Page:Popular Science Monthly Volume 68.djvu/312

308 when the temperature had risen, vast landslides—avalanches of mud filled with detritus—would be propelled for many miles over these frozen lakes, and when the ice disappeared, the same would be deposited in the form of a promiscuous aggregate of sand, gravels, pebbles and boulders.

In 1840 an immense stride in the study of drift deposits was made through the publication of Louis Agassiz's 'Etude sur les Glaciers,' a work comprising the results of his own study and observation combined with those of Jan de Charpentier, E. T. Venetz and F. G. Hugi. The work was published in both French and German, and brought to a focus, as it were, the scattered rays by which the obscure path of the glacial geologist had been heretofore illuminated. It was Agassiz's idea, as is well known, that at a period geologically very recent, the entire hemisphere north of the thirty-fifth and thirty-sixth parallels had been covered by a sheet of ice possessing all the characteristics of existing glaciers in the Swiss Alps. Through this agency he would account for the loose beds of sand and gravel, the boulder clays, erratics, and all the numerous phenomena within the region described, which had been heretofore variously ascribed to the Noachian deluge, the bursting of dams, the sudden melting of a polar ice-cap, or even to cometary collisions with the earth. These ideas were favorably received by the majority of workers, though there was, naturally, a highly commendable feeling of caution against their too hasty acceptation. As a reviewer in the American Journal of Science expressed it:

These very original and ingenious speculations of Professor Agassiz must he held for the present to be under trial. They have been deduced from the limited number of facts observed by himself and others and skillfully generalized, but they can not be considered as fully established until they have been brought to the test of observation in different parts of the world and under a great variety of circumstances.

The effect of this publication, however, soon made itself apparent in the current literature. Thus, in 1843 Professor Charles Dewey, writing on the striae and furrows on the polished rocks of western New York, argued that, while the boulders of the drift indicated that a mighty current had swept from north to south, the polishing and grooving might be due to glaciers. 'Glaciers or icebergs and the strong currents of water—a union of two powerful causes—probably offer the least objectionable solution of those wonderful changes,' he wrote. Though thus disposed to accept in part Agassiz's conclusions, Dewey yet failed to realize their full possibilities.

He could not conceive how it was possible for a glacier to transport sandstone boulders from the shore of Lake Ontario to the higher level of the hills to the southward. Boulders of graywacke removed from the hills in the adjoining part of the state of New York and scattered throughout the Housatonic Valley furnished a like difficulty, since