Page:Popular Science Monthly Volume 4.djvu/671

Rh Erratics.—These are of all shapes and sizes, occasionally reaching colossal proportions, and containing many hundred feet. Some are rounded, others are angular, and not a few exhibit marks of scarification. They may rest on base-rock, and, if carefully poised, may be made to oscillate by the form of the land, or these large blocks may appear on the till, angular débris, and hills of gravel. As a general rule, they prove to have been carried from higher to lower levels in Scotland, though many exceptions are recorded. There is one at the height of 1,020 feet on the Pentland Hills, which may have traveled westerly as much as eighty miles. It probably passed from one mountain across a wide valley before attaining its final resting-place. This is not so striking as the blocks lying nearly over the recently-completed Hoosic Tunnel, in Western Massachusetts, one of which weighs 510 tons, and has been transported from Oak Hill across a valley 1,300 feet deep. It has hundreds of lusty comrades, scattered in a southeasterly course for thirty miles.

Sometimes a large block is revealed by the washing away of the till around it. Those on the surface of gravel may have been carried by floating ice. To such blocks it is not easy to assign limits of the distance traveled, since icebergs may float for thousands of miles without melting.

—The question of the cause of the glacial cold has been discussed warmly for a long time. The opinion seems to be gaining ground that purely geological causes are not sufficient to account for the magnitude of the glacial distribution. The precession of the equinoxes, changing the times of the seasons, and the eccentricity of the earth's path around the sun, lengthening the winters and increasing precipitation of moisture, when combined with certain changes in the courses of ocean-currents, and some elevation of land in the north, may have together been instrumental in bringing around a period of intense cold. If it be possible to use the orbital changes as a guide to a chronological date for this term of cold, we can say it began about 240,000 years since, and continued for 160,000 years, terminating 80,000 years before 1260. The cold would have culminated about 30,000 years after its beginning.

Granting such figures, we can understand that the glacial must have been the dark age in the earth's history—a terrible blight upon the flourishing faunas and floras existing in tertiary times in northern latitudes. The presence of warm temperate plants in Greenland has always excited interest, even to the proposal of very wild theories to account for the genial climate there of preglacial days. It may be that the American Sequoia traveled across the bridge anciently connecting Greenland with Iceland and Scotland, and that the renowned cedars of Lebanon are the cousins of their famed relations in California; but the connection has been severed by the ruthless ice-flow, and is not likely to be reestablished, unless our sun shall carry his