Page:Popular Science Monthly Volume 15.djvu/853



NCIENT moraines, striations denoting the action of vanished glaciers, the lost rocks, clay-beds evidently of glacial origin—all these are evidences testifying that at some period not very remote, as we count geological periods, the whole northern hemisphere down to the southern limit of 40° was submerged and covered with vast glaciers and ice-floes. There is not a living scientist of any eminence who questions the truth of this assertion.

When we come to study the cause of these phenomena, we find many perplexing and contradictory theories in the field. A favorite one is that of vertical elevation. But it seems impossible to admit that the circle inclosed within the parallel of 40°—some 7,000 miles in diameter—could have been elevated to such a height as to produce this remarkable result. This would be a supposition hard to reconcile with the present proportion of land and water on the surface of the globe and with the phenomena of terrestrial contraction and gravitation. Moreover, it seems evident that an extensive submergence was one of the features of the glacial age. The frozen archipelago called Greenland is a fair picture of what northern America and Europe must have been at that time; and, of course, this precludes the idea of elevation.

If it were not true that submergence and a great lowering of temperature occurred simultaneously, we might imagine that a sort of undulation in the earth's crust, alternately raising and lowering each portion of it, could have caused this result. However, there is no evidence that such an undulatory motion has ever occurred, and we can not conceive of any force likely to produce it.

For the past fifty years, the relation of the inclination of the earth's axis to the plane of the ecliptic and its varying angle with the line of the apsides, has been the subject of careful study, from the impression that herein was a key to the mystery. Astronomical and geological works abound with hints and suggestions of this sort, but I have never yet seen any satisfactory analysis of the question. St. Pierre, Adhemar, and others have presented theories which seem strangely illogical in many of their conclusions. I have been striving to analyze the question, and will present a few of my conclusions.

The orbit of the earth is an ellipse, of which the sun occupies one of the foci. The major axis, or line of the apsides, is the longest diameter of the ellipse, passing through its two foci and through the points of perihelion and aphelion. This line is not fixed with respect