Page:Popular Science Monthly Volume 4.djvu/633

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The sides as well as the bottom of the glacier are studded with bowlders, pebbles, and sand, forming a gigantic rasp. As the glacier moves forward, this rasp grinds, furrows, and polishes the rocks over which it moves, the furrows trending in the direction in which the glacier moves. These furrows and polished surfaces, which are often observed on rocks remote from any living glaciers, are the record of the former existence of glaciers in such places. When the ground is uneven, the eminences being small, and the hollows too deep and wide to be bridged over by the glacier, the ice-rasp rounds and polishes these knolls, forming those rounded elevations which have received the name of roches moutonnées. In consequence of the rocky walls above the sides of the glacier becoming warmed by the sun, the ice is melted near them, and hence the glacier becomes convex. Into these troughs the débris from the walls fall and form long lines of bowlders, pebbles, and sand, which are called lateral moraines. When two glaciers flow together, the two lateral moraines on the adjoining sides of each unite and form what is called a medial moraine. A third form, the terminal moraine, is the accumulation of sand and rocks which the glacier pushes before in its progress down the valley. In consequence of the increased rate of progression of the centre of the glacier, these terminal moraines assume a semicircular form, which, when the glacier retreats, consequent upon an excess of liquefaction over the snow-supply, leaves a crescentic wall across the valley, usually cut in two by the river flowing from the glacier. The erratic blocks which are found over most of the globe, accompanying scratched and polished rock-surfaces, are simply the bowlders of the surface of the glacier left on or near the spot where they stood when the glacier disappears.

In the fall of 1846 Agassiz sailed for the United States, on a mission from the Prussian Government. The warm reception which greeted him here, and especially the rare field for scientific research which this country afforded, determined him, the next year, to make America permanently his home. The professorship of Natural History in the Lawrence Scientific School at Harvard College being offered him that same year, he accepted it, and held the position till his death, with the exception of two years when he occupied the chair of Natural History in the University of South Carolina, at Charleston. In 1848, in connection with H. E. Strickland, he began the publication of a "Bibliographia Zoologiæ et Geologiæ." This work, which comprises a list of all the periodicals devoted to zoology and geology, and an alphabetical list of authors and their works in the same departments, was completed in four volumes, the fourth being published in 1854.

Agassiz's studies on the glaciers of Switzerland led him to expect to find in the United States many traces of former ice-action. Nor was he disappointed. He explored the country from the Atlantic to the Rocky Mountains, from the great lakes to the Gulf of Mexico, and everywhere, north of the thirty-fifth parallel of latitude, found evi-