WHILE several different origins have been assigned for the phenomena known as "the Drift," and while one or two of these have been widely accepted and taught in our schools as established truths, yet it is not too much to say that no one of them meets all the requirements of the case, or is assented to by the profoundest thinkers of our day.
Says one authority:
"The origin of the unstratified drift is a question which has been much controverted."
Louis Figuier says, after considering one of the proposed theories:
"No such hypothesis is sufficient to explain either the cataclysms or the glacial phenomena; and we need not hesitate to confess our ignorance of this strange, this mysterious episode in the history of our globe. . . . Nevertheless, we repeat, no explanation presents itself which can be considered conclusive; and in science we should never be afraid to say, I do not know."
"Many geologists can not yet be persuaded that till has ever formed and accumulated under ice." 
A recent scientific writer, after summing up all the facts and all the arguments, makes this confession:
[1. "American Cyclopædia," vol. vi, p. 112.
2. "The World before the Deluge," pp. 435, 463.
3. "The Great Ice Age," p. 370.]
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."
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.
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.]