PEOFESSOE TYNDALL ON THE YEINED STEUCTUEE OE Q-LACIEES. 
283 
But let a circle be stamped upon the mud-stream near to its side; owing to the 
speedier flow of the centre, this circle must be distorted to an ellipse, because the part 
of the circle furthest from the side moves more quickly than the part nearest the side. 
Hence we shall have an ellipse formed with its major axis inclined downwards, indicating 
that the mud is compressed in one direction and expanded in another. An exactly 
similar state of things occurs in many glaciers ; the ice near the sides is subjected to a 
pressure and tension like that here indicated, and we have marginal crevasses as the 
result of the tension, while the veined structure is, at all events, found associated with 
the pressure. 
Fig. 2 will perhaps render my meaning 
more intelligible, in which cb^ ch represents 
the sides of a glacier moving in the direction 
of the arrow. Here, while the central circle 
retains its shape, the side ones are squeezed 
and drawn out to ellipses. Marginal cre- 
vasses occur parallel to the lines m w, or per- 
pendicular to the tension, while the dotted 
lines mark the direction of the blue veins which are at right angles, or nearly so, to the 
crevasses. I have dotted the line marking the direction of the structure along the 
margins ab, ab. In connexion with this point, I would refer to the instructive papers of 
Mr. Hopkijs'S*, who has shown that in glaciers "vyhich move through valleys of uniform 
width, the directions of maximum pressure and tension are at right angles to each other, 
each of them enclosing an angle of 45 degrees with the side of the glacier. 
I have simply said that the structure in the case described is “ associated with the 
pressure;” thus confining myself within the strict limits of the facts. But what has 
been said shows that the pressure theory afibrds, at all events, a possible solution of a 
difliculty, which, without violence to fact, is inexplicable upon the hypothesis of stratifi- 
cation ; the difliculty, namely, that a finely developed structure often exists along the 
margin of a glacier, while it is excessively feeble, or entirely absent, in the central 
portions. 
§ 4. Transverse Structure. — Glaciers of Grindelwald, the Bhone, &c. 
In many cases, however, the structure is not thus limited to the margins, but sweeps 
across the glacier from side to side, without interruption, being as well developed at the 
centre as at the margins. The stratification theory is wholly incompetent to account 
for this ; the pressure theory requires that to produce this transverse structure the 
glacier must, at some portion of its route, have been forcibly compressed longitudinally. 
It was not till after my return from the Mer de Glace in 1867, that the full mechanical 
significance of a change of inclination in the glacier occurred to me. 
Bend a prism of glass, we have compression on one side and extension on the other, 
* Philosophical Magazine, 1845, xxvi. p. 148. See also Proceedings of the Eoyal Institution, vol. ii. p. 324. 
Fig. 2. 
