traveller in this country. The intermitting character of these fountains may be, in some measure, imitated by pour ing a stream of water through a bent tube depressed about the centre, and heated in that part alone.

Under these circumstances the steam suddenly generated at bottom will force one portion of the water out in a jet from the opposite extremity to that at which it entered, driving back at the same time the current of water that continued to flow in. In this manner the water might be propelled in jerks, as happens in the case of the Geyser springs.

Such an explanation however is far from being adequate to account for the complicated phænomena of these fountains, which, after a pause of many hours, first threw up water, and afterwards vast columns of steam, to the height sometimes of 200 feet, and then immediately sunk into a temporary repose; neither is it applicable to the singular circumstance, mentioned by Mr. Henderson, as to the pos sibility of bringing on the explosion at any given time by merely throwing large stones into the orifice. The latter fact indeed seems to prove that the generation of steam is constant, and that nature has provided other vents sufficient to carry off a certain portion of the elastic vapour, unless when obstructed in the manner produced by Mr. Henderson, in which case its rapid accumulation gives rise to an almost immediate explosion.

The presence of siliceous earth surrounding the waters of the Geyser springs, is a phænomenon not altogether confined to Iceland.

It exists as I have shewn in Ischia and in other places, where it is less easy to attribute its occurrence, as has been done in the present instance, to the chemical affinity exerted for the earth by the mineral alkali present in the watersperhaps indeed in either case the pressure exerted contributes to the effect.

If we were to credit the accounts given by some travellers,

we must attribute still more extraordinary effects to the water of these Geysers than the mere solution of silica.

In the second volume of the Edinburgh Philosophical Journal, you will see an account of Iceland, by Menge, a German, in which he attributes the formation not only of siliceous sinter to these springs, but even in many instances of tuff, of basalt, of porphyry, and of obsidian. He even declares that he saw one hot spring producing lava, another forming basalt, and a third, trap porphyry, and notices a particular case where he extracted from a boiling marsh a muddy hot mass, which, when broken, exhibited the characters of basaltic lava in the centre, and towards the surface passed gradually into red and grey mud.

Such extraordinary facts however require for their belief the very best evidence, and I fear the testimony of Menge will hardly be considered sufficient to substantiate them, if at least I am rightly informed with respect to the estimation as a geologist in which he is held in his own country. That purely siliceous minerals, such as pearlstone, hyalite, and opal may be the productions of hot-springs is indeed less improbable, but all analogy is opposed to the idea that extensive strata of basalt or porphyry have ever been produced in the same manner.

Almost the only substance not connected with volcanic operations which occurs in Iceland is the surturbrand or bituminous wood, of the situation of which a recent traveller, Mr. Henderson, has given us a detailed account. The west side of a perpendicular cleft in the side of a mountain called Hagafiall exposes a section of ten or twelve horizontal strata, of which the surturbrand is undermost, occupying four layers, which are separated from each other by intermediate beds of soft sandstone and clay.

They vary in thickness from a foot and a half to three feet, and differ also in quality, the two lowest strata exhibiting the most perfect specimens mineralized wood, free from all foreign admixture and of a jet black, the numerous knots, roots, &c. leaving no doubt of its vegetable origin.

The two upper strata contain an admixture of earthy and ferruginous matters, and in the midst of them occurs a thin layer, four inches in thickness, consisting of a schistous mass which appears to be made up entirely of leaves closely pressed together, separated only by a little clay. These leaves are chiefly of poplar, a tree, Mr. Henderson says, at present not met with on the island. The beds of surturbrand support an alternation of basalt, tuff, and lava, which extend to the summit of the hill.

With the sole exception perhaps of this substance, the whole of the mineral structure of Iceland may be said to have originated more or less directly from volcanos, and there is probably no part of the globe in which operations of this kind have been going on with so much activity, and for so considerable a period. The existence of submarine lavas proves the action to have commenced before the retreat of the ocean, notwithstanding which eruptions occur here more frequently at present than they do at Vesuvius or in any other known case.

Besides Hecla, which has been twenty-two times in a state of activity during the last eight hundred years, five other volcanos are enumerated, from which the total number of recorded eruptions during the same period is no less than twenty. Some of these happened at the same time at which the volcanos of the Mediterranean were in action, but the instances of this coincidence are not sufficiently numerous to lead to any certain conclusion.

In the year 1783 a submarine eruption took place six or eight miles from Reykiavess, which gave birth to a new island a mile in circumference, which however the following year again disappeared. A submarine eruption also took place about the same time seventy miles from the same cape, which is said to have thrown up pumice sufficient to cover the sea for a space of 150 miles round.

A new island is also stated to have appeared opposite Hecla in the year 1563, but there are some doubts as to this latter fact.*

* See Raspe de novis Insulis. p. 126.

The only other volcano in the north of Europe is that in the island of Jan Mayen, off the coast of Greenland. This, when visited by Captain Scoresby* in the year 1817, exhibited the marks of a recent eruption, and was found to consist of cellular lava, of tuff, and of scoriæ; on its summit was a magnificent crater 500 feet in depth, and about 2000 in diameter.

Those who have made up their minds with regard to the volcanic origin of trap, will perhaps suppose a connexion between the rocks of Iceland, and those of Faro, the Hebrides, and the county of Antrim.

The line, however, which I have prescribed to myself in the prosecution of this inquiry, precludes me from entering into a description of these latter countries, which indeed are already made known to the public by some of the most distinguished geologists of this country, in publications at once easily accessible, and of recent date.t

It is true, there are certain phænomena noticed by Sir G. Mackenzie in his description of Faro, which remind one of those which I have noticed in my account of the volcanic rocks near Frankfort, especially the occurrence of a bed of amygdaloid having its upper surface filled with small insulated perpendicular cavities, as if caused by the escape of a gaseous fluid when the rock was in a soft pasty state.

As however I have not the same inducements, from having personally visited these spots, for entering upon the physical structure of Faro, as I had for noticing that of the trap formations in Germany, I shall return to the south of Europe, where there yet remain several tracts not less worthy of notice with reference to the present enquiry than those

* Scoresby in the Edinb. Philosophieal Journal.

Sir G. Mackenzie and Mr. Allan, Edinb. Phil. Trans. vol. 7.--Mr. Trevelyan, Ed. Phil. Trans. vol. 8.--Macculloch's Western Islands and Jameson's Mineralogical Travels.-Berger on the Geol. Features of the N. Eastern counties of Ireland, with an Introduction and Remarks by the Rev. W. Conyheare-and Descriptive Notes referring to an outline of Sections of the same Coast by Messrs. Cony beare and Buckland.

P

SANTORINO.

which have been treated of in the course of the preceding Lectures.

Thus we have strong reasons, both from the accounts of antient writers, and the observations of modern travellers, to infer the prevalence of igneous action in many parts of the Grecian Archipelago.

It is true, there may be room for questioning the sufficiency of the former source of information, where not con firmed by the physical structure of the spots themselves; for Pliny mentions Rhodes as having been thrown up from the ocean* in a manner that would lead us to infer the agency of some volcanic force, and yet it appears, that this island consists of granite and other rocks, which present no traces of any action of this kind.

In other cases, however, where the information obtained from these two distinct sources concur in assigning to the place this mode of formation, we can have no difficulty in admitting the correctness of the statements given.

The Island of Santorino, known by the antients under the name of Thera, as well as the smaller one near it called Therasia, are mentioned by Pliny as having been thrown up from the sea;t both which statements seem to have a foundation in fact, though mixed up with much inaccuracy of detail.

Thus in speaking of the larger Island Thera, the Roman naturalist sets down the time of its appearance as happening in the 135th Olympiad, or about 237 years B. C., a date quite inconsistent with the mention made of the island by Herodotus, who states that it was given by Cadmus to Membliares, one of his followers ‡

If this historian indeed be depended on, we must likewise

Plin. Hist. Nat. Lib. ii. CLXXXIX.

+ Vide Plin. Hist. Nat. hoc. cit.

Melpomene, c. 147.