Mr. J. Ball on the Structure of Glaciers. 489 
tiful lake, the Marjelen See. There it experiences resistance 
from the mass of the Aeggischhorn which lies in its direct 
course, the inclination becomes steeper, the stream is forced to 
the right, is rapidly narrowed from about three miles to little 
more than one mile in width, and finally descends through a 
steep gorge below the Bell Alp, until it pours out a stream which 
soon unites with the Rhone, and more than doubles the volume 
of its waters. 
It cannot be doubted that the part of the glacier which is 
most subjected to lateral pressure, is that below the Mirjelen 
Sec, where it abuts against the rocks of the Aeggischhorn, and 
is turned from its direct course by the resistance which it en- 
counters. Yet in that part of the glacier the veined structure is 
much less perceptible than usual; a fact which Professor Forbes 
also noticed and explained in accordance with his own theoretical 
views, by pointing out the probability, or almost certainty, that 
the motion of the glacier of Aletsch is much slower than that of 
other great glaciers; and as a consequence, that the difference 
between the rate of progress at the centre and at the sides is 
proportionately diminished. But if this explanation were cor- 
rect, it would apply with still more force to the upper part of 
the glacier, where the inclination is still less, and where we 
must assume that the onward motion is very slow and sensibly 
uniform, except close to the sides. 
In that long unbroken reach of the ice-stream, again, there 
can be far less lateral pressure than opposite the Aeggischhorn ; 
and I apprehend that if there be any pressure at work capable 
of modifying the internal structure of the ice, it must be that 
due to the weight of the mass acting vertically on its lower 
strata, compounded with that which causes its onward move- 
ment. We have, at all events, direct evidence that there is no 
appreciable amount of lateral compression. So long as the 
glacier is confined on both banks by solid rock, the point might 
remain doubtful; but at the point already referred to, the rocks 
which had confined the glacier on its eastern shore subside, and 
give place to the depression which is occupied by the Marjelen 
See. Instead of a barrier, there is a hollow partly filled by the 
blue waters of the lake, which is on one side shut in by the 
glacier. True it is, that this hollow is of moderate depth com- 
pared to that of the central part of the glacier; but above 
the level of the lake the ice-cliffs are often 50 or 60 feet in 
height, to which must be added the depth of the water below. 
If the veined structure at the surface of the glacier was due 
to lateral compression, when that pressure was removed the ice 
would be pressed outwards into the vacant space. The fact that 
no such lateral expansion takes place, and that the glacier moves 
