Transactions of the Geological Society, 1st series, vol. 3/On the Geology of Barbadoes

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On the Geology of Barbadoes
by Joseph Skey

V. Some Remarks upon the Structure of Barbadoes, as connected with Specimens of its Rocks.

By Joseph Skey, M.D. Physician to the Forces.

Read June 19th, 1812.

It is not my intention to offer to the Society a mineralogical description of those West Indian islands from which I have brought specimens; for, independently of the peculiar difficulties attending close and accurate researches of this nature in such a climate, my knowledge of the subject is too limited, and I am too little conversant with such descriptions to venture upon any but what the specimens themselves would seem to call for, in order to explain the circumstances under which they were collected. As however my friend Mr. Aikin has requested me to copy the few and imperfect notes which I have taken upon the subject, I beg leave to present them to the Society, together with the specimens, trusting that they will be considered as little else than their accompanying catalogue.

I would beg permission to state that Barbadoes, which furnished the specimens under notice, is an island totally unlike those immediately near it, both in appearance and in structure, as will be evident, when I lay before the Society specimens of the rocks of St. Vincent and St. Lucie in my possession.

The land is seen to rise in a gentle swell from the coast towards the middle of the island, excepting in a small district hereafter to be noticed; its highest hills have no great elevation, probably not exceeding eight or nine hundred feet, and their general direction is I think nearly north-west and south-east: its shores have no bold promontories nor rocky headlands, excepting in some few spots upon the windward, or north—eastern coast, which indeed is of a bolder character, as is the case with all the islands in these seas of which I have knowledge; perhaps too the shore is more abrupt at the opposite extremity of the island, the line of hills, which may be said in a general way to pass through the middle of the island, terminating here also in rocks of moderate height.

I understand that Barbadoes is similar in appearance and in structure to a few of the other islands in this Archipelago; to wit, that half of Guadaloupe which is called Grande-Terre, and which indeed forms a separate island from Basse-Terre, the two divisions having a channel, occasionally, if not always, filled. Marigalante, Antigua, and Santa Cruz also have a common structure with Barbadoes; they all agree in being of moderate elevation, have no volcanic traces, and are all formed of limestone rock; of this however I have no personal knowledge. Barbadoes is in great part composed of fossil madrepores, and traces of organic structure are to be met with in almost every part of the island. These remains are particularly discoverable along the whole of the south and south-west or leeward coast, and here I think the rocks assume a form which, although it obtains more or less in every part of the island, is here most discernible. The land, which when seen from the sea appears to rise uniformly from the coast, is observed on a nearer view to consist of successive terraces rising in two or three gradations one above the other; each terrace forming a plain of a quarter or half a mile in breadth, and terminated by a cliff of coral rock, varying in elevation from twelve to more than twenty feet; sometimes indeed having a considerably greater height. Although these terraces are in general bounded by cliffs which run parallel to the line of the coast, yet they are not wholly uninterrupted, for here and there the high grounds advance towards the shore, and break their continuity for a short space, where the terraced form of the land is again taken up.

The rocky boundaries to each terrace are formed by broken pieces of madrepores of different species thrown about in great confusion, held together by a calcareous cement of greater or less hardness. Such are the madrepore with contiguous round cells, No. 1. and the meandritical madrepore bored by mytili, No. 2.

Near the garrison of St. Ann's, and to the eastward of it, the rocks bear the character of a dull compact chalky-looking limestone, with ramose Alcyonia, No. 3, while considerably to the westward, as in the parish of St. Peter's, the rock is more distinctly coralloidal: it contains also some natural caverns, from which very perfect organic remains may be procured.

In sinking a well at the Naval Hospital, a little to the north-east of St. Ann's, the following specimens were met with in the order in which they are enumerated, exhibiting in a striking manner the increasing compactness of the rock, in proportion to the pressure of the incumbent mass.

1. A madrepore with contiguous round cells.

2. A madrepore with detached round cells.

3. A hard porous mass with terebratulites and lenticular concretions, which are perhaps organic remains.

4. A limestone analogous to the preceding but much harder.

In no part of this district, including almost all the leeward or south and south-west side of the island, did I ever detect any other rock than limestone, excepting that at very low water a bed of calcareous sandstone is to be seen. This rock is observed to dip to the south-west at an angle of about 25° or 30°. There is no coral rock incumbent upon it, but a small section of the broken strata is here and there observable. Such appearances are to be met with about a mile to the east of St. Ann's, and again to the westward of Bridgtown, near to Black Rock.

Towards the interior of the island this terraced appearance becomes less observable, and here too the limestone rock loses somewhat of its traces of organic structure, and occasionally even passes into a calcareous tufa.

In the windward parish of St. Philip, where also there is less of this terraced appearance, and where, if I mistake not, the hills commence which form the middle or main ridge, the rock assumes the external appearance of hard chalk; it is used for building, but effloresces on exposure to the atmosphere.

The island is almost destitute of running streams, excepting in a district which I shall hereafter notice. Upon the leeward coast I do not know of any constant stream; this no doubt is partly owing to the porous nature of the rocks, and partly to the numerous caves which are every where to be met with. These caves are sometimes of large dimensions, and in the parish of St. Thomas is one (usually shewn to strangers) which forms the bed of a subterranean stream, the source and termination of which are wholly unknown. As in all caves of this description, large stalactitic masses of fantastic forms depend from the roof.

I ought to notice another peculiarity in the features of this island which is particularly observable among the hills which slope from the central parts towards the leeward coast. The country is here intersected by deep fissures, called gullies, which have rent asunder the cliffs, and are continued across the terraces in irregular lines.

These rents in the rocks are sometimes of great depth but of little breadth, and, generally speaking, are very precipitous in their sides, so as to be quite impassable, excepting here and there. Their perpendicular sides exhibit the structure of the coralloidal rocks to a considerable depth. Their bottoms are the beds of rapid torrents in the rainy season, and almost the only places where any native wood is now to be met with.

This scantiness of wood, together with the little elevation of the island in any part, renders Barbadoes very liable to drought, much more so than any of its neighbouring islands, and the inhabitants already speculate upon the necessity of replanting with a view to increase the fall of rain; but they will not find it an easy matter to effect a growth of wood upon their arid rocks. Even in the island of St. Vincent, where the quantity of rain is so much greater, failure perpetually follows the attempt; probably because we are ignorant of the successive means that have been required to produce the luxuriant vegetation which, under natural operations, springs from rocks almost bare of mould; and because too, we attempt to produce in a few years effects which have required ages to accomplish.

Upon the northern and north-eastern side of the island is a district several miles in length, varying in breadth from half a mile to two or three miles, which differs wholly from the rest of the island in its general features. It is in fact a mountainous country in miniature, and indeed that part of it which has most of this character is called Scotland.

I never had an opportunity of exploring it minutely, but, as far as I could judge, the rocks are almost wholly calcareous, though less marked by organic remains than in the other districts. Sp. No. 15, which, from its texture and the pearly lustre of its recent fracture appears to be composed of shells and other organic remains extremely comminuted, is from St. Joseph's parish, which, although not strictly a part of Scotland, has features in common with it. Imbedded in this rock I found coarse flint, and a fine grained ferruginous sandstone is in contact with it: this latter is in nearly vertical strata, and is the only specimen of siliceous sandstone which I have met with in the island. The rocks near the spot are frequently found impregnated with bitumen.

The following peculiarities are observable in the district; the country is much more broken into hill and valley than any other part of the island, it has few or no gullies, is watered by some scanty streams, and has no traces of those successive stages which give so peculiar a character to the opposite side of the island. Its land boundary is in some places a very lofty cliff, and in every part a ridge of pretty considerable elevation, having a precipitous descent towards the north.

In the parish of St. Andrew this descent is the site of a spring upon whose surface floats the bitumen which is to be met with more or less in every part of the district, and is seen to exude through the soil: near some small hollows which have been made to collect this substance is a spring through whose muddy bottom carbonated hydrogen bubbles and burns with a lambent flame when a lighted taper is held above it.

The soil here appeared to be more argillaceous than that of the rest of the island, and indeed there is a pottery of coarse ware in Scotland, which I believe is the only part which furnishes the material for it.[1]

Additional Note by Mr. Parkinson.

The specimens Nos. 1. and 4. illustrate the nature of some fossil corals, showing that the forms in which they at present exist are not those which belonged to the same substances in their original state, and consequently ought not to affect their specific or generic distinctions.

In some of these specimens only circular or polygonal cavities occur, which possess no internal radiated structure, but have their insides more or less crenelated. Dr. Skey's specimen No. 4, is chiefly composed of empty cylindrical cavities with slightly crenulated sides; but in two of these cavities, near the center of the specimen, erect lamellæ are disposed in a stellular form, evincing that the corresponding lamellar of the other cavities have been removed by accident.

In other specimens are erect radiating lamellæ not surrounded by any parietes. Dr. Skey's specimen No. 1. is also thus composed of lamellæ forming stellular columns, between which no parietes are disposed. But in one part of this specimen the compleat form of the madrepore appears, two or three of the radiated columns being here surrounded by similar parietes with those which are seen without any columnar portion in specimen No. 4, showing that this was the original form of the madrepore, and proving the identity of the madrepore an remains in both Dr. Skey's specimens, Nos. 1.and 4.

  1. The nature of the soil in this district affects its productions, which are more those of the mountainous islands than is the case generally in Barbadoes: the plantain flourishes here and here only.