68 KEY. A. IRVING ON THE 



which are melted by the sun's heat or by a warm eurrent of air 

 may, in penetrating the glacier, come into contact with ice which is 

 locally colder than 0° C, and in this way undergo refrigeration. If 

 this took place anywhere around a surface of contact of the ice aud 

 the rocky bed, the surface of contact being increased (as Helmholtz 

 has shown in the valuable notes which he has appended to his 

 lecture, ' Eis und Gletscher '), the pressure on the previous plane or 

 point of contact would be distributed over a larger space, with the 

 thermal effect which can easily be deduced from the foregoing 

 principles. In this way as the glacier-mass is diminished at the 

 surface, a partially compensating formation of ice may go on at the 

 base. So far as it acted, there would be a transfer of materials to 

 some extent away from the bed. A little further reflection will 

 show us, however, that this could only happen where the water was 

 unconfined, and consequently only where the glacier was moving 

 down a slope. In such a case the result would seem to be rather 

 adverse to erosion. When however the glacier lies on a level or 

 hollow surface, such a process must soon bring itself to an end ; for, 

 the glacier being motionless at the base, all spaces would soon be 

 filled with ice and cold water kept there by gravitation, and the 

 whole pressure would bear directly upon the rock, tending, not to 

 break it up, but to compress it. 



A word or two is needed on the influence of terrestrial heat. 

 The facts connected with the flow of heat by slow conduction from 

 the interior to the exterior of the earth's mass can be learnt from 

 any good text-book of physics. The point with which we are here 

 concerned is this : — Since the rock in contact with the ice remains 

 at the same temperature as the ice, it can only do so by parting with 

 its heat to the glacier as it receives it from below. This heat must 

 do work. "What is that work ? Clearly the heat must be expended 

 in overcoming the cohesion of ice-particles in contact with the rock. 

 And this it would do whether the ice were at or below 0° C, since 

 in the latter case the heat received from the crust of the earth 

 cooperates with the pressure which is the necessary condition of a 

 temperature below 0° C. The bearing of this fact as tending to 

 diminish friction, and therefore erosion, has been pointed out above*. 



In connexion with glaciers there is, in addition to the polishing, 

 striating, and grooving work, observable everywhere on glaciated 

 rocks, a still more extensive work of erosion going on, by the action 

 of the glacier-streams which flow beneath them. Such streams, 

 while they intervene in places between the ice-mass and the trough 

 in which the glacier moves, are capable of doing much more work 

 than the ice itself, by virtue of the greater velocity with which they 

 carry stones and sand along. Since, however, this action depends 

 entirely upon the movement of the water, it is clear that a descent 

 is necessary for this movement ; and as water does not flow up-hill, 



* The conduction downwards of absorbed solar heat from the sides of the 

 valley to below the surface of the glacier produces in the summer a want of 

 contact, for some feet down, between the ice and rock — a fact familiar to every 

 observer of glaciers. 



