THE GREAT CAÑON AND LOWER FALLS OF THE YELLOWSTONE.
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."
CHAPTER XV.
MR. EVERTS'S THIRTY-SEVEN DAYS OF PERIL.