76 NINTH LETTER ON GLACIERS. [1845. 



cases (should the fact really appear to be general, as is not un- 

 ikely, provided the lower station be always chosen low enough) 

 seems to be, that the main glacier resists the interference of its 

 tributary with its course, and consequently represses its stream, 

 causing a heaping up in front, such as mere friction on a low 

 inclination alone produces, and is thus in conformity with the 

 viscous theory. In the other case, that of the free glacier of 

 the second order, the difference of velocity at the upper and 

 lower station (one-seventh part only) is not more than the dif- 

 ference of slope (15 and 25) will readily explain. 



VI. Movements of Bas-Neves or Snow-beds. One observa- 

 tion remains which completes my analysis of the measures 

 of M. Agassiz's coadjutors. It is one of considerable interest, 

 and I believe is new. It is the establishment of the fact that 

 the highly inclined beds of old snow, formed by avalanches, 

 which lie unmelted in the ravines, without assuming any external 

 trace of glacial structure, have a proper motion of their own. 

 This, though to me not unexpected, is very interesting ; for the 

 most attached advocate of either the dilatation or the sliding 

 theory, will hardly maintain, on the one hand, that the congela- 

 tion of soft snow could act here as a propelling force, or on the 

 other, that the motion can take place without acceleration in 

 the totality of a mass, inclined (in this case) at an angle even 

 of 43, over the bed on which it rests : especially since the 

 actual movement under this enormous inclination was only 7 

 millimetres, or three-tenths of an inch, per day;* or one-thirtieth 

 of that of the great glacier under an inclination of but a few 

 degrees. The velocity increased towards the lower extremity, 

 as in the free glacier of the second order. On the plastic theory, 

 this evidently presents the extreme case of a body, approaching 

 in its nature to a soft heavy powder slightly moistened, which 

 gives way by the yielding of its parts, and so far resembles a 

 fluid (as a bank of earth, slightly glutinous, rather than sand 

 does) ; and the slowness of movement is in conformity with the 

 imperfectness of the fluid pressure, and with the fact already 



* Comptes Rendus, p. 1305, line 32 ; and p 1306. 



