The manner in which I have illustrated the production of this structure will be understood from [Fig. 43]. b b are two wooden boxes, communicating by sluice-fronts with two branch canals, which unite to a common trunk at g. They are intended to represent respectively the trunk and tributaries of the Unteraar Glacier, the part g being the Abschwung, where the Lauteraar and Finsteraar glaciers unite to form the Unteraar. The mud is first permitted to flow beneath the two sluices until it has covered the bottom of the trough for some distance, when it is arrested. The end of a glass tube is then dipped into a mixture of rouge and water, and small circles are stamped upon the mud. The two branches are thickly covered with these circles. The sluices being again raised, the mud in the branches moves downwards, carrying with it the circles stamped upon it; and the manner in which these circles are distorted enables us to infer the strains and pressures to which the mud is subjected during its descent. The figure represents approximately what takes place. The side-circles, as might be expected, are squeezed to oblique ovals, but it is at the junction of the branches that the chief effect of pressure is produced. Here, by the mutual thrust of the branches, the circles are not only changed to elongated ellipses, but even squeezed to straight lines. In the case of the glacier this is the region at which the structure receives its main development. To this manifestation of the veins I have applied the term longitudinal structure.
The three main sources of the blue veins are, I think, here noted; but besides these there are many local causes which influence their production. I have seen them well formed where a glacier is opposed by the sudden bend of a valley, or by a local promontory which presents an obstacle sufficient to bring the requisite pressure into play. In the glaciers of the Tyrol and of the Oberland I have seen examples of this kind; but the three principal sources of the veins are, I think, those stated above.
EFFORTS TO SOLVE QUESTION.
It was long before I cleared my mind of doubt regarding the origin of the lamination. When on the Mer de Glace in 1857 I spared neither risk nor labour to instruct myself regarding it. I explored the Talèfre basin, its cascade, and the ice beneath it. Several days were spent amid the ice humps and cliffs at the lower portion of the fall. I suppose I traversed the Glacier du Géant twenty times, and passed eight or ten days amid the confusion of its great cascade. I visited those places where, it had been affirmed, the veins were produced. I endeavoured to satisfy myself of the mutability which had been ascribed to them; but a close examination reduced the value of each particular case so much that I quitted the glacier that year with nothing more than an opinion that the structure and the stratification were two different things. I, however, drew up a statement of the facts observed, with the view of presenting it to the Royal Society; but I afterwards felt that in thus acting I should merely swell the literature of the subject without adding anything certain. I therefore withheld the paper, and resolved to devote another year to a search among the chief glaciers of the Oberland, of the Canton Valais, and of Savoy, for proofs which should relieve my mind of all doubt upon the subject.
EXPEDITION FOR THIS PURPOSE.
Accordingly in 1858 I visited the glaciers of Rosenlaui, Schwartzwald, Grindelwald, the Aar, the Rhone, and the Aletsch, to the examination of which latter I devoted more than a week. I afterwards went to Zermatt, and, taking up my quarters at the Riffelberg, devoted eleven days to the examination of the great system of glaciers of Monte Rosa. I explored the Görner Glacier up almost to the Cima de Jazzi; and believed that in it I could trace the structure from portions of the glacier where it vanished, through various stages of perfection, up to its full development. I believe this still; but yet it is nothing but a belief, which the utmost labour that I could bestow did not raise to a certainty. The Western glacier of Monte Rosa, the Schwartze Glacier, the Trifti Glacier, the glacier of the little Mont Cervin, and of St. Théodule, were all examined in connexion with the great trunk-stream of the Görner, to which they weld themselves; and though the more I pursued the subject the stronger my conviction became that pressure was the cause of the structure, a crucial case was still wanting.
In the phenomena of slaty cleavage, it is often, if not usually, found that the true cleavage cuts the planes of stratification—sometimes at a very high angle. Had this not been proved by the observations of Sedgwick and others, geologists would not have been able to conclude that cleavage and bedding were two different things, and needed wholly different explanations. My aim, throughout the expedition of 1858, was to discover in the ice a parallel case to the above; to find a clear and undoubted instance where the veins and the stratification were simultaneously exhibited, cutting each other at an unmistakable angle. On the 6th of August, while engaged with Professor Ramsay upon the Great Aletsch Glacier, not far from its junction with the Middle Aletsch, I observed what appeared to me to be the lines of bedding running nearly horizontal along the wall of a great crevasse, while cutting them at a large angle was the true veined structure. I drew my friend's attention to the fact, and to him it appeared perfectly conclusive. It is from a sketch made by him at the place that [Fig. 44] has been taken.
CASE OF STRUCTURE ON THE ALETSCH.
