NOTES TO BOOK XVIII. 683 



Geologists differ as to the question whether these 

 changes in the inhabitants of the globe were made by 

 determinate steps or by insensible gradations. M. Agassiz 

 has been led to the conviction that the organized popula- 

 tion of the globe was renewed in the interval of each prin- 

 cipal member of its formations (Brit, Assoc. Report, 1842, 

 p. 83). Mr. Lyell, on the other hand, conceives that the 

 change in the collection of organized beings was gradual, 

 and has proposed on this subject an hypothesis which I 

 shall hereafter consider. 



(EA.) p. 602. The effects of glaciers mentioned in 

 the text are obvious ; but the mechanism of these bodies, 

 the mechanical cause of their motions, was an unsolved 

 problem till within a very few years. That they slide as 

 rigid masses; that they advance by the expansion of their 

 mass; that they advance as a collection of rigid fragments; 

 were doctrines which were held by eminent physicists ; 

 though a very slight attention to the subject shows these 

 opinions to be untenable. In Professor J. Forbes's theory 

 on the subject (published in his Travels through the Alps, 

 1843,) we find a solution of the problem, so simple and 

 yet so exact as to produce the most entire conviction. In 

 this theory, the ice of a glacier is, on a great scale, sup- 

 posed to be a plastic or viscous mass, though small por- 

 tions of it are sensibly rigid. It advances down the 

 slope of the valley in which it lies as a plastic mass 

 would do, accommodating itself to the varying shape and 

 size of its bed, and showing by its crevasses its mixed 

 character between fluid and rigid. It shows this charac- 

 ter still more curiously by a ribboned structure on a small 

 scale, which is common in the solid ice of the glacier. 

 The planes of these ribbons are, for the most part, at 



