The reader will therefore understand that, in advancing into this argument, he is not invading a realm where Science has already set up her walls and bounds and landmarks; but rather he is entering a forum in which a great debate still goes on, amid the clamor of many tongues.
There are four theories by which it has been attempted to explain the Drift.
These are:
I. The action of great waves and floods of water.
II. The action of icebergs.
III. The action of glaciers.
IV. The action of a continental ice-sheet.
We will consider these several theories in their order.
[1. "Popular Science Monthly," July, 1876, p. 290.]
{p. 10}