ISLE OF France.

either cellular or compact, together with a species of tuff or puzzolaná. Near the level of the sea is a conglomerate of recent formation, consisting of shells and sand agglutinated together. There exists a crater, called the Devil's Punch Bowl, in the interior of the island, 1000 yards from east to west, 700 across, and about 250 yards in depth; but it does not appear that any lava has been given out from it.

.

The islands on the opposite coast of Africa afford abundant evidences of igneous action, though one only is known to possess an active volcano.

Madagascar, the largest of them, is as yet but little explored, and we only know that its structure is very different from that of the adjacent continent; but of the Isles of France and of Bourbon there is an account by Bory St. Vincent, of a more circumstantial nature.*

The Isle of France is of an oval shape, about 11 leagues in its greatest length, and 8 in its greatest breadth; it rises on all sides from the circumference to the centre, so as to form a conical mountain, called Le Piton.

The other mountains, of which the most elevated is Peter Boot, constitute a chain extending across the island.

All these mountains consist of volcanic matter, either lava or basalt, which is sometimes prismatic, and intersected by dykes. The strata have a slight inclination towards the sea.

The low region to the north, and the isolated rocks off the coast, consist of a coralline limestone, produced at a very recent epoch. It lies in a sort of basin, enclosed in the midst of volcanic rocks, and intermixed with lava, so that it would appear that it was of posterior formation to certain of the latter products.

* Voyage ause Iles d'Afrique.

ISLE OF BOURBON.

The Island of Bourbon, like the Island of France, slopes on all sides upwards towards its centre.

It may be viewed as consisting of two volcanic mountains, of different dates-the southern, which is the smallest, still in a state of activity; the western, extinct.

The point of contact between the two coincides with at line drawn from N.E. to S.W., along which the country is of inferior elevation.

The mountain of Salazes, which is the active volcano, contains two craters; the first of which, the crater Commerson, is composed of basaltic lava having a prismatic form, and therefore seeming to indicate, that the expansive force had in this instance made its way through volcanic matter of more antient date.

The extinct volcano, called Les Fournaises, exhibits a number of basaltic dykes, which, it is to be remarked, are never observed in the one now burning.

Some of these dykes seem no larger than cords, whilst others are of the same thickness as the beds which they traverse. They penetrate indiscriminately all the rocks, sometimes in a direction perpendicular to their planes, at others in one more or less oblique. They also intersect each other in the figure of the letter X. They are usually divided into prisms, which, whatever may be the inclination of the dyke, range at right angles to it. Being more compact than the lavas they penetrate, they frequently stand out in bold relief.

It is worth observing, that earthquakes are only experienced in that part of the island, which is farthest from the site of the active volcano, and even there but rarely.

The most curious production of this island is the variety of pumice like spun glass, which is thrown out occasionally by its volcano.

Bory St. Vincent describes the quantity of these films as

S

FEZZAN AND TRIPOLI.

being on one occasion sufficient to form a cloud, covering the entire summit of the volcano.

Scarcely had he observed it, when the whole party found themselves covered with small shining and capillary flakes, possessing the flexibility and appearance of silk, or of a spider's web. This substance was accompanied with showers, of light, vitreous, spongy scoriæ, in fragments varying from the size of a cherry to that of an apple. It fell into powder with the slightest force.

The threads, of which we have just been speaking, appeared to him nothing but a modification of the scoriform lava peculiar to the Isle of Bourbon. He supposes, they may have been formed, owing to the extrication of elastic matter from this substance, whilst in a state of partial fusion, on the same principle that the threads are formed in sealingwax, when the stick is suddenly withdrawn from the surface of a portion, dropped upon paper, and not completely cooled. He was confirmed in this opinion by observing, attached to these threads, little pear-shaped globules, which were found on examination to be identical with the vitreous scoriæ before alluded to.*

Volcanos therefore seem pretty abundantly distributed on all sides of the African coast, but whether they exist also on the continent, we have not the means of determining.

I have hinted above at the possibility, that volcanic rocks may be met with in the chain of Mount Atlas, on the western extremity of Africa; on the northern coast we know of no active volcano, nor have any rocks been noticed even of a basaltic nature, excepting in Fezzan and Tripoli, where Captain Lyon speaks of a formation of that sort associated

*Similar specimens have been noticed as occurring in the Island of Guadaloupe, and by Mr. Ellis in Owhyhee. See his late Tour in the Sandwich Islands. London, 1826.

EGYPT.

with calcareous beds, which I have the authority of Professor Buckland for pronouncing tertiary.*

There is also an account in the 11th volume of the correspondence of Baron Zach (No. 3.) respecting some rocks in the interior of Egypt, between the Nile and the Red Sea, in the province called Kordoufan.

Rüppell, a German traveller, on whose authority these facts are given, states merely, that there exists in that country a chain of some half-extinguished volcanos of great interest, at a place called Gebel-Koldagi, where a conical mountain of great height, smokes continually on the summit, and throws out hot cinders without intermission.

Mons. Jomard, who accompanied Buonaparte in his invasion of Egypt, states, that in the centre of that country, between the Nile and the Red Sea, in the midst of the Alabaster Quarries, there exists a mountain called Djebel Dokhan, which means the Mountain of Smoke. The Arabs speak of the Petroleum observed to flow some distance off. Djebel Kebryt, or the Sulphur Mountain, lies more to the south, in the 24th degree of latitude, and on the borders of the sea. According to the reports of the Arabs, Djebel Dokhan smokes continually.

It is probable that Rüppell and Jomard both refer to the same chain of mountains, but whether the facts they notice relate to some volcano, or to phænomena similar to those of Macaluba in Sicily, or the Pietra Mala in the Appennines, may admit of doubt.

It must be remarked that neither reporter has visited the spot to which he refers, and that it is not stated even on the authority of the Arab informants, that lava had been ejected from any of the mountains specified.

* As quoted in Brongniart's work, "Sur les environs de Paris." Dr. Oudney also speaks of these rocks between Tripoli and Mourzouk. Sea Denham and Clapperton's Travels in Africa, p. xxix,

ON THE VOLCANOS OF ASIA.

WHATEVER opinion may be entertained with respect to the existence of volcanos in the interior or coast of Egypt, there is no question as to their presence on the opposite shore, as well as in the Red Sea itself.

The Island of Zibbel-Teir, in north latitude 16o. appears, from Bruce's statement, to contain an active volcano, and rocks possessing the same characters are mentioned, as occurring in a groupe of smaller islets in the same part of the Red Sea, off Loheia.*

Niebuhr likewise has given some accounts of phænomena allied to those arising from volcanos, and states in particular that in the valley of Girondel, near Suez, he met with some hot sulphureous springs, on the spot, near which, according to vulgar tradition, Pharoah and his host were swallowed up.+

Lord Valentia‡ likewise remarks, that the rocky Peninsula, on which the town of Aden, at the extremity of the Arabian Gulph, is situated, has all the appearance of being the half of a volcano, the crater of which is covered by the sea, whilst the edge of it is occupied by the present town; from which the rocks rise to a considerable height.

The unfortunate Seetzen,|| who is supposed to have been poisoned during his travels over this part of the Arabian Peninsula, notices particularly the mountain near Aden, as containing a kind of volcanic rock, nearly as light as pumice, and mentions the account which he received of an eruption

✦ Bruce's Travels, vol. i. p. 330 & 340.

+ Niebuhr, vol. i. p. 184.

Travels, vol. 2. p. 86.

Some account of Seetzen's researches may be seen in Baron Zach's Correspondence- in Hammer's publication, entitled Fundgrube des Orients,— and in a small volume entitled, "A brief account of the countries adjoining the Lake Tiberias, the Jordan, and the Dead Sea. London, 1810."