PEOPESSOE TYNDALL ON THE VEINED STEUCTUEE OF OLACIEES. 281 
ice, exposed to the sun, and to wasting atmospheric influences, melts in a manner similar 
to the wasting of the rocks ; little grooves and little ridges are formed upon the surface 
of the glacier, the latter being due to the more resisting ice, while the grooves are pro- 
duced by the melting of the less resisting mass between them. 
The consequence of this is, that the light dirt scattered by winds and avalanches over 
the surface of the glacier is gradually washed into the little grooves, thus forming fine 
hnes, which to the practised eye are an infallible indication of the structure of the ice 
underneath. Visitors to the Jardin have ample occasion to observe these striee, for they 
are finely shown upon the surface of the Mer de Glace between the Angle and Trela- 
porte. When they are followed until they are intersected by a fissure or a stream, it is 
seen that the superficial groovings always mark the dmection of the veined structure 
within the glacier. 
§ 3. Structure and Stratification '.—Marginal Structure. 
Opinions at present are very diverse as to the origin of these veins. Professor Forbes 
first regarded them as being caused by the freezing of water which filled fissures in the 
ice, but he now discards the notion of freezing, and supposes the “incipient fissures” 
to be closed by “ time and cohesion.” M. Agassiz despairs of rendering an account of 
them, but calls them “ bands of infiltration.” The Messrs. Schlagintweit have also 
treated the question, but with no greater success. In the paper published by Mr. 
Huxley and myself, pressure is referred to as the probable cause of the phenomenon, 
but we were unable at the time to furnish proofs of this. Apart from those who give 
public expression to their Hews upon the subject, I know that there are many who reject 
the pressure theory, and adopt instead of it the explanation that the blue veins of the 
glacier are merely the continuation of the strata of the neve ; a view which has recently 
been upheld by Mr. John Ball in the Philosophical Magazine. The matter indeed so 
stands, that in a recent resume of glacier investigations. Professor Mousson of Zurich 
omitted the subject of structure altogether, for the express reason that the question is 
still in complete obscurity. 
I will not take up the time of the Society in discussing the vague, involved, and often 
absurd explanations which have been given of the blue veins of glaciers, but state 
broadly that the question now rests between the pressure theory, and that of stratifica- 
tion. Taking the parallel geological phenomena, the question then is. Does the veined 
structure of glaciers correspond to the stratification or the cleavage of rocks 1 
In reply to this question, I will remark, in the first place, that the veins are not always, 
nor even generally, such as we should expect from stratification. The latter ought to 
furnish us with distinct planes extending parallel to each other for great distances 
through the glacier; this is by no means the general character of the veins. WV 
observe blue streaks, some a few inches, some a foot, and some several feet in length 
upon the walls of the same crevasse, and varying from a fraction of an inch to several 
inches in thickness. In many cases the blue spaces are definitely bounded, giving rise 
