485. The flattened mass is at first too soft to cleave sharply; but you can see, by tearing, that it is laminated. Let us chill it with ice. We find afterwards that no slate rock ever exhibited so fine a cleavage. The laminæ, it need hardly be said, are perpendicular to the pressure.
486. One cause of this lamination is that the wax is an aggregate of granules the surfaces of which are places of weak cohesion; and that by the pressure these granules are squeezed flat, thus producing planes of weakness at right angles to the pressure.
487. But the main cause of the cleavage I take to be the lateral sliding of the particles of wax over each other. Old attachments are thereby severed, which the new ones fail to make good. Thus the tangential sliding produces lamination, as the rails near a station are caused to exfoliate by the gliding of the wheel.
488. Instead of wax we may take the slate itself, grind it to fine powder, add water, and thus reproduce the pristine mud. By the proper compression of such mud, in one direction, the cleavage is restored.
489. Call now to mind the evidences we have had of the power of thawing ice to yield to pressure. Recollect the shortening of the Glacier du Géant, and the squeezing of the Glacier de Léchaud, at Trélaporte. Such a substance, slowly acted upon by pressure, will yield laterally. Its particles will slide over each other, the severed attachments being immediately made good by regelation. It will not yield uniformly, but along special planes. It will also liquefy, not uniformly, but along special surfaces. Both the sliding and the liquefaction will take place principally at right angles to the pressure, and glacier lamination is the result.
490. As long as it is sound the laminated glacier ice resists cleavage. Regelation, as I have said, makes the severed attachments good. But when such ice is exposed to the weather the structure is revealed, and the ice can then be cloven into tablets a square foot, or even a square yard in area.
[§ 67.] Conclusion.
491. Here, my friend, our labours close. It has been a true pleasure to me to have you at my side so long. In the sweat of our brows we have often reached the heights where our work lay, but you have been steadfast and industrious throughout, using in all possible cases your own muscles instead of relying upon mine. Here and there I have stretched an arm and helped you to a ledge, but the work of climbing has been almost exclusively your own. It is thus that I should like to teach you all things; showing you the way to profitable exertion, but leaving the exertion to you more anxious to bring out your manliness in the presence of difficulty than to make your way smooth by toning difficulties down.
492. Steadfast, prudent, without terror, though not at all times without awe, I have found you on rock and ice, and you have shown the still rarer quality of steadfastness in intellectual effort. As here set forth, our task seems plain enough, but you and I know how often we have had to wrangle resolutely with the facts to bring out their meaning. The work, however, is now done, and you are master of a fragment of that sure and certain knowledge which is founded on the faithful study of nature. Is it not worth the price paid for it? Or rather, was not the paying of the price the healthful, if sometimes hard, exercise of mind and body, upon alp and glacier—a portion of our delight?
493. Here then we part. And should we not meet again, the memory of these days will still unite us. Give me your hand. Good bye.