of " Glacial Erosion, in Norway," etc.) 

 without any lateral ridges being produced from the ice that filled what are now its 

 channels, such being moulded into the mass (this is plasticity). (2) A tongue of ice (see 

 fig. 3) pushing against a boulder, was bent back without rupture on either side of the 

 hanging plate — the ice on one side being in tension and on the other in compression 

 (here is viscosity). (3) A large rounded boulder (see fig. 2), held in the side of a moving 

 glacier, where the rounded ice-wall rose about thirty feet above the stone, which was 

 being rolled along as the ice moulded around it, had just been crushed. The glacier rose 

 along its winding course to the snow fields, 1,500 to 2,000 feet above the stone. Conse- 

 quently the crushing weight upon the granitoid boulder must have been derived from 

 the vertical component of the momentum of descent of the whole mass, which could be 

 transmitted thus only through a semi-fluid body. (4) The flow of the upper layers of ice 

 over the lower was seen when the glacier was impeded by a barrier (see fig. 4). 



The experiments of Herr Pfaff' prove that a solid body can be pressed into ice at a 

 temperature about freezing point as rapidly as glaciers ordinarily move ; while at a tem- 

 perature a little above, the motion is greatly accelerated, but if below 0°O., the plasticity of 

 the ice diminishes rapidly to almost zero. However, as shewn by the subglacial streams 

 in winter, the temperature of the inferior surface of a glaciers is not below freezing-point. 



The effects of increased summer sunlight, as well as direct heat, as proved by the ex- 

 periments of Rev. A. Irving, ^ in which he transmitted both sunlight and heat waves 

 through ice, is to accelerate the movement as the former is converted into heat undula- 

 tions, and radiated against the lower part of the glacier from the adjacent rocks, thus 

 increasing the fluidity of the ice and flow of the glacier, owing to increase of tempera- 

 ture. 



The temperature of the lower surface of the glacier as also increased by the radiation 

 of the internal heat of the earth ; yet this is slight, as the amount radiated per annum is 

 only enough to melt a layer of ice, 6'5 millimetres in thickness. * 



Although glaciers do not conform to all the inequalities of their beds, and at the ice- 

 falls aad elsewhere become fractured, and subsequently reunited, whether by heat 

 regelation or plastic flow, the fluidity theory is the most acceptable explanation of the 

 motion of glaciers, especially when the angle of descent is reduced almost to zero, and 

 modern observations only supplement the good reasons upon which Prof. Forbes pro- 

 posed his theory more than forty years ago. 



1 Nature, Aug. 19th, 1875. " Quart. Jour. Geo. Soc, Feb., 1883. 



■' Élie de Beaumont, Thomson, Woodward and others give the range as from five to eight millimetres. 



