Mr. J. G. Jeffreys on the Descent of Glaciers. 123 



gestion which may be improved by some abler pen. My atten- 

 tion has been somewhat directed to the question in consequence 

 of my having resided during a considerable part of last year in 

 Switzerland, the land of glaciers, where I had the good fortune 

 of making the acquaintance of that veteran geologist, M. do 

 Charpentier. 



These theories appear to be five in number. 



1st. That of De Saussure, who supposed that glaciers de- 

 scended solely by their own weight ; and this has been called 

 the " Gravitation '^ theory. His observations have justly had 

 the credit of being most accurate; and they extended over a 

 great number of years, and were conducted with much labour 

 and at considerable expense. They will be found in his ' Voyage 

 dans les Alpes,' published in 1779. 



2nd. That of De Charpentier, and adopted by Agassiz, which 

 supposed that the phsenomenon was caused by the surface of the 

 glacier being thawed during the day ; that the water thus pro- 

 duced percolated the porous material ; and that upon conge- 

 lation taking place at night the whole structure expanded in 

 every direction, naturally occasioning or accelerating a down- 

 ward movement in the direction of the slope. This is called the 

 " Dilatation '^ theory. It was first propounded by Charpentier 

 at a Meeting of the Helvetic Society held at Lucerne in 1834 ; 

 and it appeared in the 8th volume of the ' Annales des Mines.' 

 It was afterwards (in 1841) published by him in a more elabo- 

 rate form under the title of ' Essai sur les Glaciers.^ Agassiz' 

 memoir was read at a Meeting of the same Society held at Neu- 

 chatel in 1837; and it was, I believe, published in their Transac- 

 tions. In his work entitled ^ Etudes sur les Glaciers,' and pub- 

 lished in 1840, this theory is further developed. 



3rd. That of Professor James Forbes, which attributed the 

 phaenomenon to the viscous or plastic nature of the glacier, 

 causing the descent suis viribus. This has been called in Ger- 

 many the "Pech" theory, and was published in 1843 by our 

 distinguished countryman in his work on Glaciers. 



4th. That of Mr. Hopkins of Cambridge (mentioned by Pro- 

 fessor Forbes), who referred the motion of a glacier to the dis- 

 solution of the ice in contact with the rock; although Char- 

 pentier had previously instanced some striking facts to prove 

 that the glacier bed never thaws. 



And 5th. The theory lately ofi^ered by the Rev. Henry Moseley 

 and published in the 7th volume of the Royal Society's Proceed- 

 ings, which (assimilating a glacier to a sheet of lead) supposed 

 that the phaenomenon was owing to the heat of the sun, and 

 consequently to an alternate expansion and contraction of the 

 material. 



