Mr. Hopkins on Glacial Theories. 309 



position. He and those who were associated with him, or had pre- 

 ceded him, brought forward incontestible evidence of the former ex- 

 tension of glaciers in the Alps, and of their efficiency in the trans- 

 port of enormous angular fragments of rock from their original sites 

 to other localities, not only in the same Alpine valley, but even on 

 the flanks of the Jura on the opposite side of the great central valley 

 of Switzerland. These were the great facts which bore upon specu- 

 lative geology. Glaciers were still engaged In the work of trans- 

 port, and they had been so on a much larger scale at some former 

 epoch ; the masses of Ice of which they were composed had been 

 then of much larger dimensions, and consequently the mean climate 

 of Western Europe must have been at that period considerably lower 

 than at present. No one was so active as M. Agassiz in pressing 

 these facts on the notice of geologists, or insisted more strongly on 

 their geological Importance ; and by the energy of his own character, 

 his great reputation, and extensive personal acquaintance with men 

 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 v/ith 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 es])ecially those portions near Its lower extremity, were urged 

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

 Few persons ever received tliis 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 phsenomenathan perhajis all 

 other works together on that sul)ject. But in this lecture, Mr. Hop- 

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

 scriptive details. AI. Agassiz's second work on glaciers, his )Sy5<p»ie 

 Glacilrc, 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 ciuinge its form under 

 the same circumstances. Hence It was called the viscous theoi'i/. 

 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 j)ortions 

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



