It will be observed that in no part of the tube does the water reach the boiling point. The nearest approach to it is at A, thirty feet from the bottom; out even here the water is some four degrees below the temperature at which it could boil. How then is an eruption possible?

Professor Tyndall's explanation is in substance this: Suppose that by the entrance of steam from the ducts near the bottom of the tube the geyser column is elevated six feet, a height quite within the limits of actual observation; the water at A is thereby transferred to B. Its boiling point at A is 255°, and its actual temperature is 251°; but at B its boiling point is only 249°, hence when transferred from A to B, the heat which it possesses is in excess of that necessary to make it boil. This excess of heat is instantly applied to the generation of steam; the column is thus lifted higher, and the water below is relieved of pressure, and its boiling point lowered. More steam is generated; this lifts the column still higher, and compels the generation of more and more steam, until the whole upper portion of the column bursts into ebullition, and the water, mixed with steam-clouds, is projected into the atmosphere, and we have the geyser eruption in all its grandeur.

One confirmation of this theory of Bunsen's is that small stones suspended in the lower part of the geyser-tube are not thrown out during an eruption; and a stronger confirmation lies in the fact that all the peculiarities of geyser action can be imitated. Professor Tyndall uses for this purpose an apparatus consisting of a tube of iron six feet long, surmounted by a basin, and heated by fires underneath. To imitate, as far as possible, the conditions of the geyser, he encircles the tube with a second fire, two feet from the bottom. As the water in the tube becomes heated, the phenomena of geyser eruption are repeated in miniature with beautiful regularity. By stopping the mouth of the tube with a cork, the enforced explosions of the Strokr are reproduced; and by similar simple devices the action of all other eruptive springs may be accurately imitated.

All through the Firehole Basin and around Yellowstone Lake are many extinct geysers; sometimes, as in the case of Old Faithful, an active geyser is surrounded by a number of deserted cones, the remains of ancient "roarers." What occasions their decline? Earthquakes may, and no doubt frequently do, derange their mechanism, as observed in the old Strokr. But most of them probably die a natural death, from old age and decrepitude.

"A moment's reflection," says Professor Tyndall, "will suggest that there must be a limit to the operations of the geyser. When the tube has reached such an altitude that the water in the depths below, owing to the increased pressure, cannot attain its boiling point, the eruptions of necessity cease. The spring, however, continues to deposit its silica, and often forms a Laug, or cistern. Some of these, in Iceland, are forty feet deep. Their beauty, according to Bunsen, is indescribable. Over the surface curls a light vapor; the water is of the purest azure, and tints with its lovely hue the fantastic incrustations on the cistern walls; while at the bottom is often seen the mouth of the once mighty geyser. There are in Iceland vast, but now extinct geyser operations. Mounds are observed whose shafts are filled with rubbish, the water having forced a passage underneath and retired to other scenes of action. We have, in fact, the geyser in its youth, manhood, old age, and death here presented to us. In its youth as a simple thermal spring; in its manhood, as an eruptive column; in its old age, as the tranquil Laug; while its death is recorded by the ruined shaft and mound, which testify the fact of its once active existence."

All that Professor Tyndall describes so eloquently of Iceland, exists in our Grand National Park in infinitely greater variety, and magnitude, and splendor. And much more: Iceland has no Gardiner's River. To find the nearest approach to the marvels of White Mountain Hot Spring, we must go to the opposite side of the globe—to New Zealand. In the celebrated Lake District of the North Island is a region of hot springs, far exceeding in extent and variety all the others in the world, save those of the Yellowstone. First of all, says Hochstetter, the most marvellous of the Rotomahana marvels is the Te Tarata—the Tattooed Rock—with its terraced marble steps projecting into the lake. The spring lies about eighty feet above the lake, on a fern-clad hill-slope, in a crater-like excavation, with steep reddish sides, from thirty to forty feet high, and open only toward the lake. The basin of the spring is about eighty feet long and sixty wide, filled to the brim with perfectly transparent water, which in its snow-white basin appears of a beautiful blue, like the blue turquoise. Immense clouds of steam curl up from the surface, obstructing the view, but the noise of boiling and seething is always audible. The water is slightly salt, but not unpleasant to the taste, chemically neutral, and possesses petrifying, or rather incrusting qualities, in a high degree. The deposit is silicious, like that of the Iceland springs and the springs around Yellowstone Lake, not calcareous, like those of Gardiner's River; yet the system of terraces built up by the deposit on the hill-slope has the same appearance of a cataract plunging over a series of natural shelves and suddenly turned to stone. The deposits cover an area of about three acres, a mere trifle compared with the square miles of similar formations on Gardiner's River and in the Yellowstone Basin.

In the same neighborhood is a system of bubbling mud-pools, miniature copies of those on the Yellowstone above the falls. The principal group, lying in a ravine nearly a quarter of a mile long, is described by Dr. Hochstetter as follows:

"The entrance to the ravine is overgrown with a thicket and rather difficult of access; it also requires considerable caution, as suspicious places have to be passed, where the visitor is in danger of being swallowed up in heated mud. Inside, the ravine has the appearance of a volcanic crater. The bare walls, utterly destitute of vegetation, are terribly fissured and torn, and odd-looking rocky serratures, threatening every moment to break loose, loom up like dismal spectres from red, white and blue fumarole-clay—evidently the last remains of decomposed rocks. The bottom of the ravine is of fine mud, scattered with blocks of silicious deposit, like cakes of floating ice after a thaw. Here, a big caldron of mud is simmering; there, lies a deep basin of boiling water; next to this is a terrible hole, emitting hissing jets of steam, and further on are mud-cones from two to five feet high, vomiting hot mud from their craters with dull rumblings, and imitating on a small scale the play of large fire volcanoes." The gay colors of the Yellowstone mud-springs are frequently exhibited in the volcanic lake district of New Zealand, and so indeed are most of the other phenomena we have been studying, though on a far less magnificent scale. For example, the grandest "Firehole basin" on the island occupies the Shallow valley of a little stream the Waikato, for the distance of a mile. It is but a cabinet exhibition comparatively, yet the learned geologist of the Novara expedition grows eloquent in his description of it. "In the morning a dense fog lay upon the Waikato, but it soon vanished, the sun shone brightly into the valley, and now—what a sight! In its swift course, forming rapids after rapids, the Waikato was plunging through the deep valley between steep-rising mountains; its floods whirling and foaming round two small rocky islands in the middle of the river, were dashing with a loud uproar through the defile of the valley. Along its banks white clouds of steam were ascending from hot cascades falling into the river, and from basins full of boiling water shut in by a white mass of stone. Yonder a steaming fountain was rising and falling; now there sprung from another place a second fountain; this also ceased in its turn; then two commenced playing simultaneously, one quite low at the river bank, the other opposite on a terrace; and thus the play continued with endless changes, as though experiments were being made with grand waterworks, to see whether the fountains were all in perfect order, and whether the waterfalls had a sufficient supply. I began to count the places where a boiling waterbasin was visible, or where a cloud of steam indicated the existence of one. I counted seventy-six points, without, however, being able to survey the whole region, and among them were numerous intermittent geyser-like fountains with periodical eruptions of water."

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The picture is admirably drawn, but could the artist have done so well with the stupendous chasm of the Grand CaÑon? or the thousand volcanic vents of Firehole Basin with their deafening detonations, their immeasurable evolutions of water and steam? It is possible, but scarcely probable. The incomprehensible grandeur of the scene would have awed, astounded, bewildered him, and like our Yellowstone explorers, he would have despaired of grouping the myriad marvels into one grand effect, and contented himself with setting down a few details of form and color.

In following the exploration of the Yellowstone country and Firehole Basin, the reader has doubtless observed, in the passage from the quiet springs of Gardiner's River to the erupting fountains further on, that there is a complete change in the nature of the deposits. The mounds and terraces built up by the former have for their basis lime, those of the latter silica. Dr. Hayden attributes the calcareous deposits to the deep bed of limestone underlying the springs, but not all waters have the power of disolving lime so freely, nor could ordinary water take from the trachytic lavas below the silicious springs around Yellowstone Lake and in the Firehole Basin, the silica that appears so abundantly in their deposits. There must be other forces at work. What are they? "Both kinds of springs," says Dr. Hochstetter, in his chapter on New Zealand springs, "owe their origin to the water permeating the surface and sinking through fissures into the bowels of the earth, where it becomes heated by the still existing volcanic fires. High-pressure steam is thus generated, which, accompanied by volcanic gases—such as muriatic acid, sulphurous acid, sulphuretted hydrogen and carbonic acid—rises again toward the colder surface and is there condensed into hot water. The overheated steam and the gases decompose the rock beneath, dissolving certain ingredients which are deposited on the surface. According to Bunsen's ingenious observations, a chronological succession takes place in the coÖperation of the gases. The sulphurous acid acts first. It is generated where rising sulphur vapor comes in contact with glowing masses of rock. Wherever vapors of sulphurous acid are constantly formed, there acid-springs or solfataras arise. Incrustations of alum are very common in such places, arising from the action of sulphuric acid on the alumina and alkali of the lavas; another product of the decomposition of the lavas is gypsum, or sulphate of lime, the residuum being a more or less ferruginous fumarole clay, the material of the mud-pools. After the sulphurous acid comes sulphuretted hydrogen, produced by the action of steam on sulphids; and by the mutual decomposition of sulphuretted hydrogen and sulphurous acid sulphur is formed, the characteristic precipitate in all solfataras, while the deposition of silica is either entirely wanting or quite inconsiderable, and the smell of sulphuretted hydrogen is but rarely noticed. These acid springs have no periodical outbursts of water.

In course of time the source of sulphurous acid becomes exhausted, and sulphuretted hydrogen alone remains active. The acid reaction of the soil disappears, yielding to an alkaline reaction by the formation of sulphids. At the same time carbonic acid begins to act upon the rocks, and the alkaline bi-carbonates thus produced dissolve the silica, which on the evaporation of the water is deposited in the form of opal or quartz or silicious earth, and thus the shell of the springs is formed, on the structure of which the periodicity of the outburst depends.... The deposition of silica in quantities sufficient for the formation of this spring-apparatus in the course of years, takes place only in the alkaline springs. Their water is either neutral or has a slightly alkaline reaction. Silica, common salt, carbonates and sulphates are the chief ingredients dissolved in it. In the place of sulphurous acid the odor of sulphuretted hydrogen is sometimes observed in these springs.... By the gradual cooling of the volcanic rocks under the surface of the earth the hot springs themselves gradually die out, for they too are but a transient phenomenon in the eternal change of created things."