PHYSICAL HISTORY OF THE AMAZONS. 403 
There seems thus far to be an inextricable confusion in 
the ideas of many geologists as to the respective action of 
currents, icebergs, and glaciers. It is time that they should 
learn to distinguish between classes of facts so different 
from each other, and so easily recognized after the discrim- 
ination has once been made. As to the southward move- 
ment of an immense field of ice, extending over the whole 
North, it seems inevitable, the moment we admit that snow 
may accumulate around the pole in such quantities as to 
initiate a pressure radiating in every direction. Snow, 
alternately thawing and freezing, must, like water, find its 
level at last. A sheet of snow ten or fifteen thousand feet 
in thickness, extending all over the northern and southern 
portions of the globe, must necessarily lead, in the end, to 
the formation of a northern and southern cap of ice, moving 
toward the equator. 
I have spoken of Tijuca and the Dom Pedro Railroad as 
favorable localities for studying the peculiar southern drift ; 
but one meets it in every direction. A sheet of drift, con- 
sisting of the same homogeneous, unstratified paste, and 
containing loose materials of all sorts and sizes, covers the 
country. It is of very uneven thickness, - sometimes 
thrown into relief, as it were, by the surrounding denuda- 
tions, and rising into hills ; sometimes reduced to a thin 
layer ; sometimes, as, for instance, on steep slopes, washed 
entirely away, leaving the bare face of the rock exposed. 
It has, however, remained comparatively undisturbed on 
some very abrupt ascents ; as may be seen on the Corcovado, 
along the path leading up the mountain, where there are 
some very fine banks of drift, the more striking from the 
contrast of their deep-red color with the surrounding vege- 
tation. I have myself followed this sheet of drift from Rio 
