VALLEY OF THE AMAZONS. 
159 
to fifteen hundred feet in height, and descend¬ 
ing into the intervening valleys only two or 
three hundred feet above the sea, or sometimes 
even on a level with it. I take it to be impos¬ 
sible that a floating mass of ice should travel 
onward in one rectilinear direction, turning 
neither to the right nor to the left, for such a 
distance. Equally impossible would it be for 
a detached mass of ice, swimming on the sur¬ 
face of the water, or even with its base sunk 
considerably below it, to furrow in a straight 
line the summits and sides of the hills, and 
the beds of the valleys. It would be carried 
over the depressions without touching bottom. 
Instead of ascending the mountains, it would 
remain stranded against any elevation which 
rose greatly above its own basis, and, if caught 
between two parallel ridges, would float up 
and down between them. Moreover, the action 
of solid, unbroken ice, moving over the ground 
in immediate contact with it, is so different 
from that of floating ice-rafts or icebergs, that, 
though the latter have unquestionably dropped 
erratic boulders, and made furrows and striae 
on the surface where they happened to he 
grounded, these phenomena will easily be dis- 
