Page:Popular Science Monthly Volume 8.djvu/728

708 vast ice weights from one end of the world to the other? On all the mountains of New England there are sea-lines at elevations of 2,000 and 3,000 feet, and Lyell himself has recorded the facts. When the ocean was that deep over Boston, there were no continents in the northern hemisphere. Undoubtedly the height and direction of mountain-ranges, the trending of sea-shores, and the course of ocean currents, have much to do with local climates. But, instead of the relative quantity or location of land and sea having any agency in producing the glacial periods, it is these periods which produce the land and the sea.

So much for the causes and conditions which pertain to the geography of the present and the future. When, now, we turn back a few of the leaves which tell of the past condition of our planet, we immediately see that the same causes have been at work in recent geological times on a much more extensive scale—in fact, that they have been the chief agents in composing and modifying the present surface of the earth outside of the tropics. Over all the northern portions of Europe, Asia, and North America, are found the unmistakable evidences of extensive and recent ice-work. Bowlders of every size, some worn and some angular, are scattered in immense quantities over all the country, on the hills, on the plains, in places where the only possible explanation is that they were lifted up, carried, and dropped, just where they are found; and the great iceberg was the carrier. The face of the rock-beds, wherever brought to view, in the valleys or on the mountains, is almost always found to be ground or polished, and, over that, grooved and furrowed with nearly parallel scratches. The Alpine glaciers are doing exactly the same work today. Erratic blocks of foreign origin, and sometimes of enormous dimensions, are frequently found perched on the very tops of hills, or stranded high up the mountain-sides; and the quarries from which they came are invariably found to the northward, sometimes fifty or even a hundred miles. It is argued that nothing but polar glaciers could thus have moved them in uniformly meridional lines. The scrapings of grounding ice-floes, the marks of ancient sea-shores, and marine relics and shells, are found at elevations of several thousand feet above the present ocean-level. There is no escaping the conclusion that the northern continents have been, in not remote ages, deeply submerged beneath an ice-laden sea; and that the entire polar and north temperate regions, extending in some places south of the fortieth parallel of latitude, have been capped with one massive covering of ice of great thickness. Precisely the same evidences are found in South America, and, according to Agassiz, even much nearer the equator than in North America. We have again to search our astronomy for causes many times more powerful than any thing we have yet found, for differences of polar temperatures.

The earth is made to revolve in an orbit drawn out of the circular