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 calca reous 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 colour.-Dr. Webster, Ed. Phil. Jour.

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, subterra nean 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 be comes more violent, until at length a column of water is

hrown 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 explo sive 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 symp toms 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 spring, only that the Geyser acts by steam, while the other is explained on the principles 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

be

low, finds its

way into the subterranean

cavity d, by the fissures f f, while at the same time

steam of an

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, whatever 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 choked 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

a cavity exists, and it is certain that steam is the moving power.

Mr. Lyell forms a theory of earthquakes on this expla nation. See Seat and Theory of Earthquakes.

DELTAS IN LAKES.

Considerable changes have taken place by causes now going on, in consequence of the deposition of earthy matter at the mouths of rivers where they enter lakes, or seas. We have already given an account of the accumulation of land along the shores of the Adriatic in consequence chiefly of depositions from the river Po. The quantity of matter thus carried down by different rivers, of similar magnitudes, differs exceedingly; this difference depending much on the rapidity of the stream, and its liability to overflow its banks at certain seasons.

Delta of the Lake of Geneva. The Lake of Geneva is thirty seven miles long, and from two to nine miles broad. The Rhone enters at one end of this lake and the city of Geneva stands at the other. The water where it discharges itself near the city is exceedingly clear and transparent, but at the upper end it is commonly turbid, in consequence of the matter brought down by the Rhone.

Mr. De La Beche, after numerous soundings, found that the depth of the water in the middle of the lake was from one hundred and twenty, to one hundred and sixty fathoms; but on approaching the mouth of the Rhone, the water began to grow shallower at the distance of a mile and three quarters from that end of the lake. It may be stated therefore that the strata annually produced by the river are about two miles in length. From soundings it has been ascertained that in some places the deposites from the Rhone are probably from six to nine hundred feet in thickness; and from the remains of some Roman buildings on the border of the lake, Mr. Lyell judges that this accumulation has taken place within the last eight hundred years. 'If," says he "we could obtain the depth of this accumulation formed in the last eight centuries, we should see a great series of strata, probably from

66

six to nine hundred feet thick, and nearly two miles in length, inclined at a very slight angle."

Mr. Lyell proposes a plan for estimating the time when the Lake of Geneva, or the Leman Lake will become dry land by the accumulations from the Rhone.

The capacity of the lake being obtained, "it would,” says he, "be an interesting subject of inquiry, to determine in what number of years the Leman lake would be converted to dry land. It would not be difficult to obtain the elements for such a calculation, so as to approximate at least to the quantity of time required for the accomplishment of this result. The number of cubic feet of water annually discharged by the river into the lake being known, experiments might be made in winter and summer to determine the proportion of matter held in suspension, or in chemical solution, by the Rhone."

Such calculations, however, after all the data that could be obtained, would be exceedingly uncertain, and since the elements proposed by the author have not been obtained, we do not extract his speculations on this subject.

But were it ascertained exactly how much alluvial matter is carried down by the Rhone at the present day, still this would decide nothing definitely with respect to the time during which this accumulation has been forming. According to Mr. Lyell's supposition above cited, a part of the delta has formed at the rate of about a foot in a year, namely, from six to nine hundred feet in eight hundred years. Now allowing that the Rhone has, on an average, deposited a foot of matter a year in the lake, and has continued to do so ever since the deluge, then the accumulation ought to be at least four thousand feet thick, which would long ago have filled up the Leman lake, and made it solid ground. The phenomena of this lake, therefore clearly shows that either it has not received the Rhone for so many years, or if so, that its waters contained less solid matter anciently than at present. In either case, it is quite certain that no argument can be derived from the present condition of this delta, in favour of the high antiquity of the present form of the earth. But on the contrary, if any conclusions can be drawn from this source, they are in direct coincidence with the idea that the present order of things are of recent origin, and therefore in confirmation of the truth of the sacred history of the deluge.