6 
GLACIAL PERIOD. 
my readers, I must leave the Scotch hills for 
a while, and beg them to return with me to 
Switzerland once more. 
Thus far I have spoken chiefly of the ad¬ 
vance of glaciers, and very justly, since they 
are in constant onward motion, being kept 
within their limits only by a waste at their 
lower extremity proportionate to their ad¬ 
vance. But in considering the past history 
of glaciers, we must think of their changes 
as retrograde, not progressive movements; 
since, if the glacial theory be true, a great 
mass of ice, of which the present glaciers 
are but the remnants, formerly spread over 
the whole northern hemisphere, and has grad¬ 
ually disappeared, until now no traces of it 
are to be found, except in the Arctic regions 
and in lofty mountain-ranges. Every ter¬ 
minal moraine is the retreating footprint of 
some glacier, as it slowly yielded possession 
of the plain, and betook itself to the moun¬ 
tains ; wherever we find one of these ancient 
semicircular walls of unusual size, there we 
may be sure the glacier resolutely set its 
icy foot, disputing the ground inch by inch, 
while heat and cold strove for the mastery. 
