238 Professor Forbes' s Sixth Letter on Glaciers. 



sides and front, and nearly the same ground-plan, all bespeak 

 a similarity in the circumstances of motion. I may add, that 

 in some experiments which I made some years ago upon the 

 flowing of melted iron in narrow channels, and upon small 

 slopes, with a view to illustrate some phenomena of lava 

 streams, before I had commenced a particular study of gla- 

 ciers, I arrived at similar results, and obtained the same con- 

 vexity of surface which is produced in the plaster models be- 

 fore cited. 



It is very interesting to observe how many intelligent per- 

 sons have been struck with the similarity between glaciers and 

 lava streams, without, however, pushing the parallel beyond 

 a general resemblance. M. Elie de Beaumont, we have seen, 

 speaks of the moraines of volcanoes ; but in various parts of 

 his writings, as well as those of his colleague, M. Dufrenoy, we 

 find the mention of glaciers as continually suggested to his 

 mind w-hen surveying the wastes of Etna and Vesuvius. One 

 of these passages is the following : *' L'ecorce superieure d'une 

 coulee separee de l'ecorce inferieure et du sol sousjacent par 

 une certaine epaisseur de lave liquide ou du moins visqueuse, 

 se trouve dans un etat comparable ^ celui d'un glacier, qui, 

 ne pouvant adherer au sol sousjacent a cause de la fusion 

 continuelle de sa couche inferieure, se trouve contraint h. 

 glisser ;"* shewing that the author then adopted the theory 

 of Saussure (since ably defended by Mr Hopkins), in which the 

 fusion of the ice by the heat of the earth, might be said, in 

 some sense, to jfoat down the superincumbent solid ; an opinion 

 best controverted by the fact which M. E. de Beaumont has 

 since clearly brought into notice, that under existing circum- 

 stances such fusion is perfectly insignificant.f 



The writer of a popular Italian guide-book, Mrs Starke, is 

 perhaps one of the first who indicated the striking general 

 resemblance of a stream of lava to a glacier. She describes 

 the former (which she saw during a small eruption of Vesuvius) 

 as " rolling, wave after wave, slowly down the mountain with 



* Recherches sur I'Etna, p. 177. 



t Annales des Sciences G^ologiques par Riviere, 



