54 General Notes. 
channels, as it had been discovered that ice can be melted by pres- 
sure (Thomson). He attributes the motion to plastic 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, in my Glacier Erosion in Norway’) 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 by weight. The glacier 
rore along its winding course to the snow fields, 1,500 to 2,000 feet 
above the stone. Consequently the crushing weight upon the gran- 
itoid 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 Plaff? show that a solid body can be 
pressed into ice at a temperature about freezing point as rapidly as 
glaciers ordinarily move ; whilst at a temperature a little above, the 
motion is greatly accelerated, but if below 0° ©, the plasticity of the 
ice diminishes rapidly to almost zero. However, as shown by the 
sub-glacial streams in winter, the temperature of the inferior sur- 
face glacier is not below freezing point. 
The effects of increased summer sunlight, as well as direct heat, 
as shown by the experiments of Rev. A. Irving, in which he trans- 
mitted both sunlight and heat waves through ice, is to accelerate 
the movement as the former is converted into heat undulations, 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 temperature. 
The temperature of the lower surface of the glacier is also slight- 
ly increased by the radiation of the internal heat of the earth, yet 
1 See American Naturalist, March, 1888. S 
2 Nature, Aug. 19th, 1875. 
3 Q. J. @. S., Feb., 1883 
