But the notions of M. Agassiz as to the turning up of the strata so as to expose their edges at the surface, and the acute remarks and arguments of Mr. John Ball on the same subject, might still cast a doubt upon the pressure theory, by suggesting a possible, though extremely improbable, explanation of the structure in accordance with the theory of stratification.
Hence my strong desire to discover some crucial phenomenon which should set this question for ever at rest, and leave no room for doubt, even on the minds of those who never saw a glacier. On Wednesday, August 18, I was fortunate enough to make this discovery upon the Furgge glacier.
This ice-field spreads out as an almost level plain at the base of Mont Cervin. The strata pile themselves one above the other without disturbance, and hence with great regularity. The ice at length reaches a brow, over which it is precipitated, forming in its descent four great terraces, and shutting up the lower valley as a cul de sac. When I reached this place huge blocks of ice stood, like rocking stones, upon the topmost ledge, and numbers, which had fallen, had been caught by the other ledges, and occupied very threatening positions: the base of the fall was cumbered with crushed ice, and large boulders of the substance had been cast a considerable way down the glacier.
On the faces of the terraces horizontal lines of stratification were shown in the most perfect manner. Here and there the exertion of a powerful lateral squeeze was manifest, causing the beds to crumple, and producing numerous faults. Examining the fall from a distance through an opera-glass, I thought I could discover lines of veining running through the strata, at a high angle, exactly as the planes of cleavage often run at a high angle to the bedding of slate rocks. The surface of the ice was, however, weathered; and I was unwilling to accept an observation upon such a cardinal point with a shade of doubt attached to it. Leaving my field-glass with my guide, who was to give me warning should the blocks overhead give way, I advanced to the wall of ice, and at several places cut away with my axe the weathered superficial portions. Underneath I found the true veined structure, running nearly at right angles to the planes of stratification.
I afterwards climbed the glacier to the right, and, as I ascended, still better illustrations of the coexistence of the structure and the strata than those observed upon the terraces exhibited themselves. The ice was greatly dislocated, and on the faces of the crevasses the beds were distinctly shown, with the veins crossing them. The idea that the veins could be due to the turning up of the strata is plainly irreconcileable with these observations.
The same year I visited the Mer de Glace and its tributaries, and found the pressure key applicable to their phenomena also. The transverse structure of the Glacier du Géant is formed at the base of the séracs; that of the Talèfre branch of the Mer de Glace at the base of the Talèfre ice-fall, where the change of inclination and the thrust from behind produce the requisite longitudinal compression. I have already had occasion to remark upon the peculiar dipping of the structure, and the scaling-off of the protuberances, which are effects of the same cause. These phenomena are exhibited at the base of all the ice-cascades.
The principal kinds of structure may be divided into three; as follows:
1st, Marginal structure, developed by pressure due to the swifter motion of the centre of the glacier.
2nd, Longitudinal structure, due to mutual pressure of two tributary glaciers; the structure here is parallel to the medial moraine which divides the tributaries.
3rd, Transverse structure, produced by pressure due to the change of inclination, and to the longitudinal thrust endured by the glacier at the base of an ice-fall.