TRANSACTIONS OF THE SECTIONS. 65 
beds of the glaciers does not exceed 25°, or of the supereminent plane 
of the snow above 35° to 40°; so that the portion of the weight resolved 
on mechanical principles in a direction parallel to the inclined plane, or 
in the direction of motion, would be extremely small. Weight alone, 
therefore, or pressure from it being inadequate, Mr. Mallet ascribes 
the movement to the hydrostatic pressure of water accumulating between 
- the masses of ice and the rocky bed on which they rest, whereby the 
__ ice is as it were floated or transferred (at intervals) upon liquid rollers 
_ from a higher to a lower level. From the nature of the isogeothermal 
euryes, the bed on which the glacier rests is warmer than the glacier itself, 
_ hence a continued melting of the lower surface, and a constant produc- 
tion of torrents which rush out at the lower extremity of every glacier. 
_ This melting is independent of season, whilst the melting of the superfi- 
_ cial snow and of the external surface of ice depends solely upon it ; hence 
in summer there is no obstruction to the flowing of the torrents, whilst 
in winter their embouchures are closed by ice, and an accumulation of 
_ water below is the result, by the pressure of which the glacier is raised 
until a sufficient vent has been formed for the éscape of the waters, on 
the sinking of which the glacier descends for a certain distance into the 
valley. An example of a striking kind occurred in 1814-1815, in the 
Glacier de Bois, or the “Mer de Glace”. Its torrent, the Aveyron, being 
dammed up by the falling masses of ice, and partly frozen, could no 
longer discharge itself at its usual icy opening, but accumulated under 
the ice until it forced itself a new passage, 700 feet above the former one; 
the pressure being so great that for some months mountains of ice 
_-were continually falling. In short, the ice is by this hydrostatic pressure 
raised up perpendicularly to the inclined plane on which it rests; but, 
on the removal of the pressure, sinks perpendicularly to the horizon, 
and hence advances forwards. Next, with respect to the rifts or 
crevices which intersect the glaciers, they have, allowing for slight 
perturbations depending on the steepness of the slope of the beds, a 
general direction transverse to the line of motion, or across the valley 
in which the glacier lies, and the form of a curve or vast wave-like line 
 erossing the ice from side to side, and having its convex side down- 
wards. The glacier itself may be considered as a bed of indurated 
snow, deposited at the bottom of a sloping valley, and surcharged 
with infiltrated water, either the result of rain or its own slow 
_ liquefaction, and must be presumed therefore to have originally 
formed one unbroken mass, beneath which flowed a current of ice- 
water. But whenever this stream became obstructed either by frost or 
_ debris, the water would begin to rise beneath, until the hydrostatic 
“pressure at the lower end becoming sufficient to overcome the cohe- 
sion of the ice, the huge mass would be broken in two or more parts, 
_ somewhere above the upper line of the subglacial waters, just as a 
stranded vessel, supported at stern and stern, becomes, in technical 
language, hogged. And according as this process is modified by the 
_aceidental staggering of one mass against another, &c., so is the aspect 
of the whole strangely and capriciously varied. Sometimes an isolated 
rock resting on the surface, melts, from its superior conducting power, 
VOL. vi. 1837. EF 
