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advance more than 4 or 5 inches a day at the most.* Masses of rock 
of different sizes, from huge boulders to stones, are constantly broken by the 
frost from the sides of the valley of the glacier, and are carried down slowly 
on its back to the level where its head melts away, and there are deposited. 
These are called moraines. They lie along the course of the glacier in ridges 
protecting the ice beneath them from the sun's rays. That ice does not 
therefore melt as the rest of the ice does, and so it forms a ridge of ice. A 
moraine is therefore a ridge of stones standing on a ridge of ice. 
The descent of a glacier is not a descent of the whole together, or bodily 
like that of a block of stone. There is an internal descent of every particle 
in the glacier over and alongside of every other particle. If a plane section 
be imagined to be made across it ; the particles of ice passing through that 
section at any given time, must be conceived to be all moving at different 
rates so as to be sliding over and beside one another ; the particles at the 
surface moving faster than those below, and the particles nearer the centre 
moving faster than those at a distance from it, exactly as the particles of a 
stream of water move. 
The cause of the descent of glaciers has long been, and still is, the subject 
of controversy. Some philosophers say that they descend by their weight 
only. Others say that their weight does not supply power enough to bring 
them down. To the first class belonged the Swiss philosopher De Saussure, 
who was the first to study the subject with care, and wrote on it about 60 
years ago. He held that glaciers slip down the slopes on which they rest by 
their weight, just as other bodies slip down inclined planes. This explanation 
is simple and was generally accepted as long as it was thought that glaciers 
slipped down bodily like blocks of stone would, with an equal motion of all 
their particles ; but when the iaternal motion of their particles upon one 
another, like that of running water, came to be discovered, and when it was 
found that the high pitched tributary glaciers moved slower than the low 
pitched principal ones, it was brought into doubt, for it was in direct 
contradiction to these facts. 
Looking (like De Saussure) for no other force than the obvious one of their 
weight as the cause, M. Rendu Bishop of Annecy, who had made glaciers 
the subject of a very profound study, thought that solid as they seemed, they 
were not so ; but viscous, and descended as mud would descend, or soft 
plaister, or honey, or pitch ; and this is the famous viscous theory taken up 
and advocated with remarkable knowledge of the whole question, and great 
* The motion of the Glunberg, a tributary of the Aar glacier inclined at 
30° to 50 ti was found by Desor to be 22 metres a year, while that of the Aar 
glacier inclined at 4 y was 77 metres. 
