44 
THE PARALLEL ROADS OF 
the inequality in the surfaces presented by the 
opposite sides of any obstacle in the path of the 
ice is often an important means of determining 
the direction of its motion. 
The other characteristic peculiarity of these 
roches moutonnees consists in the direction of 
the glacier-scratches, which ascend the slope 
to its summit in a direct line on one side, 
while they deviate to the right and left on the 
other sides of the knoll, more or less obliquely 
according to its steepness. Occasionally, large 
boulders may be found perched on the very 
summit of such prominences. Their position 
is inexplicable by the supposition of currents 
as the cause of their transportation. Any 
current strong enough to carry a boulder to 
such a height would of course sweep it on with 
it. This phenomenon finds, however, an easy 
explanation in the glacial theory. The thick¬ 
ness of such a sheet of ice is of course less 
above such a hill or mound than over the lower 
levels adjoining it. Not only will the ice melt, 
therefore, more readily at this spot, but, as ice 
is transparent to heat, the summit of the prom¬ 
inence will become warmed by the rays of the 
sun, and will itself facilitate the melting of 
