Page:Popular Science Monthly Volume 9.djvu/201

Rh was of the greatest extent and severity. Ten cycles back—about 210,000 years ago—one of the periods of maximum eccentricity had just commenced, the highest since four times that number of years. The perigee, or nearest approach to the sun, happened then as now, a few days after the winter solstice of our half of the world. It was the great summer of the northern hemisphere. But over the southern hemisphere at this time, almost if not quite to the tropics, extended one vast sheet of ice. It reached far into Brazil, it covered Southern Africa, and lapped over on Australia. The marks are all there, scored on the solid rocks, to show how it crept up the southern slopes of the hills, and how far it pushed its icy arms. In South America at least there is ample proof that the great glacier spanned the southern ocean to reach it; for the furrows on the rock-beds of Patagonia are from the pole toward the equator, whereas in any other case they would have been from the mountains to the sea. With such a state of things at the southern end of the world, with probably miles in depth of ice and sea in its higher latitudes, there could have been but little water left for the opposite northern regions. What is called the Atlantic-cable plateau, between Newfoundland and Ireland, was very possibly the north shore of the Atlantic Ocean; and probably no considerable bodies of water existed anywhere north of that parallel. The present continents were all mountain tablelands, far from the vicinity of evaporating surfaces. Like all such elevated regions not exposed to specially moist winds, they were doubtless dry and arid deserts. However warm may have been the climate of the north temperate and arctic zones during this their great summer, their great elevation and the want of any kind of water-supply must have made them barren of all forms of animal or vegetable life. Consequently there would be, as is notably the case, but few if any traces of this part of the great season left in the geological records, at least above the present seas.

Five thousand years pass, and the perigee has advanced to meet the vernal equinox. The spring season is now the shortest of all; but, as the autumnal is correspondingly lengthened, the average climate is about that of the present time. But it is the season of the great thaw—the breaking-up time—of the southern hemisphere, and the waters are returning to fill the northern ocean-beds. Imperceptibly a permanent white cap begins to fasten itself to the heights of the boreal zone, to extend its outline, and to increase its depth. Slowly the lands are being submerged and the oceans broaden out, till there comes a time when land and water are equalized in the two hemispheres, and the climates are substantially alike.

Another 5,000 years pass, and the perigee now coincides with the summer solstice of the northern hemisphere. This is the position there of greatest cold: the winters are twenty-eight days longer than the summers; and the extra days are in great part those