WE come now to a much more reasonable hypothesis, and one not without numerous advocates even to this day, to wit: that the drift-deposits were caused by icebergs floating down in deep water over the sunken land, loaded with débris from the Arctic shores, which they shed as they melted in the warmer seas of the south.
This hypothesis explains the carriage of enormous blocks weighing hundreds of tons from their original site to where they are now found; but it is open to many unanswerable objections.
In the first place, if the Drift had been deposited under water deep enough to float icebergs, it would present throughout unquestionable evidences of stratification, for the reason that the larger masses of stone would fall more rapidly than the smaller, and would be found at the bottom of the deposit. If, for instance, you were to go to the top of a shot-tower, filled with water, and let loose at the same moment a quantity of cannon-balls, musket-balls, pistol-balls, duck-shot, reed-bird shot, and fine sand, all mixed together, the cannon-balls would reach the bottom first, and the other missiles in the order of their size; and the deposit at the bottom would be found to be regularly stratified, with the sand and the finest shot on top. But nothing of this kind is found in the Drift, especially in the "till"; clay, sand, gravel, stones,
and bowlders are all found mixed together in the utmost confusion, "higgledy-piggledy, pell-mell."
"Neither can till owe its origin to icebergs. If it had been distributed over the sea-bottom, it would assuredly have shown some kind of arrangement. When an iceberg drops its rubbish, it stands to reason that the heavier blocks will reach the bottom first, then the smaller stones, and lastly the finer ingredients. There is no such assortment visible, however, in the normal 'till,' but large and small stones are scattered pretty equally through the clay, which, moreover, is quite unstratified."
This fact alone disposes of the iceberg theory as an explanation of the Drift.
Again: whenever deposits are dropped in the sea, they fall uniformly and cover the surface below with a regular sheet, conforming to the inequalities of the ground, no thicker in one place than another. But in the Drift this is not the case. The deposit is thicker in the valleys and thinner on the hills, sometimes absent altogether on the higher elevations.
"The true bowlder-clay is spread out over the region under consideration as a somewhat widely extended and uniform sheet, yet it may be said to fill up all small valleys and depressions, and to be thin or absent on ridges or rising grounds."
That is to say, it fell as a snow-storm falls, driven by high winds; or as a semi-fluid mass might be supposed to fall, draining down from the elevations and filling up the hollows.
Again: the same difficulty presents itself which we found in the case of "the waves of transplantation." Where did the material of the Drift come from? On what sea-shore, in what river-beds, was this incalculable mass of clay, gravel, and stones found?
[1. "The Great Ice Age," p. 72.
2. "American Cyclopædia," vol. vi, p. 112.]
Again: if we suppose the supply to have existed on the Arctic coasts, the question comes,
Would the icebergs have carried it over the face of the continents?
Mr. Croll has shown very clearly that the icebergs nowadays usually sail down into the oceans without a scrap of débris of any kind upon them.
Again: how could the icebergs have made the continuous scratchings or striæ, found under the Drift nearly all over the continents of Europe and America? Why, say the advocates of this theory, the icebergs press upon the bottom of the sea, and with the stones adhering to their base they make those striæ.
But two things are necessary to this: First, that there should be a force great enough to drive the berg over the bottom of the sea when it has once grounded. We know of no such force. On the contrary, we do know that wherever a berg grounds it stays until it rocks itself to pieces or melts away. But, suppose there was such a propelling force, then it is evident that whenever the iceberg floated clear of the bottom it would cease to make the strive, and would resume them only when it nearly stranded again. That is to say, when the water was deep enough for the berg to float clear of the bottom of the sea, there could be no striæ; when the water was too shallow, the berg would not float at all, and there would be no striæ. The berg would mark the rocks only where it neither floated clear nor stranded. Hence we would find striæ only at a certain elevation, while the rocks below or above that level would be free from them. But this is not the case with the drift-markings. They pass over mountains and down into the deepest valleys; they are
[1. "Climate and Time," p. 282.]
universal within very large areas; they cover the face of continents and disappear under the waves of the sea.
It is simply impossible that the Drift was caused by icebergs. I repeat, when they floated clear of the rocks, of course they would not mark them; when the water was too shallow to permit them to float at all, and so move onward, of course they could not mark them. The striations would occur only when the water was; just deep enough to float the berg, and not deep enough to raise the berg clear of the rocks; and but a small part of the bottom of the sea could fulfill these conditions.
Moreover, when the waters were six thousand feet deep in New England, and four thousand feet deep in Scotland, and over the tops of the Rocky Mountains, where was the rest of the world, and the life it contained?