222 ` Glacial Erosion in Norway. 
ice has a great deal to do with its flow about obstacles. Below 
freezing-point, the movement is scarcely more than appreciable, 
while above that point, but not below, it may reach twenty-eight 
inches a day, or more. The conditions arising from the tempera- 
ture beneath the glaciers are more or less favorable for the move- 
ment of the ice, as the lower surfaces are never entirely below 
freezing-point, even in winter. Professor S. A. Sexe! found that 
the water flowing from a Folgefond glacier, in February, 1861, 
had a temperature of 1°R., whilst that of the air was 7°R. below 
freezing-point. 
The movement or flow of the ice about detached stones, rest- 
ing upon rocks, has been observed by Professor Sexe beneath the 
- Buarbre, and by Professor J. W. Niles beneath the Aletsch gla- 
cier.? Professor Sexe illustrates the moulding of the ice about a 
loose stone, which was held beneath the glacier by a projection 
of the rock. My observations were upon stones, not held up by 
rocky projections, but upon surfaces often sloping downward. 
Although Professor Niles did not record observations showing 
that there was definite movement of the stone, yet he concluded 
that there was a differential movement of the ice and the block. 
Whatever differential movement there is, it must be very incon- 
siderable, not only upon horizontal plains, but upon inclined sur- 
faces. In the former case the movement of the ice is reduced 
almost to zero, as shown by the measurements of Professor Tyndall 
upon the Morteracht, where the velocity of the surface, some dis- 
tance from its end, was fourteen inches, whilst that of the tongue 
of the glacier, as it reached the plain, was only two inches a day.” 
The most important condition favorable for holding stones in ice 
as graving tools is low temperature, which impedes its progress; 
but this condition beneath glaciers does not generally exist. At 
higher temperatures, the velocity of the glacier is not great enough 
to overcome its plastic movement and to drag along detached blocks. 
However, when the whole mass of ice is charged with sand and 
stones, there is no doubt that polishing and scratching are effected ; 
but when there are only occasional fragments in the bottom of the 
ice, as is commonly the case, the erosion from the sliding ceases as 
1 Om Sneebreen Folgefon, af $. A. Sexe. 
2 American Journal of Science, Nov., 1878. 
3 Tyndall’s Forms of Water. 
