On Drift. 3 



as much propriety as those which were condemned as extravagant 

 in the agency of water. It is manifest, however, that these ex- 

 treme views are gradually but surely giving way in favour of those 

 more moderate, and, as I believe, sounder views to which we ap- 

 pear to be rapidly converging. 



The glacial theories of transport of erratic blocks made rapid 

 progress among us soon after their first announcement, although 

 received by many geologists in the first instance with great reser- 

 vation. One reason of this reserve was, I imagine, the difficulty 

 of conceiving a change of temperature such as required by those 

 theories, exactly opposite to the changes which the geologist had 

 ever contemplated — a change after the glacial epoch from a lower 

 to a higher temperature. Increasing knowledge, however, of the 

 causes affecting climatal conditions have enabled us to remove in 

 great measure this source of doubt. Another reason for hesitation 

 in accepting these theories was, perhaps, to be found in the incau- 

 tious manner in which their claims were asserted by some of their 

 first advocates, and the unlimited application which was made of 

 them to account for the phenomena of transported materials of all 

 kinds. Whatever truth might belong to the facts adduced in sup- 

 port of these theories, it was clear that much of the reasoning 

 founded upon them was untenable. Overstrained applications, 

 however, of physical theories, are almost the necessary conse- 

 quences of their early reception by minds animated by an ardent 

 zeal for the discovery of new scientific truths ; and perhaps this 

 tendency, in certain stages in the progress of science, may be almost 

 necessary to counteract the hesitation of those whom natural timi- 

 dity, or possibly severer mental discipline and more accurate phy- 

 sical knowledge, may have rendered too slow in the recognition 

 of the germs of new theories, while supported, perhaps, by little 

 of demonstrative evidence. All doubts, however, as to these 

 theories being founded in truth, whether there might be more or 

 less of exaggeration in the advocacy of them, soon gave way before 

 the evidence collected by northern voyagers respecting the action 

 of icebergs, and that supplied by Agassiz, Charpentier, Forbes, 

 and others, who devoted themselves to the study of the constitu- 

 tion and motion of glaciers. Almost all geologists, I conceive, 

 now agree in the opinion that both floating and terrestrial ice have 

 played their part to a greater or less extent in the transport of 

 erratic blocks. 



The theories of Agassiz and Charpentier as to the causes of 

 glacier motion have been refuted by the exact admeasurements 

 made not only by Professor Forbes, but by those of Agassiz him- 

 self; and the speculative views of the latter philosopher on the 

 former extension of glaciers over the surface of a large portion of 

 the northern hemisphere are no longer received. But, gentlemen, 

 geologists would be ungrateful if, while they acknowledge, as we 



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