Page:Popular Science Monthly Volume 9.djvu/202

182 of the briefest sunshine. Besides this, the earth is 10,500,000 miles farther from the sun in winter than in summer. According to the most careful calculations, the temperature of extreme northern regions would be lowered 50°, and the mean annual range would be fully 60° below zero. This in all probability would carry the isothermal line of Labrador, South Greenland, and Iceland (32° Fahr.),down to Charleston and the Gulf of Mexico. The late Prof. Agassiz found ice-marks as far south as this, though it can hardly be supposed that the permanent glacier extended so far. There are, however, abundant signs of the permanent ice-layer all over the State of New York, and both east and west of it. The same distinguished authority was wont to claim in his lectures that all the beautiful north and south lakes of Western New York—the Cayuga, the Seneca, the Canandaigua—were ploughed out of the solid rock and walled around with their clay and gravel hills by advancing and retreating glaciers. The rocky summits of New England are found to be grooved and scored all over their sides and tops with markings always in nearly a north and south direction. They have been traced on Mount Washington to within 300 feet of the highest point. There can be no doubt that at the time we are writing of, about 200,000 years ago, there was one solid ice-stratum of immense thickness—Agassiz said from two to three miles—slowly being pushed from the northward by the power of freezing water, over all of New England and the lake States.

Again the perigee proceeds to meet the autumnal equinox. The winter and the summer seasons have again become equal in length; and the sun is just half its time on the north side of the equator. The great ice-shroud is now being gradually withdrawn. Where it abuts on deep waters, enormous icebergs are broken off and float away to the south, carrying bowlders and soil and whatever it may have picked up in its slow course down to the sea. Where it terminates in shallow waters or on the land, its effect is to produce such an arrangement and diversity of soils and such a peculiar outline of country as no other agency could ever have brought about. So different is the nature and work of the great polar glacier from anything with which we are familiar at the present day, that it has seemed to me to require a few words of more particular description.

As is well known, the glacier is an accumulation of many winters' snows consolidated by pressure into a clear blue ice. In this condition it manifests the peculiar property of viscous bodies—it is in continual slow motion in the direction of least resistance. Whether it is by the expansion produced by the repeated thawing and freezing of water in its interstices, as Agassiz claimed, or whether by the pressure of the mass and glacial regelation, which is the constant freezing together of ice-surfaces in contact, after breaking under unequal pressures, or crashing against obstacles, which is the theory of Prof. Tyndall, or whether by both causes combined, certain it is that large