Mr. J. Ball on the Structure of Glaciers. 491 
possible reply, that, although there are portions of the glacier 
where the frontal resistance must greatly predominate over the 
lateral pressure, this new force does not operate long enough to 
modify sensibly the structure already impressed upon the ice*. 
Even with this qualification it is hard to reconcile the new theory 
with such facts as are presented on the lower part of the glacier 
of La Brenva. Professor Forbes has well described the way in 
which this great glacier, descending through a steep gorge on 
the southern side of Mont Blane into the Allée Blanche, a 
valley running nearly at right angles to its original course, 
encounters first an enormous moraine, which it has itself piled 
up below, then abuts against the rocks on the opposite side of 
the main valley, bridging over the torrent of the Doire, and is 
thus gradually bent round, until the lower part of its course 
follows the direction of that valley. Now if it were possible to 
examine the conditions of internal pressure and tension across 
the glacier opposite to the point where it abuts against the south 
side of the Allée Blanche, and where the process of flexure is 
chiefly accomplished, it cannot be doubted that these conditions 
would be found to be far other than symmetrical as regards the 
centre of the glacier. On the south side, we must admit the 
existence of powerful lateral pressure from the resistance of the 
rocks to the onward movement of the glacier+, while on the 
* This view of the permanence, within certain limits, of the veined 
structure, which I assume to be an essential portion of Mr. Tyndall’s 
theory, is, of course, at variance with the belief expressed by Professor 
Forbes, that that structure is destroyed in ice-cataracts, where the surface 
is completely cut up by deep crevasses, and redeveloped within a short di- 
stance, where the glacier resumes its normal mode of progress, and of which 
he gave one instance in the Mer de Glace, opposite to the promontory of 
Trelaporte. I have already stated reasons for doubting this conclusion ; 
and I may here add, that considering the great difficulty of obtaining 
actual access to the most crevassed parts of the ice, observations on its 
condition are to be received with great caution. Where glacier ice presents 
a moderate slope, the action of running water in warm weather acts 
so as to scour away the disintegrated portions of the surface, and to 
bring to light the real structure of the interior ; but where the form of the 
ice is such that the water melting from the surface runs off at once without 
forming rills, as is the case on the steep faces of ice pinnacles, and on the 
upper edges of crevasses, the whole surface’ is weathered into a crust of 
disintegrated fragments of ice, and no sign of internal structure is visible 
to the eye. This is even true, in some states of the weather, on parts of the 
lacier where the veined structure is generally visible, and affords ground 
or doubting some alleged instances of its non-appearance. 
+ “Intense pressure” Professor Forbes calls it in his Twelfth Letter on 
Glaciers. I may remark that this most interesting glacier has subsided at 
the present time (September 1857) nearly to the dimensions described and 
gured by Professor Forbes in 1842, while in September 1853 it had pretty 
nearly those which he found in 1846, On both occasions, however, it 
seemed to me that his sketches somewhat underrated the length of the 
