From the foregoing facts, it seems to me that we are justified in
concluding:

"1. That however simple and plausible the Lyellian hypothesis may be,
or however ingenious the extension or application of it suggested by
Dana, it is not sustained by any proof, and the testimony of the
rocks seems to be decidedly against it.

"2. Though much may yet be learned from a more extended and careful
study of the glacial phenomena of all parts of both hemispheres, the
facts already gathered _seem to be incompatible with any theory yet
advanced_ which makes the Ice period simply a series of telluric
phenomena, and so far strengthens the arguments of those who look to
extraneous and cosmical causes for the origin of these phenomena."[1]

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.