Page:Popular Science Monthly Volume 44.djvu/634

618 than or as fast as it accumulated; if the mean temperature is low, the snowfall may be light and yet the glaciers accumulate, as the heat would be insufficient to melt that which did fall; but if the mean temperature is so high as to prevent the accumulation of snow, or so low as to prevent the formation of aqueous vapor, there can be no glaciers formed, the last conclusion being subject to the qualification that a vapor-laden atmosphere may be carried by prevailing winds from a warm climate to a cold one, and the vapor there condensed and precipitated.

One hypothesis is that the whole solar system passes at times through stretches of space of different temperatures, and that the Glacial period coincides with a time when the solar system was passing through a low-temperature area; another is that the heat of the sun varies, such variations being the result mainly of contact with meteorites, as the impact of bodies generates heat, the idea being that the Glacial epoch coincides with a time when



few meteorites were colliding with the sun, the heat emanating therefrom being therefore decreased. These two hypotheses agree in one particular—they can neither be proved nor disproved, consequently their only value is speculative.

Again, it is supposed that the earth's axis has shifted—that during the Glacial period the north pole was in Greenland. This seems to be negatived by the slight observed shifting of the pole, and the fact that Tertiary fossil flora, immediately preceding the Glacial period, of both Greenland and the present arctic regions, indicate a temperate climate.

Adhémar the astronomer advanced the hypothesis, also advocated by the late Dr. James Croll and Prof. James Geikie, that, as the earth's orbit is elliptical and as the sun is not central to this orbit but some three million miles nearer one end than the other, this fact in connection with the precession of the equinoxes may explain the Glacial epoch; it being held that during that