210 Correspondence. 
Forbes-]- describes the glacier of Brenva and others as flowing over their 
moraines. Certainly none knew better the movement of glaciers than 
Forbes, for although combatted, there has been no advance in the knowl- 
edge of the causes of their flow beyond his own plastic theory, except to 
support his views. 
CollombJ "i who was with Agassiz) said that the Aar glacier(the basis of 
Agassiz's work) pushed against its moraines, scarcely deranging, and slid 
over without excavating them. He also says that the glaciers of the Rhone, 
AUee Blanche, and those about Zermatt and Chamouni did not penetrate 
the soil, although affecting the surface of a meadow very slightly. Char. 
pentier made similar observations, and generalizes thus : "When a glacier 
reaches the bottom of a large valley, so that it can expand freely on ali 
sides, it ceases to dig and to raise the flat earth which it meets, especially 
if it be deprived of vegetable soil, and gravelly enough not to retain tbe 
water which it absorbes."§ 
Dr. Charles Martins Xt speaking with equal authority, says: "Un glacier 
ne penetre pas dans terrain meuble a la maniSre d'un soc de churrue qui 
entame le soc et 1' af^ouille. II agit comme grande polissoir qui le nivell© 
dans surfaces nf^glues." 
In the region of lake Geneva and in Val d' Aosta, where the ancient 
glaciers were at least from 2200 to 2700 feet thick the older subjacent 
gravels were not disturbed as has often been shown by the Swiss school of 
glacialists. This shows that the action of the great ancient glaciers upon 
subjacent gravels was one in kind with that of the modern living glaciers. 
If we ignore those phenomena, w^e must pass into the fleld of speculation 
which admits of no proof. The ploughshare of the glacier cannot be the 
bottom but the snout, as the pressure of the mass is a great leveller. 
We know but little of the action of glaciers under partial flotation 
•when they are projecting into the sea, (but here I am willing provisionally 
to allow, if necessary, a different action from that of land glaciers,) which 
are the only ones that are known to have the phenomenal velocity ; and 
these are gathered from great basins, whose aggregate discharge through 
a single channel must produce an accelerated velocity greatly in excess of 
that of the parts. 
My reviewer asks if the boulders, which I have described as causing the 
subjacent parts of the glacier to be channelled, when these come in con- 
tact with the bed-rocks, owing to the adhesion of the stones to the rock 
from friction, may not have melted through the ice? Go and see; for I 
have seen them in so many stages of contact and enclosure in the ice as> 
to preclude the idea that the cliannellings were due to the melting of the 
ice owing to the greater conductivity of heat by the stone, for the enclosed 
stones have the same temperature as the enclosing ice ; and any increase 
of heat in the caverns, visited, under the ice affected the body of the 
tTravels in the Alps, by J. D. Forbes, 1843. 
jat. by Sir R. I. Murchison, Add. Roy. Geog. Soc, 1864. 
gEssai sur Les Glaciers etc., by J. de Charpentier, 1841 
JtRevue de Deux Mondes, March, 1864, p. 87. 
