ISi 



THE FORMS OF WATER 



Sharpe, Mr. Sorby added other powerful evi- 

 dence, founded upon the microscopic exam- 

 ination of slate rock. Taking both into ac- 

 count, the conclusion is irresistible that such 

 rocks have suffered enormous pressure at 

 right angles to the planes of cleavage, exactly 

 as the glacier has demonstrably suffered 

 great pressure at right angles to its planes of 

 lamination. 



488. The association of pressure and cleav- 

 age is thus demonstrated ; but the question 

 arises, Do they stand to each other in the re- 

 lation of cause and effect ? The only way of 

 replying to this question is to combine artifi- 

 cially the conditions of nature, and see 

 whether we cannot produce her result*. 



484. The substance of slate rocks was once 

 a plastic mud, in which fossils were imbed- 

 ded. Let us imitate the action of pressure 

 upon such mud by employing, instead of it, 

 softened while wax. Placing a ball of the 

 wax between t\\o glass plates, wetted to pre- 

 vent it from slicking, we apply pressure and 

 flatten out the wax. 



485. The flattened mass is sit first too soft 

 to cleave sharply ; but you can see, by tear- 

 ing, that it is laminated. Let us chill it with 

 ice. We find aflerwaid that no slate rock 

 ever exhibited so fine a cleavage. The lam- 

 ina?, it need hardly be said, are perpendicular 

 to the pressure. 



486. One cause of this laminnticn is that 

 the wax is an aggregate of gtanules the sur- 

 faces of which are places of weak cohesion ; 

 and that by the pressure these granules are 

 squeezed flat, thus producing planes of weak- 

 ness 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 sta- 

 tion 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 piistine 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 Geant, and the squeezing of the 

 Glacier de Lechaud, at Trelaporte. Such a 

 substance, slowly acted upon by pressure, 



will yield laterally. Its part ides wih slide 

 over each other, the severed attachments be- 

 ing immediately made good by legi-lation 

 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 princi- 

 pally at right angles to the pressure, and gla- 

 cier lamination is the result. 



490. As long as it is sound Ihe laminated 

 glacier ice resists cleavage. Regelatiou, as I 

 have said, makes the severed "attachments 

 good. But when such ice is exposed to ike 

 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 labors 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 in- 

 stead 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 al- 

 most exclusively your own. It is thus that 

 I should like to teach you all things ; show- 

 ing 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 snioclh 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 an;i 

 certain knowledge which is founded on the 

 faithful study of nature. Is it not worth th 

 price paid for it ? Or rather, was not tho 

 paying of the price the healthful, if some 

 times hard, exercise of mind and body, upon 

 alp and glacier a portion of our delight ? 



493. Here then we part. And should we 

 liot meet again, the memory of these days 

 will still unite us. Give rue your hand. 

 Good-by. 



