H́nh ảnh trang
PDF
ePub

river Don and Sea of Azof may have acted the same part in pro ducing these appearances in the one case, as the Orinoco and Gulf of Paria appear to have done in the other. It may be supposed that the destruction of a forest, or perhaps even a great savanna on the spot, would be a more obvious mode of accounting for this singu

system of geology, I confess an explanation similar to this appears to me sufficiently probable, and consonant with the known phænomena of nature. A vast river, like the Orinoco, must for ages have rolled down great quantities of woody and vegetable bodies, which from certain causes, as the influence of currents and eddies,-may have arrested and accumulated in particular phænomenon; but, as I shall lar places; they may there have immediately state, all this part of undergone those transformations the island is of recent alluvial forand chemical changes which va- mation, and the land all along this rious vegetable substances similarly coast is daily receiving a considersituated have been proved to suffer able accession from the surroundin other parts of the world. An ing water. The pitch lake with accidental fire, such as is known the circumjacent tract being now frequently to occur in the bowels on the margin of the sea, must in of the earth, may then have ope- like manner have bad an origin of rated in separating and driving off no very distant date; besides, acthe newly formed bitumen more cording to the above representation or less combined with siliceous and of Capt. Mallet, and which has argillaceous earths, which forcing been frequently corroborated, a its way through the surface, and fluid bitumen oozes up and rises to afterwards becoming inspissated the surface of the water on both by exposure to the air, may have sides of the island, not where the occasioned such scenes as I have sea has encroached on and overventured to describe. The only whelmed the ready-formed land, other country accurately resem- but where it is obviously in a very bling this part of Trinidad, of rapid manner depositing and formwhich I recollect to have read, is ing a new soil. that which borders on the Gulf of Taman in Crim Tartary: from the representation of travellers, springs of naphtha, and petroleum equally abound, and they describe volcanic mounds precisely similar to those of Point Icaque. Pallas's explanation of their origin seems to me very satisfactory; and I think it not improbable that the

From a consideration of the great hardness, the specific gravity, and the general external characters of the specimens submitted a few years ago to the examination of Mr. Hatchett, that gentleman was led to suppose that a considerable part of the aggregate mass at Trinidad was not pure mineral pitch or asphaltum, but rà

Vide Universal Magazine for February, 1808, Mrs. Guthrie's. Tour in the Tauride, or Voyages de Aallas.

[ocr errors][merged small]

ther a porous stone of the argillaceous genus, much impregnated with bitumen. Two specimens of the more compact and earthy sort, analysed by Mr. Hatchett, yielded about 32 and 36 per cent. of pure bitumen: the residuum in the crucible consisted of a spongy, friable and ochraceous stone; and 100 parts of it afforded, as far as could be determined by a single trial, of silica 60, alumina 10, oxide of iron 10, carbonaceous matter by estimation 11; not the smallest traces of lime could be discovered; so that the substance has no similarity to the bituminous limestones which have been noticed in different parts of the world.* I have already remarked, that this mineral production differs considerably in different places. The specimens examined by Mr. Hatchett by no means correspond in character with the great mass of the lake, which, in most cases, would doubtless be found to be infinitely more free from combination with earthy substances; though from the mode of origin which I have assigned to it, this intermixture may be regarded as more or less unavoidable. The analysis of the stone after the separation of the bitumen, as Mr. Hatchett very correctly observes, accords with the prevalent soil of the country; and I may add, with the soil daily deposited by the gulf, and with the composition of the porcelain jasper in immediate -contact with the bituminous mass.

All the country which I have .visited in Trinidad is either decidedly primitive or alluvial. The great northern range of mountains

which runs from east to west, and is connected with the Highlands of Paria on the continent by Islands at the Bocas, consists of gneiss, of mica slate containing great masses of quartz, and in many places approaching so much to the nature of talc as to render the soil quite unctuous by its decomposition, and of compact blueish gray limestone, with frequent veins of white crystallized carbonate of lime. From the foot of these mountains, for many leagues to the southward, there is little else than a thick fertile argillaceous soil, without a stone or a single pebble. This tract of land, which is low and perfectly level, is evidently formed by the detritus of the mountains, and by the copious tribute of the waters of the Orinoco, which being deposited by the influence of currents, gradually accumulates; and, in a climate where vegetation is astonishingly rapid, is speedily covered with the mangrove and other woods. It is accordingly observed, that the leeward side of the island constantly encroaches on the gulf, and marine shells are frequently found on the land at a considerable distance from the sea. This is the character of Naparima and the greater part of the country I saw along the coast to la Braye. It is not only in forming and extending the coast of Trinidad, that the Orinoco exerts its powerful agency; cooperating with its mighty sister flood, the Amazons, it has manifestly formed all that line of coast and vast extent of country included between the extreme branches of each river. To use

* Vide Linnean Trans. vol. viji.

the

the language of a writer in the Philosophical Transactions of Edinburgh: "If you cast your eye upon the map, you will observe from Cayenne to the bottom of the Gulf of Paria this immense tract of swamp, formed by the sedi ment of these rivers, and a similar tract of shallow muddy coast, which their continued operation will one day elevate. The sediment of the Amazons is carried down thus to leeward (the westaward) by the constant currents which set along from the southward and the coast of Brazil. That of the Oroonoko is detained and allowed to settle near its mouths by the opposite island of Trinidad, and still more by the mountains on the main, which are only separated from that island by the Bocos del Drago. The coast of Guiana has remained, as it were, the great eddy or resting-place for the washings of great part of South America for ages; and its own comparatively small streams have but modified here and there the grand deposit."*

Having been amply gratified with our visit to this singular place, which to the usual magnificence of the West Indian landscape unites the striking peculiarity of the local scene, we re-embarked in our vessel, and stood along the

coast on our return.

On the way we landed, and visited the plantations of several gentlemen, who received us with hospitality, and made us more fully acquainted with the state of this island: a colony which may with truth be described as fortunate in its situa tion, fertile in its soil, and rich beyond measure in the productions

of nature; presenting, in shorty by a rare combination, all which can gratify the curiosity of the naturalist, or the cupidity of the planter; restrained in the develope ment of its astonishing resources, only by the inadequacy of population, the tedious aud ill-defined forms of Spanish justice, and the severe, though we may hope transient, pressure of the times.

ACCOUNT OF THE SOUFFRIERL
OF MONTSERRAT. By Nicholas
Nugent, M. D.

From the same Work,

On my voyage last year (Oct. 1810) from Antigua to England, the packet touched at Montserrat, and my curiosity having been excited by the accounts I received of a place in the island called the Sulphur, and which, from the descriptions of several persons, I conceived might be the crater of an inconsiderable volcano, I determined to avail myself of the stay of the packet to visit that place. A

The island of Montserrat, so called by the Spaniards from a fancied resemblance to the cele brated mountain of Catalonia, is every where extremely rugged and mountainous, and the only roads, except in one direction, are narrow bridle-paths winding through the recesses of the mountains there is hardly a possibility of using wheeled carriages, and the produce of the estates is brought to the place of shipment on the backs of mules Accompanied by a friend, I ac cordingly set out on horseback from the town of Plymouth, which is situated at the foot of the moun

[ocr errors]

Vide Mr. Lochhead's Obsery. on the Nat. Hist. of Guiana. Edin. Trans. vol. lv.

tain

tains on the sea-shore. We proceeded by a circuitous and steep route about six miles, gradually ascending the mountain, which consisted entirely of an uniform porphyritic rock, broken every where into fragments and large blocks, and which in many places was so denuded of soil, as to render it a matter of astonishment how vegetation, and particularly that of the cane, should thrive so well. The far greater part of the whole island is made up of this porphyry, which by some systematics would be considered as referable to the newest floëtz trap formation, and by others would be regarded only as a variety of lava. It is a compact and highly indurated argillaceous rock of a grey colour, replete with large and perfect crystals of white felspar and black hornblende. Rocks of this description generally pass in the West Indies by the vague denomination of fire-stone, from the useful property they possess of resisting the operation of intense heat. A considerable quantity of this stone is accordingly exported from Montserrat to the other islands which do not contain it, being essential in forming the masonry around the copper-boilers in sugarworks. We continued our ride a considerable distance beyond the estate called Galloway's (where we procured a guide), till we came to the side of a very deep ravine which extends in a winding direction the whole way from one of the higher mountains to the sea. A rugged borse path was traced along the brink of the ravine, which we followed amidst the most beautiful and romantic scenery. At the head of this ravine is a small am

[ocr errors]

phitheatre formed by lofty surrounding mountains, and here is situated what is termed The Sulphur. Though the scene was extremely grand, and well worthy of observation, yet I confess I could not help feeling a good deal disappointed, as there was nothing like a crater to be seen, or any thing else that could lead me to suppose the place had any connexion with a volcano. On the north, east and west sides were lofty mountains wooded to the tops, composed apparently of the same kind of porphyry we had noticed all along the way. On the south, the same kind of rock of no great height, quite bare of: vegetation, and in a very peculiar. state of decomposition. And on the south-eastern side, our path. and the outlet into the ravine. The whole area, thus included, might be three or four hundred yards in length, and half that distance in breadth. The surface of the ground, not occupied by the ravine, was broken and strewed with fragments and masses of the porphyritic rock, for the most part so exceedingly decomposed, as to be friable, and to crumble on the smallest pressure. For some time I thought that this substance, which is perfectly white, and in some instances exhibits an arrangement like crystals, was a peculiar mineral; but afterwards became couvinced that it was merely the porphyritic rock singularly altered, not by the action of the air or weather, but, as I conjecture, by a strong sulphureous or sulphuric acid vapour, which is generated here, and which is probably driven more against one side by the eddy wind up the ravine, the breeze

from

from any other quarter being shut out by the surrounding hills.

Amidst the loose stones and fragments of decomposed rock are many fissures and crevices, whence very strong sulphureous exhalations arise, and which are diffused to a considerable distance: these exbalations are so powerful, as to impede respiration, and near any of the fissures are quite intolerable and suffocating. The buttons of my coat, and some silver and keys in my pockets, were instantaneously discoloured. An intense degree of heat is at the same time evolved, which, added to the apprehension of the ground crumbling and giving way, renders it difficult and painful to walk near any of these fissures. The water of a rivulet, which flows down the sides of the mountain, and passes over this place, is made to boil with violence, and becomes loaded with sulphureous impregnations. Other branches of the same rivulet,which do not pass immediately near these fissures, remain cool and limpid; and thus you may with one hand touch one rill, which is at the boiling point, and with the other band touch another rill, which is of the usual temperature of water in that climate. The exhalations of sulphur do not at all times proceed from the same fissures, but new ones appear to be daily formed, others becoming, as it were, extinct. On the margins of these fissures, and indeed almost over the whole place, are to be seen most beautiful crystallizations of sulphur, in many spots quite as fine and perfect as those from Vesuvius, or indeed as any other specimens I have ever met with. The

whole mass of decomposed rock in the vicinity is, in like manner, quite penetrated by sulphur. The specimens which I collected of the crystalized sulphur, as well as of the decomposed and undecom. posed porphyry, were left inad. vertently on board the packet at Falmouth, which prevents my hav ing the pleasure of exhibiting them to the society. I did not perceive at this place any trace of pyrites, or any other metallic substance, except indeed two or three small fragments of day iron-stone at a little distance, but did not discover even this substance any where in situ. It is very probable that the bed of the glen or ravine might throw some light on the internal structure of the place; but it was too deep, and its banks infinitely too precipitous, for me to venture down to it. I understood that there was a similar exhalation and deposition of sulphur on the side of a mountain not more than a mile distant in a straight line; and a subterranean communication is supposed to exist between the two places.

Almost every island in the western Archipelago, particularly those which have the highest land, has, in like manner, its Sulphur, or, as the French better express it, its Souffrière. This is particularly the case with Nevis, St. Kitt's, Gusdaloupe, Dominica, Martinico, St. Lucia, and St. Vincent's. Some islands have several such places, analogous, I presume, to this of Montserrat; but in others, as Guadaloupe, St. Lucia, and St. Vincent's, there are decided and well characterized volcanos, which are occasionally active, and throw out

ashes,

« TrướcTiếp tục »