The sand and gravel of Sahara are underlaid by a deposit of clay.

Bayard Taylor describes in the center of Africa

[1. "The Palæolithic Implements of the Valley of the Delaware,"
Cambridge, 1881.]

{p. 40}

great plains of coarse gravel, dotted with gray granite bowlders.[1]

In the United States Professor Winchell shows that the drift-deposits
_extend to the Gulf of Mexico_. At Jackson, in Southern Alabama, be
found deposits of pebbles one hundred feet in thickness.[2]

If there are no drift-deposits except where the great ice-sheet
ground them out of the rocks, then a shroud of death once wrapped the
entire globe, and _all life ceased_.

But we know that all life,--vegetable, animal, and human,--is derived
from pre-glacial sources; therefore animal, vegetable, and human life
did not perish in the Drift age; therefore an ice-sheet did not wrap
the world in its death-pall; therefore the drift-deposits of the
tropics were not due to an ice-sheet; therefore the drift-deposits of
the rest of the world were not due to ice-sheets: therefore we must
look elsewhere for their origin.

There is no escaping these conclusions. Agassiz himself says,
describing the Glacial age: