3o8 POPULAR SCIENCE MONTHLY 



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 de- 

 posited 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 com- 

 bined 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 re- 

 ceived 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 



