ICE-PERIOD IN AMERICA. 
93 
mined by laws of temperature, and so have we 
also seen t-liat the motion of glaciers is mainly 
determined by conditions of temperature, al¬ 
though, in this case, an internal mechanical 
action is combined with external influences; 
and while it is true that glaciers, as they now 
exist, are dependent upon the shape of the 
valleys in lofty mountain-chains, yet under 
different geographical conditions the same phe¬ 
nomena may be produced over level, open 
countries. 
I believe that circumstances similar to those 
determining the more rapid advance of the 
glaciers from higher to lower levels at that 
point where the alternate thawing and freez¬ 
ing, the infiltration of water and consequent 
expansion of the ice under frost, are greatest, 
would also determine the motion of a large 
body of ice from north to south, since it would 
be along its southern limits that these condi¬ 
tions would prevail; while the great reservoir 
of snow at the north would correspond to the 
upper troughs of the present glaciers, from 
which their lower ranges are constantly fed. 
The change of snow into ice is owing to alter¬ 
nations of temperature, to partial melting and 
