penines, innumerable mineral springs, chiefly containing carbonate of lime, issue from the ground. As the water evaporates, the lime is left on the surface, and thus the ground in some parts of Tuscany, is covered to a considerable extent with the kind of deposite called Travertine, already noticed. In some places these deposites are solid and smooth on the surface, much resembling currents of lava.

Baths of San Vignone. This spring is also in Tuscany, and affords a striking example of the rapid precipitation of carbonate of lime from thermal waters. The spring issues from near the summit of a hill about one hundred feet high. The water is hot, but Mr. Lyell, from whom this account is taken, does not give its temperature.

So rapid is the deposition from this water, that a pipe leading from the spring to the baths, and inclined at an angle of thirty degrees, is found to contain a coat of solid limestone half a foot thick every year. A mass of solid rock below the hill, formed by this water, is two hundred feet thick. This is employed as a building stone, and in quarrying it, Roman remains of art, such as tiles, have been found five or six feet below the surface, being covered by the deposite.

Baths of San Filippo. These baths are situated only a few miles from those already described. The waters which supply them are impregnated with carbonate of lime, and sulphate of lime, (gypsum.) They flow from the spring immediately into a pond, where in twenty years a solid rock has been deposited thirty feet thick. A curious manufactory, which produces medallions in basso-relievo is carried on at this place.

The water is first allowed to stand in a cistern where the sulphate of lime is deposited. It is then conveyed to a chamber through a tube, from the end of which it falls ten or twelve feet, the current being broken by numerous small sticks crossing each other, and by which means the spray is dispersed around the room. Here are placed the moulds of the medallions to be formed, which are first rubbed over with a little soap. The water striking on these moulds leaves particles of carbonate of lime, which gradually increasing, leaves exact and beautifully white casts of their figures.

1

The solid matter left by this spring, is a mass of limestone and gypsum rock, a mile and a quarter long, the third of a mile in breadth, and in some places at least two hundred and fifty feet in thickness. The length of this deposite terminates abruptly, being crossed by a small stream, which carries away the undeposited matter with the waters of the spring, otherwise it would have been much more extensive.

The quantity of matter deposited from these springs, show the newness of the earth, or at least of the present order of things on its surface; for had they existed at the period when Mr. Lyell supposes the cataract of Niagara was at Queenstown, and discharged their waters, and formed depositions as they do at the present day, and which it is certain they did at the time of the Romans, these strata ought to have been at least ten thousand feet thick. It is true, however, that these thermal springs being caused by volcanic heat, might have been formed within the last two thousand years.

It is apparent from what has been stated concerning calcareous springs, that in the lapse of ages, considerable changes must have been made in the earth's surface from this source. But it must not be forgotten that this cause is local in its nature, being confined chiefly to volcanic districts; and that even such districts seldom contain springs which work such changes as are above described.

Silicious Springs. Although we possess no chemical process by which water can be made to dissolve pure silex, or flint, yet in the great labratory of nature, this effect is produced. There is, however, a process in chemistry, in which, by a previous combination, silex becomes soluble in water, and which, perhaps, affords an analogy to the process employed by nature. If silex be finely pulverized, and then melted with a quantity of common alkali, the whole becomes soluble in hot water. Now springs containing any considerable quantity of silex, are always of high temperatures; and it is to the great degree of heat which exists at their sources, together with small portions of alkali which volcanic rocks contain, and which the water dissolves, that we are to attribute the property these waters possess, of holding silex in solution. Springs containing any considerable quantity of silex, are however, exceedingly rare, and are mentioned here, rather on this

1

account, than for the changes they have produced on the earth's surface.

Springs of St. Michael. The hot springs of St. Michael, one of the Azores, have been long celebrated. These waters rise from among volcanic rocks and hold large quantities of silex in solution. As the waters descend from the fountain, they deposite their silex in the form of what is termed silicious sinter, which may be considered as answering to the travertine, or tufa of calcareous springs.

The herbage and leaves along the course of the stream are more or less encrusted with silex, and exhibit all the successive steps of petrifaction, from a soft state to a complete conversion into stone. Branches of ferns, such as now grow in the vicinity, are thus changed, still preserving their appearance of vegetation, except that they have acquired an ash grey color.-Dr. Webster, Ed. Phil. Journal.

Geysers of Iceland. But the Geysers of Iceland afford the most remarkable examples of the deposition of silex. These springs are situated in a volcanic district; the surface of the ground out of which they rise being covered with streams of ancient lava, through the fissures of which, steam and hot water are emitted in various places.

The great Geyser, which has excited so much interest, on account of the singular phenomena which it exhibits, rises out of a basin at the summit of a circular mound, composed of silicious incrustations deposited from the spray of its waters. The diameter of this basin or crater is 56 feet in one direction, and 46 in the other.

In the centre of this basin is a natural pipe seventyeight feet in perpendicular depth, and from eight to ten feet in diameter, gradually widening as it opens into the basin. The basin, as the spring intermits, is sometimes empty, but is more commonly filled with beautifully transparent boiling hot water, which is often in a state of violent ebullition. During the rise of the water up the pipe, especially when the ebullition is most violent, subterranean noises are heard, like the distant firing of cannon, and a slight tremor of the earth is felt near the place. The sound then increases, and the motion of the earth becomes more violent, until at length a column of water is

thrown up from the pipe in a perpendicular direction, to the height of from one to two hundred feet, attended with loud explosions. This is continued, with interruptions like an artificial fountain, for a few minutes, the water at the same time giving off immense quantities of steam, and vapor, when the pipe is evacuated by the discharge of its whole contents of water, and there follows an immense column of steam, which rushes up with amazing force and a loud thundering noise, after which the eruption, or paroxysm terminates, and the Geyser becomes quiet.

If stones are thrown into the pipe, or crater, during an eruption, they are instantly ejected, and such is the explosive force of the steam, that masses of hard rock thrown in, are returned into the air, shivered into small fragments. Mr. Henderson late a resident in Iceland, and well acquainted with these phenomena, states that by throwing stones into the pipe of the Geyser, he could bring on an eruption in a few minutes, and that in such cases the fragments of stone as well as the water, were thrown much higher than usual. When an eruption had been brought on in this manner, and the water had been ejected, the steam continued to rush up, with amazing force, and attended by a deafening roar, for nearly an hour; but the Geyser as if exhausted by this effort, did not give symptoms of a fresh eruption when its usual interval had elapsed.

In the different explanations which have been offered to account for phenomena so singular, and astonishing, and which have been no where else observed, most writers agree in supposing a subterranean cavity, where water and steam collect, and where the free escape of the steam is interrupted at intervals, or until it acquires sufficient force to overcome the resistance occasioned by the pressure of the water. This will be readily understood by the annexed diagram, reduced from Mr. Lyell, and we may remark that the theory is the same with that of intermitting springs, only that the Geyser acts by steam, while the other is explained on the principle of the syphon.-See the Author's Nat. Philosophy, p. 107.

In explaining this cut, suppose water percolating from the surface of the

earth, or from springs below, finds its way into the subterranean cav

ity d, by the fissures f fr while at the

extremely high temperature, emanates from volcanic rocks into the same cavity through the fissures c c. A portion of the steam in the first place would be condensed into water, but its temperature continuing to increase by the latent heat of the steam, the lower part of the cavity would soon be filled with the boiling fluid, while the upper part would be filled by steam under considerable pressure. The steam continuing to form, the water being now too hot to condense it, would soon by its expansive force, drive the water up the pipe or fissure e, b, what ever might be its height, and thus the basin at the surface would be filled and an eruption take place. When the pressure is thus diminished, the steam in the upper part of the cavity a, would expand, or probably a portion of the boiling water under diminished pressure would be instantly converted into steam, and the passage being free, would rush up the pipe in the same manner as is seen and heard on opening the safety valve of a steam boiler. If the pipe be choaked up artificially with stones, even for a few minutes, a great increase of heat would be occasioned, since the steam would thus be prevented from escaping, so that the water would be made to boil in a few minutes, and thus an eruption would be brought on, as stated by Mr. Henderson.

This explanation accounts for all the phenomena observed in the Geysers, and although we cannot be certain of its truth, still there is every reason to believe that such