THE VARIATIONS OF GLACIERS. 
THE great interest which the physical study of living glaciers 
has to the geologist is the hght it may throw on causes produc- 
ing, and conditions prevailing during, the Ice Age. One of 
the habits of living glaciers bearing most directly on the Ice Age 
is the variation continually occurring in their length, thickness 
and velocity of motion. All students of glaciers have collected 
reports and made observations themselves on the advance and 
retreat of the ends of particular glaciers, but it is only within about 
twenty years that anything like systematic work has been done 
in getting together records which enable us, in some cases, to 
exhibit roughly the variations in the extent of certain glaciers 
for three hundred years, during which period there has been 
quite a number of advances and retreats; observations since the 
fifties give us a fair record of the movements of some Swiss 
glaciers. 
What are the causes of these variations? The answer of 
course is that they are meteorological, for it is quite evident 
that the extent of a given glacier depends on the snow-fall and 
the rate of melting; the ice continues to flow down its valley 
until it is all melted away. Anything that increases its velocity 
will make it flow further, and anything that increases its rate of 
melting will cut it off shorter. But which of these factors has 
been most important in determining the changes which have 
actually occurred? M. Forel has argued ably* that it is the 
change in velocity. He has shown that glaciers have varied 
very much in extent when there has been no great change in the 
rate of melting; that this has been due to a change in the 
velocity, which in its turn depended ona change in snow-fall. 
If this proves to be universally the case we have the following 
interesting application to make to the ice of the Ice Age: If the 
* Bibliotheque Universelle de Geneve, VI., pp. 5-49, 448-460. 
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