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of science, he had undoubtedly made the first great steps in giving 

 currency to the glacial theories of geology — theories which, though 

 viewed at that time with much distrust, had since, with such modi- 

 fications as enlarged knowledge and sober judgment had imposed 

 on them, been universally recognized by geologists. 



For his unflinching advocacy of the glacial theory in its broad out- 

 lines, geologists had been unquestionably much indebted to M. 

 Agassiz ; but in his physical theories respecting glacial phenomena, 

 he had not shown that caution or acquaintance with physical science 

 which the subject demanded. With respect to the motion of glaciers, 

 it may be sufficient to state that he regarded it as due to the infil- 

 tration and subsequent freezing of water within the glacier, and a 

 consequent expansion of its mass, by which the glacier in general, 

 and especially those portions near its lower extremity, were urged 

 forwards in the direction in which the bed of the glacier descended. 

 Few persons ever received this theory, and it is no longer considered 

 as deserving of serious attention. 



A few years later Professor Forbes commenced his researches 

 among the Alpine glaciers. His ' Travels through the Alps ' 

 was published in 1843, and contained a greater amount of well- 

 arranged information respecting glacial phenomena than perhaps all 

 other works together on that subject. But in this lecture, Mr. Hop- 

 kins remarked, he professed to deal with theories, and not with de- 

 scriptive details. M. Agassiz's second work on glaciers, his Systeme 

 Glaciere, also appeared in 1847. Professor Forbes introduced a new 

 view of the motion of glaciers, which he attributes to a certain fa- 

 cility with which he supposes glacial ice to be capable of changing 

 its form under the pressures to which it is subjected, in a manner 

 similar to that in which a viscous mass would change its form under 

 the same circumstances. Hence it was called the viscous theory. 

 It was founded on the fact (distinctly ascertained, Mr. Hopkins be- 

 lieved, the same year, both by Agassiz on the glacier of the Aar, 

 and by Prof. Forbes on the Mer de Glace) that the central portions 

 of a glacier moved considerably faster than its lateral portions, as a 

 viscous mass would move along a trough inclined at a small angle 

 to the horizon ; and, moreover, it was obvious that the general mass 

 of a glacier did so change its form as to accommodate itself to the 

 changing dimensions of the valley down which it moved. 



On the other hand, it was contended that a substance so hard and 

 brittle as glacial ice could not be said to have the property of visco- 

 sity, and that the different velocities of the central and lateral por- 

 tions of a glacier, and the changes of form which the general mass 

 might undergo, were more attributable to the formation of crevasses 

 and to discontinuous ruptures of the mass, than to any continuous 

 change of form in each infinitesimal portion of it, like that which 

 takes place in a mass which can be properly termed viscous. That 

 this view was partly true was obvious, since ruptures and crevasses 

 were actually formed by the unequal motions of different portions of 

 the mass. Those who maintained this latter view held that the 

 glacier moved by actually sliding over its bed ; while those who sup- 



