it~ 
Professor Forbes’s Ninth Letter on Glaciers. 341 
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 congelation of soft snow could act here as a propelling 
force, or on the other, that the motion can take place with- 
out 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 pre- 
sents 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 
doves) ; and the slowness of movement is in conformity with 
the imperfectness of the fluid pressure, and with the fact 
already stated (under Head III. above) that the velocity of 
any glacier is proportional to the completeness of its satura- 
tion with water at the time. The bas-névés, or old avalanches, 
furnish very little water at their lower extremities. 
I have now gone through these observations, made by per- 
sons, it may be assumed, not particularly desirous to find 
results confirming a theory which they have opposed, but 
which it may be hoped they will oppose no longer, when 
their own results speak in language so unequivocal. My 
analysis has been succinct, but complete and impartial. The 
facts are stated as given, without selection or suppression.— 
I am, dear Sir, yours sincerely, 
JAMES D. ForBEs. 
Professor Jameson. 
* Comptes Rendus, p. 1305, line 32; and p. 1396, 
