ART. XV. Observations on the Geology of the West India Islands, from Barbadoes to Santa Cruz, inclusive. By William Maclure.

[From the Journal of the Academy of Natural Sciences at

Philadelphia.]

THIS range of islands may, in a geological point of view, be

divided into two distinct parte, one of which, occupying the eastern side, consists of a stratification of transition of rocks, partially crowned by secondary, and embraces the islands of Barbadoes, Mariegalante, Grandterre in Guadaloupe, Deseada, Antigua, St. Bartholomew, St. Martin, Anguilla, and Santa Cruz; the other part, consisting of volcanic formations, with a few partial coverings of secondary, occupies the western side of the range, including the Grenadines, St. Vincent, St. Lucia, Martinico, Dominica, Basseterre in Guadaloupe, Monserrat, Nevis, St. Christopher, St. Eustatia, and Saba, where the volcanic formation appears to terminate.

Barbadoes. The northern, southern, and western sections of this island consist of rocks, formed of an aggregate of shells and madrepore rocks, mixed with different kinds of corals, being partly consolidated into a mass by the attrition of the water, having the interstices filled by the particles that have been broken, and washed into them, sometimes even losing the marks of their original formation; and partly porous and full of cavities formed by the washing away of the shells and madrepores, and by the natural shelving of these rocks. This shell limestone is deposited in four or five horizontal strata, rising gradually to the height of eight hundred feet towards the centre of the island, and forming as many plateaux as there are strata, resembling at a distant view, the steps of stairs. Thence to the eastward or windward is the district of Scotland, composed of strata of slate alternating with limestone, and an aggregate cemented with lime, in grains of various sizes, and resembling much the different. kinds of graywacke slate, dipping to the east, northerly, and running to the north, westerly; having every appearance of VOL. V. Y

being the transition rock on which the madrepores and corals were formed.

Muriegalante, Grandterre in Guadaloupe, and Deseada, are all formed of the madrepore rock, in horizontal strata, resembling the same formation in Barbadoes, the strata being elevated, one above another, and forming a plateau or table of land, at the summit of each, but not rising so high as in Barbadoes. Grandterre in Guadaloupe has this formation, exhibiting more the appearance of undulations, with gentle ascents and declivities, containing some small streams and marshes, which would rather encourage the supposition that it rests on a volcanic basis, and is therefore more liable to have its rocks deranged from their present natural horizontal position.

Antigua. This island not having been visited by the writer, he must take its description from the specimens brought from it, by which it may be concluded, that it is similar, in some of its geological traits, to the island of Barbadoes; having the same formation of madrepore rocks, some of which contain silex in the form of agates, &c.; which are valued, as beautiful specimens, by the curious. A part of the island consists of a stratified rock, in the form of a green schist, crossing the island from north to south, in a zone of three or four miles width, affording the inhabitants a useful building stone. The southern side of the island is rugged and mountainous, and is described as being volcanic.

St. Bartholomew. The formation throughout this island is evidently stratified, though in great confusion, (the word stratified is here used in contradistinction to volcanic) the strata running in a direction a little to the west of north, and dipping generally to the eastward, as far as could be ascertained from the disturbed and irregular position of the broken rocks. These rocks are found to consist of three or four species of limestone, two of them containing shells; some aggregates, which are cemented with limestone, and present much the appearance of transition formation; several species of hornblend rock, a little crystalline; amigdaloid, containing small nodules of calcareous spar and zeolite, which, when the stone is fresh

broken, are undistinguishable from the mass, and discover their difference only when in a state of decomposition; a soft argillaceous mass, with spots of green, resembling the green earth of Verona; porphyry, with crystals of quartz and feldspar, imbedded in a red argillaceous base, &c. all of them alternating one with another occasionally, and assuming the appearance of a transition formation. But the various aspects which these rocks present, and the different stages of decomposition in which they are found, and in which they differ much from the rocks of a continent, or of northern climates, render it extremely difficult to determine which part may be secondary, and which transition.

St. Martin and Anguilla are two small stratified islands, on a line with St. Bartholomew, and consisting of a similar formation.

The island of St. Thomas may also be classed in this range. It is stratified, though in much confusion, and so deranged as to render it difficult to ascertain the general direction, which appears to be from northwest to southeast, dipping easterly. The rocks consist of a variety of aggregates, resembling the transition, some of which when fresh, have the appearance of hornblend rocks, but when beginning to decompose, the aggregate appears, with a few plates of a black crystalline rock like hornblend. I found a yellowish brown quartzy aggregate, resembling a rock, in the transition, at the Lehigh Falls in Pennsylvania.

Santa Cruz. This island, though included in our first division, agrees rather with the direction of the volcanic islands; it appears, however, that the volcanic formation ceases at Saba, and that Santa Cruz is composed of madrepore rocks at the west, and on the eastern side, of rocks similar to those of St. Thomas and St. Bartholomew. The west end and the middle of the island are low, and covered with a shell limestone and madrepore rock. The foundation on which this rock reposes a stratum that retains water, and may be a compact limestone, as the bases of many of the little hills rest on solid limestone. The east end is composed of different kinds of limestone, alternating with amygdaloid, hornblend rock, and porphyry, like the rocks of

is

St. Bartholomew; it is likewise hilly and broken, being stratified in a direction nearly north and south.

All the islands that have been described have a striking similarity both in their structure and the nature of their materials; those that are partly or wholly covered with the horizontal shell limestone, or madrepore rocks, are exactly the same; those partly or wholly formed of stratified rocks, consist of rocks more than half of which are limestone, or have considerable quantities of lime in them, and the remainder of the rocks differ very little, they have nearly the same dip and direction, have a strong characteristic mark of belonging to the transition class; though from their deranged state, and the peculiar mode of their decomposition, they differ a little in their appearance from the transition rocks of Europe, for the limestone is remarkably hard, dry, and brittle, breaking into sharp pieces, which sound like a bell, when struck with a hammer: this may perhaps be the effect of the constant heat of the climate. The different appearance which these rocks assume, when in a state of decomposition, from those of northern latitudes, may in part be attributed to the climate, and partly to the same cause which produced the great confusion in which they are now found, particularly if that cause raised them from the bottom of the ocean, and exposed them to the influence of a perpetual sun. But this, like every cause which we cannot discover, must remain only problematical; for nature has so many modes of operating, and we are as yet acquainted with so small a number of them, that our speculations beyond what we actually know, can at the best but reach to probable conjecture.

The Grenadines. This group of islands is the commencement of the second or western range: we sailed through them without stopping, so that their geological character must be taken from their general appearance, which was completely volcanic, having rocks rising perpendicularly out of the ocean, one of which is called, from its form, the organ rock, being composed of columns of basalt. The rocks are in general rugged, and so deranged, that their volcanic character could not be mistaken.

St. Vincent, like all the other volcanic islands, is composed of

a mixture of lava and cinders, in all proportions. South of Kingston, there appears to be more solid and porous lava, and less cinders than at the north. The Bay of Kingston has the appearance of being the remains of an ancient crater, the beds of lava inclining irregularly from the centre, at a considerable dip, as if it had been ejected from it. On every side, the rocks are aggregates of various kinds of roasted stones, cemented with cinders, and small atoms of scoria; and though many of the rolled rocks neither bear strong marks of fusion, nor resemble much recent lavas, yet they all have a family feature, and must be considered of volcanic origin. A substance like hornblend, with feldspar imbedded in it, forms the principal part of these rocks, which vary in colour, from nearly black to gray, the feldspar being generally crystallized, and frequently diaphanous, passing through the porous or scorious rocks without indications of having undergone much change. There are two principal modes by which the production of cinders or ashes may be accounted for; they may be thrown from the crater of a volcano during an eruption of lava, and in that case they consist of small pieces of scoria, pumice, &c. and are placed in strata of various thicknesses and colours, as if deposited by water or they may be ejected from volcanoes nearly exhausted mixed with water and rocks, forming large beds or currents of an aggregate, which is in time cemented, and wears the appearance of a breccia. A third mode is, perhaps the eruption of lava into the sea, at the commencement of submarine volcanoes, when by means of the sudden cooling, the melted lava might crumble into small angular sand, and form beds of cinders. From Kingston to the north end of the island, the same alternation of cinders and solid lava obtains, forming steep precipices and narrow valleys, the wearing and excavation of which, by the mountain torrents, is facilitated by the prevalence of the cinders which increases as you approach the Soufriere, a name given in the West Indies, to spots which indicate the remains of a subsiding volcano, and whence hot sulphureous vapours are ejected through fumerols, depositing sulphur, and converting the surrounding aluminous rocks into alum-stone, as at Solfaterra near Naples.