Page:Popular Science Monthly Volume 5.djvu/391

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the Association, whose report we have before us, held at Atlanta, Georgia, April, 1873, Mr. W. Leconte Stevens, of the Boys' High School of Savannah, delivered an address on "Scientific School Studies," which has in it the ring of the true metal. Scientific education is justly prized, not only for the practical knowledge that it imparts, but also for the discipline it affords to the mind in drawing out and strengthening the perceptive faculties, and inducing clear and accurate habits of thought.

The Great Ice Age and its Relation to the Antiquity of Man. By James Geikie, F. R. S. E., etc. New York: D. Appleton & Co. 8vo., 525 pp. Price $2.50.

There are many persons who would listen with an air of scornful incredulity to the statement that the hills and valleys of New York and New England, which summer now clothes with a mantle of luxuriant verdure, were once a dreary, desolate waste, covered up by a crust of ice many hundred feet thick. Nevertheless, the fact is indisputable; the mountains, the rocks, the configuration of the soil, even the fragmentary stones that lie upon the surface, point silently, eloquently, and immovably, to the fact. Geologists had long noticed, in valleys adjoining mountainous districts, certain long, low ridges, called "sow-backs," running parallel to each other and trending down the valley. They had dug into these ridges and picked out flat, oblong stones, with strange scratches upon their surfaces. They had noticed that, while the mountainsides looked jagged and rugged from below, from above they presented a rounded and undulating outline to the very base of the mountain. It was also noticed that the rocks on the mountain-sides displayed on their undulating or upper surface the same mysterious scratches or striæ that were observed on the stones embedded in the ridges below. All of these signs greatly puzzled the geologists, and various theories were invented to account for them, but their true significance was not dreamed of until the late Prof. Agassiz, from the study of Alpine geology, announced that they were the results of one and the same cause—glacial action; that is, that the whole face of the country was covered to the depth of two or three thousand feet with solid ice, which, in gradually creeping toward the ocean to shed its bergs, had worn the mountain-sides into waves; broken, scratched, and transported the rock to distant points, and furrowed up the soil of the valleys through which it continued to crawl seaward. Unmistakable evidence existed that this arctic condition of climate prevailed all over Europe, Asia, and America, northward of 45° north latitude; that is to say, that the vast area comprehended within that circle was once covered with ice as completely as parts of Greenland and the rest of the country immediately around the north-pole is now covered. Of course no life could exist under such conditions, and it was therefore supposed that the advent of man, within that circle at least, must have occurred subsequent to their passing away. It is on this point that Mr. Geikie's book throws a flood of light. He describes the evidences of the glacial condition with admirable skill and clearness, and then proceeds to consider its bearing on the antiquity of man. The earth and stones, or "rubbish," that covers the rocky foundation of the countries comprehended in the circle described, is called the "Drift or Glacial Formation," to indicate that it was deposited thereon by glacial action. The drift is divided into two parts, the upper drift and the lower drift. The lower is, of course, the oldest formation. It is composed of a "tough, stony clay," colored like the rocks about which it lies, and small, fragmentary stones, flattened and scratched. The mass of clay and stones is called "till." The till is not laminated, but pressed down in a confused mass, and its coloring shows it to have been produced by comminution of the rock upon which it lies, while the rock itself corroborates that testimony by being scratched and polished like the stones in the till. Thus the till was formed by the grinding of the ice against the rock. Deeply embedded within the till, occur at intervals deposits of sand and gravel, such as we find at the bottom of lakes and rivers. But how could there be lakes and rivers to deposit sediment, while the whole country was covered by a crust of ice more than a thousand feet thick? This is a question that has long puzzled geologists. But only because