Page:Popular Science Monthly Volume 4.djvu/631

 Alpine regions, were the result of the action of former glaciers descending from the Alps and reaching even the upper portions of the Jura. This theory Agassiz deemed improbable, till, having visited Charpentier and investigated the phenomena, he not only became convinced of the correctness of Charpentier's views, but deduced from these and other phenomena a theory which, at the time (1837), was startlingly novel. It was that, previous to the elevation of the Alps, the globe experienced a very great reduction of temperature, and that the appearance of those mountains found the surface of the globe, from the north-pole to the Mediterranean Sea, covered with an immense sheet of ice. An elevation of temperature, consequent upon the upheaval of the Alps, caused this ice slowly to disappear, remaining longest in the valleys, where it gradually retreated to its present limits, leaving behind it, as a record, the peculiar phenomena which have attracted the attention of so many observers.

Of course a theory so novel at once raised a storm of opposition, and it became necessary for Agassiz, if he would prove the correctness of his views, to make the most careful and thorough investigations on living glaciers. For this purpose Agassiz, in company with Desor and several others, made visits in 1838 and 1839 to the glaciers of Mont Blanc and the Bernese Oberland, and in 1840 established himself for the summer on the glacier of the Aar. That year he published his "Études sur les Glaciers," giving the results of his investigations up to that time. He also visited England, Scotland, and Ireland, and studied the evidences of ice-action in those countries.

But his labors were not finished. Doubting the sufficiency of the theory of De Saussure—that the cause of the motion of the glacier depends upon gravity—and inclined to accept the dilatation theory of Schenchzer, it became necessary for him to examine with care the structure, form, distribution, and rate of motion of the glacier. Thus it was that, in 1841, he began a second series of observations for the purpose of determining these points. He chose, for the theatre of his investigations, the glacier of the Aar, which, by its extent and accessibility, promised the most favorable results. In 1845 he had completed his work, and in 1847 appeared his "Système Glaciaire," which embodied the final results of his researches upon the structure of glaciers, and their effects upon the soil. The results at which he arrived may be summarized as follows: The glacier is a mass of ice reclining on the side of a mountain-ridge, or inclosed in a mountain-valley; it is always descending, and, while wasting away from heat at its lower extremity, is continually augmented at its source. The primary material of glacier-ice is the snow which falls in the high regions of the mountain. The yearly addition of snow in the higher cold regions gradually forces the snow down the valley; here, subject to alternate thawing and freezing, it undergoes a second crystallization into what is called névé snow, and still farther down, under increased pressure,