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 Pom 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 arc 

 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 



