140 Professor Forbes's Fifteenth Letter on Glaciers. 



logies with the structure of glaciers. Again, last August, 

 you communicated some farther reflections and observations 

 by Mr Milward, and you invited me to send any remarks on 

 the same subject which occurred to me, to be communicated, 

 along with Mr Mil ward's papers, to the meeting of the Bri- 

 tish Association. I sent you, on the 11th of August, a let- 

 ter, the chief parts of which I shall embody in this one, but 

 which was not read at Swansea, in consequence of the pres- 

 sure of business in the Geological Section, which barely ad- 

 mitted (as I afterwards heard) of Mr Milward' s papers being 

 read, and consequently no discussion took place. Since that 

 time, Mr Milward, before returning to Malta, was kind 

 enough to place his papers at my disposal, which I then of- 

 fered to Professor Jameson for publication in his Jour- 

 nal, which he accepted, and now allows me to add my re- 

 marks on the same subject, which I address to you, as 

 having been the introducer of Mr Milward' s facts, and as 

 having first desired my opinion with regard to them. 



The phenomena presented by mud-slides on a large scale, 

 are not now studied quite for the first time. About two 

 years ago, I obtained a French work, entitled, " Eecherches 

 Experimentales sur les Glissements spontanes des terrains 

 argileux, par Alexandre Collin, Ingenieur des Ponts et 

 Chaussees." Paris, 1846. 4to. This interesting work, illus- 

 trated by plates, contains no allusion to the subject of gla- 

 ciers ; — the phenomena of mud-slides being considered solely 

 in an engineering point of view. The principal object of the 

 work is to investigate the form of the surface of sliding^ 

 which separates the solid from the moving soil of railway 

 cuttings, embankments, and the like. That subject is not 

 particularly connected with the one before us ; but M. Collin 

 has, at the same time, presented us with excellent and de- 

 tailed sections of land-slips, the mere inspection of which re- 

 calls forcibly the outline of glaciers, and, although evidently 

 unaware of my theory of the latter, his remarks confirm, in 

 a very satisfactory way, several of my anticipations respect- 

 ing the internal movements of viscous bodies. Thus, in the 

 transverse section of an embankment of the Paris and Ver- 

 sailles railway (fiive gauche), Plate III., fig. 1., we find the 



