Part 12

472. That this is so will be evident to you in a moment. Let the adjacent figure represent the channel of the glacier moving in the direction of the arrow. Suppose three circles to be marked upon the ice, one at the centre and the two others at the sides. In a glacier of uniform inclination all these circles would move downward, the central one only remaining a circle. By the retardation of the sides the marginal circles would be drawn out to ovals. The two circles would be _elongated_ in one direction, and _compressed_ in another. Across the long diameter, which is the direction of strain, we have the marginal crevasses; across the short diameter _m n_, which is the direction of pressure, we have the _marginal veined structure_.

473. This association of pressure and structure is invariable. At the bases of the cascades, where the inclination of the bed of the glacier suddenly changes, the pressure in many cases suffices not only to close the crevasses but to violently squeeze the ice. At such places the structure always appears, sweeping quite across the glacier. When two branch glaciers unite, their mutual thrust intensifies the pre-existing marginal structure of the branches, and developes new planes of lamination. Under the medial moraines, therefore, we have usually a good development of the structure. It is finely displayed, for example, under the great medial moraine of the glacier of the Aar.

474. Upon this glacier, indeed, the blue veins were observed independently three years after M. Guyot had first described them. I say independently, because M. Guyot's description, though written in 1838, remained imprinted, and was unknown in 1841 to the observers on the Aar. These were M. Agassiz and Professor Forbes. To the question of structure Professor Forbes subsequently devoted much attention, and it was mainly his observations and reasonings that gave it the important position now assigned to it in the phenomena of glaciers.

475. Thus without quitting the glaciers themselves, we establish the connexion between pressure and structure. Is there anything in our previous scientific experience with which these facts may be connected? The new knowledge of nature must always strike its roots into the old, and spring from it as an organic growth.

§ 66. _Slate Cleavage and Glacier Lamination_.

476. M. Guyot threw out an exceedingly sagacious hint, when he compared the veined structure to the cleavage of slate rocks. We must learn something of this cleavage, for it really furnishes the key to the problem which now occupies us. Let us go then to the quarries of Bangor or Cumberland, and observe the quarrymen in their sheds splitting the rocks. With a sharp point struck skilfully into the edge of the slate, they cause it to divide into thin plates, fit for roofing or ciphering, as the case may be. The surfaces along which the rock cleaves are called its _planes of cleavage_.

477. All through the quarry you notice the direction of these planes to be perfectly constant. How is this laminated structure to be accounted for?

478. You might be disposed to consider that cleavage is a case of stratification or bedding; for it is true that in various parts of England there are rocks which can be cloven into thin flags along the planes of bedding. But when we examine these slate rocks we verify the observation, first I believe made by the eminent and venerable Professor Sedgwick, that the planes of bedding usually run across the planes of cleavage.

479. We have here, as you observe, a case exactly similar to that of glacier lamination, which we were at first disposed to regard as due to stratification. We afterwards, however, found planes of lamination crossing the layers of the névé, exactly as the planes of cleavage cross the beds of slate rocks.

480. But the analogy extends further. Slate cleavage continued to be a puzzle to geologists till the late Mr. Daniel Sharpe made the discovery that shells and other fossils and bodies found in slate rocks are invariably flattened out in the planes of cleavage.

481. Turn into any well-arranged museum--for example, into the School of Mines in Jermyn Street, and observe the evidence there collected. Look particularly to the fossil trilobites taken from the slate rock. They are in some cases squeezed to one third of their primitive thickness. Numerous other specimens show in the most striking manner the flattening out of shells.

482. To the evidence adduced by Mr. Sharpe, Mr. Sorby added other powerful evidence, founded upon the microscopic examination of slate rock. Taking both into account, 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.

483. The association of pressure and cleavage is thus demonstrated; but the question arises, do they stand to each other in the relation of cause and effect? The only way of replying to this question is to combine artificially the conditions of nature, and see whether we cannot produce her results.

484. The substance of slate rocks was once a plastic mud, in which fossils were embedded. Let us imitate the action of pressure upon such mud by employing, instead of it, softened white wax. Placing a ball of the wax between two glass plates, wetted to prevent it from sticking, we apply pressure and flatten out the wax.

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.

INDEX.

A

Accurate measurements of the motions of glaciers, by Agassiz and Forbes, 60-62. Æggischhorn, view from the, 137. Agassiz's measurements, 60; conclusions, 107; discovery by, 150; observations made by, 187. Aiguille du Dru, pyramid of, 43; cloud-banner of, 90. ---- des Charmoz, 43; clouds about, 90. ---- Noire, 51. ---- Verte, height of, 53 ---- du Midi, stone avalanches of, 56. Air, its expansion, 24; a chilling process, 25; experiments illustrating, 25, 26. Aletsch glacier, 136; length of, 136; arm of, 137. Alpine ice, origin in the sun's heat, 7. Ancient glaciers of England, Ireland, Scotland, and Wales, 150, 151. Architecture of snow, 29-34, 91; of lake-ice, 35, 169. Arveiron, vault of, 66; description and cause of, 92.

B

Bel Alp, description of the, 139, 140. Bergschrund, formation of the, 102, 103, 179. Blue veins of glaciers, 176; whiteness of snow, 176; whiteness of ice, 177; freezing of lake-ice, 177; explanation of cause of whiteness of glaciers, 178; translucency converted into transparency, 178; vertical veins, 179; structure and bedding on glaciers, 181, 182; stratification theory, 183; observations, 187. Bodies of guides found on Glacier des Bossons, 57, 144. Boulders, size of, 148, 149.

C

Changes of volume resulting from heat and cold, 118-122; illustrations of, 119, 120; consequences from, 122; opinions of Count Rumford, 12, 124. Chapeau, refreshment at, 41. Cleavage and glacier lamination, 187; analogy between, 188, 189; planes of, 188; observations of Prof. Sedgwick, 188; discovery by Daniel Sharp, 188; additional evidence of Mr. Sorby, 189; association of pressure and cleavage established, 189: relation of cause and effect, 189; artificial conditions of Nature combined, 189, 190. Clouds, their formation, 3-6; in tropical regions, 25; illustration of the formation of, 26. Col du Géant, snows and ice-cascade of the, 46, 47; snow-fall on the plateau of, 48, 49; cracks on, 179. Conclusion, 191, 192. Condensers needed, 154. Conditions necessary for the production of natural phenomena, 99. Conscience, scientific, 180. Crevasses, 41; work among the, 52; widening of, 54; drifting of bodies buried in, 57; birth of, 98; features of, 100; characteristic, 102; transverse, 103; stalactites of Alpine, 100; marginal, 105-107; longitudinal, 109; curvature of glacier related to number of, 110-112. Crystallization of metals, 29; of sugar, 30; of saltpetre, 30; of alum, 30; of chalk, 30; of carbon, 30; reversal of the process of, 36.

D

De Saussure's theory of glacier-motion, 156. Dilatation theory, 155, 156. Dirt-bands of Mer de Glace, observed by Prof. Forbes, 130; description and explanation of, 131, 132. Distillation, oceanic, 18; ordinary, 21. Dôme du Goûté, broken crags of, 50. Drifting of huts on ice, 59, 60. Dr. William Hopkins's conclusions regarding the obliquity of the lateral crevasses, 107.

E

Égralets, passage through the, 53. Electric light, dark waves of, 14. Equivalent points, comparison of velocities of, 75, 76, 107. Erratic blocks, 147-150. Evaporation, caused by the heat-rays of the sun, 13. Expansion of water, 121, 155. Experiments to show the heat-power of the dark rays, 14-19; illustrative, 22, 23, 25, 20; Dr. Franklin's experiment, 112. Extract from Bordier's book, 157, 162.

F

Faraday's theory regarding regelation, 171; special solidifying power exerted by substances upon their own molecules, 172, 173; opinion of Prof. James Thomson, 174, 175; experiments illustrating the subject, 174; quotation from Faraday, 175. Fog, its formation in ball-rooms, 5. Forces, of crystallization, 30; of gravitation, 30; of Nature, 31; attractive and repulsive, 127. Freezing mixture, 120, 168.

G

Glacier, the source of the Rhone, 7; fed by mountain-snow, 7, 21; melted by the sun's dark rays, 13; terminal moraines of, 38; questions regarding motions of, 54-58; action of the ends of, 58; motion at top and bottom of, 80, 81; lateral compression of, 81, 82; longitudinal compression of, 84, 85; slow movement in winter of, 87; motion of Grindelwald, 94; motion of Great Aletsch, 94; motion of Morteratsch, 95; crevasses at the side of, 106; action, 146, 147; ice, 155; veined or ribboned structure of, 178; blue veins of, 170; tables, 113, 114; mills, 116-118; theory of Scheuchzer regarding, 155; ancient, 145-147; impurities thrust out by, 144; whiteness of a clean, 43, 170; measurements by Hugi and Agassiz of, 59, 60; epoch, 152-167. Glacier des Bois, description of, 38-43. ---- des Bossons, 56; mass of ice upon, 134. ---- of Aletsch, sand-cones of, 116. ---- des Périades, 51. ---- du Talèfre, boundary of, 53; width of, 92; chasms of the, 98. ---- de Léchaud, motion of, 80; width of, 82; compression of, 190. ---- du Géant, _névé_ of the, 49; motion of, 79; width of, 82; cracks above the ice-falls of, 119; honey-combing of, 115; shortening of, 190. ---- Unteraar, movement of, 61; appearance of Prof. Forbes on, 161; measurements by Agassiz on, 162. ---- Görner, sand-cones of, 116; description of, 140-144; moraines of, 143; advance and retreat of, 144; objects of interest on, 144; structure and bedding on, 181. Grand Plateau, crevasse on, 57, 118. Greenland's icy mountains, 21. Growth of knowledge, 59. Guesses in science, 74.

H

Harmony of life and its conditions, 125. Heat, waves of, 12; invisible, 14; office of the invisible waves of, 36; absorption of solar, 100; demanded for the liquefaction of ice, 131; latent, 153. Hoar-frost, 5, 6; not melted by light-waves, 13, 18. Hôtel des Neuchâtelois, movements of, at Riffelberg, 141.

I

Ice, structure of, 35, 36, 119; towers of, 104; sea, 132-134; retreat of, 145; development in the Alps of, 150; freezing of pieces of, 164; liquefaction by pressure of, 108, 160; translucent, 177; difference between hard and soft, 87. Icebergs, of arctic seas, 133; described by Sir Leopold McClintock, 133, 134; drifting of, 134; origin of, 134; of Switzerland, 136-138; colors of, 138; formation of chains of, 165. Ice-lens, concentration of sun's rays by means of, 37. Ice-river through the vale of Hasli, 146. Icicles, 99; how produced, 100; a theory of, 100-102 Imagination, scientific use of, 34. Impurities thrust out by glaciers, 144. Infinite Wisdom, designs of, 124.

J

Jardin, description of, 53. Jungfrau, 136, 137.

K

Killarney, luxuriant vegetation of, 27, 151.

L

La Grande Jorasse, crests of, 43; roses of cloud about, 90. Lake of Geneva, an expansion of the river Rhone, 6. Latent heat, 153, 167. Lateral moraines, origin of, 54. Light, wave-theory of, 10, 11; inference from the phenomena of, 11; length of wave of, 12. Likeness of glacier-motion to river-motion, 72-76. Liquefaction of ice, 168, 169; experiment by Mr. Bottomley, 170; experiment by Mr. Boussingault, 170, 171. Locus of the point of swiftest motion, 76. Longitudinal crevasses, how formed, 109; examples of, 110.

M

Magillicuddy's Reeks, 27, 150, 151. Magnet, poles of, 32; repelling corners and ends of, 126. Märgelin See, 138; icebergs in the, 138. Marginal crevasses, explanation of, 106-109. Mauvais Pas, 41. Measurements of glaciers by Hugi and Agassiz, 59, 60. Medial moraines, how accounted for, 55. Mer de Glace, 41; its sources, 43-45; view of the, 44; branching of the, 45; medial moraines of the, 51, 52; triangulation of, 62; motion of, 66, 70; daily motion of, 67; unequal motion of the two sides of, 70-72; motion of axes of, 78; summer condition of, 87; winter on, 88-92; dirt-bands of, 127; dimensions of, 145; winter motion of, 93; greater number of crevasses on eastern side of, 112; glacier tables of, 113; grand moulin of, 117, 118; approximate weight of, 154. Molecules, of water, 31, 34; expansion of, 125; forces acting upon, 127; exclusiveness of water, 132, 133. Montanvert, auberge of the, 41, 43; appearance in winter, 89. Moraine, lateral, 41; medial moraines of the Mer de Glace, 51, 112; cedars of Lebanon growing on ancient, 150; explanation of the cause of ridges on, 112, 113. Morteratsch glacier, cause of the widening of the medial moraine of, 97; motion of, 95, 96; sand-cones of, 116. Motion, of Mer de Glace, 93; of Grindelwald, 94; of Great Aletsch glacier, 94; of Morteratsch glacier, 95; sand-cones of, 116. Moulins, description of, 116; dangers from, 117; sounding of, 118. Mountain condensers, 27, 150.

N

_Névé_, explanation of term, 49; stratification of the, 179.

O

Obliquity of the lateral crevasses, 167; illustration, 107, 108.

P

Petit Plateau, 56. Piz Bernina, route to, 95. Place de la Concorde of Nature, 136. Plastic theory, 156; advocated by Bordier, 157; advocated by Rendu, 158. Poles, atomic, 32, 126; attractive and repellent, 30. Pontresina, village of, 95. Precious stones, examples of crystallizing power, 30. Precipitation, 23, 25; atmospheric, 27. Promontory of Trélaporte, 44; of Tacul, 51. Proofs of glacier-motions, 58. Pyramid of Aiguille du Dru, 43; of Aiguille des Charmoz, 43.

Q

Quotation from Rendu, 158.

R

Rain, its source, 3; tropical, 23, 25. Rainfall, observations on amount of, 28. Regulation, theory, 163-167; observations made by Mr. Faraday, 164; formation of chain of icebergs by, 165; cause of, 167; Faraday's view of, 171-176. Relation of structure to pressure, 183; veined structure described by Guyot, 183-185; illustration, 186; connection established, 187. Retina, how excited, 8; theory of Sir Isaac Newton, 9. Riffelberg Hotel, location of, 144. Rivers, their sources, 1, 7, 19.

S

Sand-cones, 116. Scientific tacts, connection of, 72. Siedelhorn, view from the summit of, 146. Snow, its conversion into ice, 156; consolidation in Alpine regions, 156; line, 49; its formation in Russian ball-rooms, 5; in subterranean stables, 5, 14; in polar regions, 21; its architecture, 29-32, 34, 91; absorbs solar heat, 100. Solidifying power of camphor and metals, 17. Source of the Severn, 2; the Thames, 2; the Danube, 2; the Rhine, 2; the Rhone, 2; the Gauges, 2; the Euphrates, 2; the Garonne, 2; the Elbe, 2; the Missouri, 2; the Amazon, 2; Albula, 2; the Arveiron, 38; the Aar, 50. Stalactites of Alpine crevasses, 100. Steam, its condensation, 3, 4. Sunbeams, office of, 10-21. Sun, its heat the source of Alpine ice, 7; vibratory motion of the atoms of the, 11; position of, 19; indirect heat, of the, 28. Switzerland, ancient glaciers of, 145-147.

T

Theodolite, description of, 63; use of, 64, 65. Theory, of Dilatation, 155; developed by De Charpentier, 156; sliding, 156; plastic, 156-160; viscous, 161-103; regulation, 163-167; of glacial epoch, 155-167. Trade-winds, 20. Transverse crevasses, formation of, 104, 105. Trélaporte, promontory of, 44; motion of the water through the narrows of, 79.

U

Universe, order of the, 32.

V

Valley, of Hasli, 146, 147; Black, 151. Vapor, in the atmosphere, 5. Viscous theory, advocated by Prof. Forbes, 161; rejected by M. Agassiz, 162.

W

Water, changes of volume of, 118-122; maximum density of, 120; effects of expansion of, 121; not a solitary exception to general law, 124; molecular expansion of, 125-127; temperature necessary to freeze the sea, 132; special solidifying power of, 173; freezing-point of, 168. Waves, of light, 8-11, 127; length of, 12; of heat, 12. Whiteness of a clean glacier, 43, 176; of Märgelin See, 138, 176; of snow, 176; of ice formed in freezing mixtures, 177.

THE END.

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Transcriber Note

On page 61 (153), Principal was changed to Professor. Minor typos were corrected. Some images were moved to avoid splitting paragraphs.