mazonian Valley. I am aware that this suggestion will appear
extravagant. But is it, after all, so improbable that, when Central
Europe was covered with ice thousands of feet thick; when the glaciers
of Great Britain ploughed into the sea, and when those of the Swiss
mountains had ten times their present altitude; when every lake in
Northern Italy was filled with ice, and these frozen masses extended
even into Northern Africa; when a sheet of ice, reaching nearly to the
summit of Mount Washington in the White Mountains (that is, having a
thickness of nearly six thousand feet), moved over the continent of
North America,--is it so improbable that, in this epoch of universal
cold, the Valley of the Amazons also had its glacier poured down into it
from the accumulations of snow in the Cordilleras, and swollen
laterally by the tributary glaciers descending from the table-lands of
Guiana and Brazil? The movement of this immense glacier would be
eastward, and determined as well by the vast reservoirs of snow in the
Andes as by the direction of the valley itself. It must have ploughed
the valley bottom over and over again, grinding all the materials
beneath it into a fine powder or reducing them to small pebbles, and it
must have accumulated at its lower end a moraine of proportions as
gigantic as its own; thus building a colossal sea-wall across the mouth
of the valley. I shall be asked at once whether I have found here also
the glacial inscriptions,--the furrows, striae, and polished surfaces so
characteristic of the ground over which glaciers have travelled. I
answer, not a trace of them; for the simple reason that there is not a
natural rock surface to be found throughout the whole Amazonian Valley.
The rocks themselves are of so friable a nature, and the decomposition
caused by the warm torrential rains and by exposure to the burning sun
of the tropics so great and unceasing, that it is hopeless to look for
marks which in colder climates and on harder substances are preserved
through ages unchanged. With the exception of the rounded surfaces so
well known in Switzerland as the _roches moutonnees_ heretofore alluded
to, which may be seen in many localities, and the boulders of Errere,
the direct traces of glaciers as seen in other countries are wanting
here. I am, indeed, quite willing to admit that, from the nature of the
circumstances, I have not here the positive evidence which has guided me
in my previous glacial investig