Part 22

§ 54. Erratic Blocks.

368. What are these other evidences? We have seen mighty rocks poised on the moraines of the Mer de Glace, and we now know that, unless they are split and shattered by the frost, these rocks will, at some distant day, be landed bodily by the Glacier des Bois in the valley of Chamouni. You have already learned that these boulders often reveal the mineralogical nature of the mountains among which the glacier has passed; that specimens are thus brought down of a character totally different from the rocks among which they are finally landed; this is strikingly the case with the erratic blocks stranded along the Jura.

369. For the Jura itself, as already stated, is limestone; there is no trace of native granite to be found amongst these hills. Still along the breast of the mountain above the town of Neufchâtel, and at about 800 feet above the lake of Neufchâtel, we find stranded a belt of granite boulders from Mont Blanc. And when we clear the soil away from the adjacent mountain side, we find upon the limestone rocks the scarrings of the ancient glacier which brought the boulders here.

370. The most famous of these rocks, called the Pierre à Bôt, measures 50 feet in length, 40 in height, and 20 in width. Multiplying these three numbers together, we obtain 40,000 cubic feet as the volume of the boulder.

371. But this is small compared with some of the rocks which constitute the freight of even recent glaciers. Let us visit another of them. We have already been to Stalden, where the valley divides into two branches, the right branch running to St. Nicholas and Zermatt, and the left one to Saas and the Monte Moro. Three hours above Saas we come upon the end of the Allelein glacier, not filling the main valley, but thrown athwart it so as to stop its drainage like a dam. Above this ice-dam we have the Mattmark Lake, and at the head of the lake a small inn well known to travellers over the Monte Moro.

372. Close to this inn is the greatest boulder that we have ever seen. It measures 240,000 cubic feet. Looking across the valley we notice a glacier with its present end half a mile from the boulder. The stone, I believe, is serpentine, and were you and I to explore the Schwartzberg glacier to its upper fastnesses, we should find among them the birthplace of this gigantic stone. Four-and-twenty years ago, when the glacier reached the place now occupied by the boulder, it landed there its mighty freight, and then retreated. There is a second ice-borne rock at hand which would be considered vast were it not dwarfed by the aspect of its huger neighbour.

373. Evidence of this kind might be multiplied to any extent. In fact, at this moment, distinguished men, like Professor Favre of Geneva, are determining from the distribution of the erratic blocks the extent of the ancient glaciers of Switzerland. It was, however, an engineer named Venetz that first brought these evidences to light, and announced to an incredulous world the vast extension of the ancient ice. M. Agassiz afterwards developed and wonderfully expanded the discovery. Perhaps the most interesting observation regarding ancient glaciers is that of Dr. Hooker, who, during a recent visit to Palestine, found the celebrated Cedars of Lebanon growing upon ancient moraines.

§ 55. Ancient Glaciers of England, Ireland, Scotland, and Wales.

374. At the time the ice attained this extraordinary development in the Alps, many other portions of Europe, where no glaciers now exist, were covered with them. In the Highlands of Scotland, among the mountains of England, Ireland, and Wales, the ancient glaciers have written their story as plainly as in the Alps themselves. I should like to wander with you through Borrodale in Cumberland, or through the valleys near Bethgellert in Wales. Under all the beauty of the present scenery we should discover the memorials of a time when the whole region was locked in the embrace of ice. Professor Ramsay is especially distinguished by his writings on the ancient glaciers of Wales.

375. We have made the acquaintance of the Reeks of Magillicuddy as the great condensers of Atlantic vapour. At the time now referred to, this moisture did not fall as soft and fructifying rain, but as snow, which formed the nutriment of great glaciers. A chain of lakes now constitutes the chief attraction of Killarney, the Lower, the Middle, and the Upper Lake. Let us suppose ourselves rowing towards the head of the Upper Lake with the Purple Mountain to our left. Remembering our travels in the Alps, you would infallibly call my attention to the planing of the rocks, and declare the action to be unmistakably that of glaciers. With our attention thus sharpened, we land at the head of the lake, and walk up the Black Valley to the base of Magillicuddy's Reeks. Your conclusion would be, that this valley tells a tale as wonderful as that of Hasli.

376. We reach our boat and row homewards along the Upper Lake. Its islands now possess a new interest for us. Some of them are bare, others are covered wholly or in part with luxuriant vegetation; but both the naked and clothed islands are glaciated. The weathering of ages has not altered their forms: there are the Cannon Rock, the Giant's Coffin, the Man of War, all sculptured as if the chisel had passed over them in our own lifetime. These lakes, now fringed with tender woodland beauty, were all occupied by the ancient ice. It has disappeared, and seeds from other regions have been wafted thither to sow the trees, the shrubs, the ferns, and the grasses which now beautify Killarney. Man himself, they say, has made his appearance in the world since that time of ice; but of the real period and manner of man's introduction little is professed to be known since, to make them square with science, new meanings have been found for the beautiful myths and stories of the Bible.

377. It is the nature and tendency of the human mind to look backward and forward; to endeavour to restore the past and predict the future. Thus endowed, from data patiently and painfully won, we recover in idea a state of things which existed thousands, it may be millions, of years before the history of the human race began.

§ 56. The Glacial Epoch.

378. This period of ice-extension has been named the Glacial Epoch. In accounting for it great minds have fallen into grave errors, as we shall presently see.

379. The substance on which we have thus far been working exists in three different states: as a solid in ice; as a liquid in water; as a gas in vapour. To cause it to pass from one of these states to the next following one, heat is necessary.

380. Dig a hole in the ice of the Mer de Glace in summer, and place a thermometer in the hole; it will stand at 32° Fahr. Dip your thermometer into one of the glacier streams; it will still mark 32°. The water is therefore as cold as ice.

381. Hence the whole of the heat poured by the sun upon the glacier, and which has been absorbed by the glacier, is expended in simply liquefying the ice, and not in rendering either ice or water a single degree warmer.

382. Expose water to a fire; it becomes hotter for a time. It boils, and from that moment it ceases to get hotter. After it has begun to boil, all the heat communicated by the fire is carried away by the steam, though the steam itself is not the least fraction of a degree hotter than the water.

383. In fact, simply to liquefy ice a large quantity of heat is necessary, and to vaporize water a still larger quantity is necessary. And inasmuch as this heat does not render the water warmer than the ice, nor the steam warmer than the water, it was at one time supposed to be hidden in the water and in the steam. And it was therefore called latent heat.

384. Let us ask how much heat must the sun expend in order to convert a pound weight of the tropical ocean into vapour? This problem has been accurately solved by experiment. It would require in round numbers 1,000 times the amount of heat necessary to raise one pound of water one degree in temperature.

385. But the quantity of heat which would raise the temperature of a pound of water one degree would raise the temperature of a pound of iron ten degrees. This has been also proved by experiment. Hence to convert one pound of the tropical ocean into vapour the sun must expend 10,000 times as much heat as would raise one pound of iron one degree in temperature.

386. This quantity of heat would raise the temperature of 5 lbs. of iron 2,000 degrees, which is the fusing point of cast iron; at this temperature the metal would not only be white hot, but would be passing into the molten condition.

387. Consider the conclusions at which we have now arrived. For every pound of tropical vapour, or for every pound of Alpine ice produced by the congelation of that vapour, an amount of heat has been expended by the sun sufficient to raise 5 lbs. of cast iron to its melting-point.

388. It would not be difficult to calculate approximately the weight of the Mer de Glace and its tributaries--to say, for example, that they contained so many millions of millions of tons of ice and snow. Let the place of the ice be taken by a mass of white-hot iron of quintuple the weight; with such a picture before your mind you get some notion of the enormous amount of heat paid out by the sun to produce the present glacier.

389. You must think over this, until it is as clear as sunshine. For you must never henceforth fall into the error already referred to, and which has entangled so many. So natural was the association of ice and cold, that even celebrated men assumed that all that is needed to produce a great extension of our glaciers is a diminution of the sun's temperature. Had they gone through the foregoing reflections and calculations, they would probably have demanded more heat instead of less for the production of a "glacial epoch." What they really needed were condensers sufficiently powerful to congeal the vapour generated by the heat of the sun.

§ 57. Glacier Theories.

390. You have not forgotten, and hardly ever can forget, our climbs to the Cleft Station. Thoughts were then suggested which we have not yet discussed. We saw the branch glaciers coming down from their névés, welding themselves together, pushing through Trélaporte, and afterwards moving through the sinuous valley of the Mer de Glace. These appearances alone, without taking into account subsequent observations, were sufficient to suggest the idea that glacier ice, however hard and brittle it may appear, is really a viscous substance, resembling treacle, or honey, or tar, or lava.

§ 58. Dilatation and Sliding Theories.