Occurrence of Stones on the surface of Glaciers. 173 



reported have so much the more interest in my eyes, as they 

 agree, in a very satisfactory manner, with the observations 

 made during the last year upon the glacier of the Aar, and 

 reported in the new work of M. Agassiz upon the glaciers.* 



It is easy to perceive, in reading over the most recent pub- 

 lications upon the glaciers, that the opposition between the 

 different theories tends constantly to disappear, in as much as 

 facts take the place of speculation ; for there can be no doubt 

 that the different physical laws, as gravitation, plasticity, 

 expansibility, which have been insisted upon in a too exclu- 

 sive manner by authors of different theories, have each some 

 influence upon the mechanism of the glaciers. 



Nevertheless, there are some points which are still viewed 

 very differently in the different systems, and especially those 

 concerning the structure of the glaciers. M. Agassiz, as you 

 know, makes a distinction between the strata, which are the 

 result of the annual deposit of snow, and the blue bands. The 

 first, according to him, extend entirely through tlie glacier, 

 and very often sand and fragments of rock are found between 

 them. These are the so-called dirt bands of Mr Forbes. The 

 blue bands, which are so frequent in the middle and lower 

 part of the glacier, have not the same importance, being 

 merely the result of superficial fissures parallel to each other, 

 and usually agreeing with the general direction of the strata. 



Mr Forbes, on the contrary, looks upon them as the con- 

 sequence of the unequal rapidity of the movement in the dif- 

 ferent parts of the glacier. He mentions, in his 12th letter, 

 a fact concerning the glacier of La Brenva, which, according 

 to him, would be an ^^ experimentum crucis^^ in favour of his 

 theory. The glacier of La Brenva, in the valley of Cha- 

 mouni, like most of the glaciers of the Alps, has increased 

 very rapidly of late years. Mr Forbes has noticed that the 

 blue bands are the most striking in the most advanced por- 

 tion of the glacier, at the part below the promontory marked 

 B in his figure (p. 100 of volume xlii. of this Journal). Ac- 

 cording to him, this is owing to a " longitudinal tearing," and 



* Systeme Glaciare, ou Kecherches surles Glaciers actuels, &c., with an Atlas, 

 1847 ; not to be confounded with his former work, entitled. Etudes sur les 

 Glaciers, 1840. 



