« PreviousContinue »
in large quantities out of the rocks near the beach. Among the shingle are found large cylindrical bodies consisting of ferruginous clay coated with bitumen. Springs impregnated with sulphuretted hydrogen ooze from under the rocks. I found on the beach a worn or rounded specimen consisting of volcanic ashes, similar in character to the stratum near Skeete's Bay.
Bituminous coal, more compact than any I have anywhere else observed in Barbados, occurs to the south of the new estate at Codrington, under similar circumstances as at Grove's1. I received from the Rev. E. P. Smith, tutor of the college, a substance which one of the labourers, employed to dig for the coal, stated to have found lying among or near that substance. It has the appearance of coke, or as it were some other artificial production. It is to be regretted that we have no better evidence than that of the labourer with regard to its having been found among the coal2. On advancing further southward, we find again large masses of earthy marls on the little hill on which the chapel of St. Mark is standing; it is capped by coralline limestone, which now prevails, until near Skeete's Bay, where we discover again traces of the clays and marls of the Scotland formation. On the north-eastern bent of this bay is a seam of volcanic ashes lying under the coral rock. In the neighbourhood are large blocks of a conglomerate consisting of minute particles of quartz and comminuted shells; some of these rocks are coarser in texture than others in the same neighbourhood.
There are instances on record, that in different situations of the world portions of rocks have been found differing so much from those in the locality where they were met with, that they must have got there by accident. When examining the peculiar formation at Skeete's Bay, I found on the beach, which consists of shingle of coral, a large elliptical piece of red granite, perfectly smooth, the circumference of its longer side being three feet, and that of the shorter two feet five inches. The occurrence of this rock, which belongs to a series of which not the slightest trace is to be observed in Barbados, astonished me; and I can come to no other conclusion, but that it is part of the ballast of a ship wrecked in the neighbourhood, as no vessel could have entered the bay itself, which is entirely barricaded by coral reefs: the force of the waves, or breakers, may perhaps have thrown it on shore, where I found it imbedded3. This is not the only instance of foreign rocks having been
1 1 According to Dr. Davy, it consists of 66·7 bitumen, and 33.3 per cent. coke, with an exceedingly minute portion of ashes.
2 It was pronounced in Barbados to be the true anthracite of mineralogists: this is doubted by Professor Gustave Rose, to whom I sent a specimen for examination, and who cannot be persuaded but that it is an artificial production.
3 Mr. Darwin has drawn my attention to a circumstance which he mentions in his journal. Captain Ross found on a small "atoll" a few miles north of Keeling, in the conglomerate of coral mud, a well-rounded fragment of greenstone, rather larger
found in Barbados: Dr. Goding possesses a specimen of a primary rock, which, as far as I recollect, he picked up on a beach in Scotland, and the specimen which I found near Mount All is equally curious. In a small collection of rocks from Barbados at the Literary Society in Bridgetown, I observed a piece of porphyry, the parent rock of which appears to be foreign to Barbados.
The observations of Professor Ehrenberg on the relative age of the Scotland series from Barbados, are to the following effect :
"The comparison of the recent forms of Polycystina with those from the remarkable rocks of Barbados, and furthermore a comparison of the so-called tertiary forms of the halibiolithic Tripoli (formed entirely of marine organic remains) from Oran in Africa, Engia, Zante, as well as from several localities in Virginia and from Bermuda; finally, a comparison with the forms obtained from the marls of Caltanisetta and Castrogiovanni in Sicily; and the results of my examination of numerous samples of mud from the bottom of the sea to a depth of 1620 feet,-have induced me to believe that the forms which compose the rocks of Barbados are comparatively more foreign to the present organization of beings, and to that of the tertiary period, than to the calcareous formation of Sicily."
Professor Ehrenberg considers the calcareous formation of Sicily as belonging to the secondary period, and upon this supposition he bases his opinion respecting the age of the rocks of Barbados. A large proportion of microscopical animals from the latter place bear comparatively a greater resemblance to those from Caltanisetta than to those from any other locality he is acquainted with.
A calcareous compact gray sandstone, with numerous specks of mica (chiefly on the layers) and of fissile structure, which I found on the beach at Springfield, is considered by Ehrenberg to belong to an older formation than the other rocks from Scotland district. It is certainly the lowest in the series, and it appears rather as if it were thrown up against the other rocks.
than a man's head: he and the men with him were so much surprised at this that they brought it away and preserved it as a curiosity. "The occurrence of this one stone," says Mr. Darwin, "where every other particle of matter is calcareous, certainly is very puzzling. The island has scarcely ever been visited, nor is it probable that a ship had been wrecked there." He concluded therefore that it became entangled in the roots of some large tree, in which supposition he was confirmed by a statement of Chamisso's, who observes that the inhabitants of the Radack Archipelago, a group of lagoon islands in the midst of the Pacific, obtained stones for sharpening their instruments by searching the trees which are cast upon the beach. It is therefore probable, that since we have it on record that stones were carried on trees to the Keeling island and the isolated position of Radack, the Barbados granite, greenstones, and porphyry may have been brought in a similar way from the Orinoco. I have already stated at page 180 that the current brings the seeds of Manicaria and Astrocaryum to its shores; it is therefore no impossibility that trees are likewise carried to its coast. The specimen of granite is now in possession of Dr. Cutting, to whom I gave it before I left Barbados.
The Scalaria which I found on the summit of Bissex Hill, and the Nucula from Springfield, induced Professor E. Forbes to consider the Scotland rocks as belonging to the miocene period of tertiary strata. The mineralogical character of rocks is considered at present of little importance when conclusions respecting their age are to be formed. Still my observations on the spot, combined with the mineralogical character of the rocks, lead me to coincide in Professor Forbes's opinion. The chalks of Caltanisetta, on which Professor Ehrenberg rests his opinion that the Scotland formation in Barbados belongs to an older period than the Miocene group, have been considered by different geologists as belonging to different periods; by some they have been regarded as secondary, by others as tertiary rocks. I have no doubt that Ehrenberg's discovery of the new class of animalcules, and an examination of other rocks from the West Indies, the age of which has been better ascertained than those in Barbados, will lead to firmer conclusions than we are at present warranted to form. There can be no doubt that the whole Scotland district is an old sea bottom, of which fact the masses of marine animalcules, without a single form belonging to the freshwater organization, give the most convincing evidence.
If we now inquire into the circumstances which produced the elevation of the Scotland series of rocks, and transformed the horizontal strata of reddish-coloured and white sandstones into almost vertical and contorted series, we cannot doubt that submarine movements, or volcanic agency acting violently from a given point, gave rise to the local derangements of this formation. The strata of sandstone are more disturbed near Chalky Mount than anywhere else. The siliceous limestones have been here contorted in the most extraordinary manner without the lamination having been obliterated. Close to the contorted strata is ferruginous sandstone; otherwise the general structure of Chalky Mount, as has been mentioned previously, consists of calcareous sandstones, which contain Polythalamia. Ehrenberg makes the following remarks on this sandstone
"Where the siliceous rocks contain a combination of calcareous matter, (consequently where they are real marls) and are of a whiter appearance and more friable like chalk, I found they contained calcareous microscopical Polythalamia, which were not in such good preservation as the siliceous-shielded Polycystina. They are surrounded with calcareous morpholites, resembling those which form the finest parts of chalk used for writing. It is by no means rare to find short, slender prisms of microscopical crystals of calcareous spar among this mass."
The presence of pumiceous particles which are disseminated through the marl; and the existence of strata of volcanic ashes in the parish of St. Joseph and at Skeete's Bay render the volcanic action less doubtful, and I suppose that the line of convulsion followed from Chalky Mount a south-west direction towards Mount Hillaby. It must be observed
however that the pumiceous particles might have arisen from a similar fall of "dust" as on the 1st of May 1812.
The isolated rocks of the coral formation, which are found lying on the summit and declivity of hills in the Scotland district, remain to be considered. Their size is considerable, sometimes as much as from twelve to fifteen feet in height, never rounded, but always angular, and of the same character as the cliffs of coral-rocks. They lie generally half-buried in the ground, with their edges upturned, and as if tossed there by force. Although the cliffs of the coralline formation exceed in height (excepting Mount Hillaby) the hills in Scotland, the intervening valleys do not allow us to suppose that these masses of coral rock were detached from these cliffs, and, falling down the mountains, rolled up again to their present situation, traversing valleys and ascending acclivities before they perched upon the top of distant hills. I conjectured that the coralline crust might have once extended over the Scotland district, and that the volcanic force which produced the upheaval of the Scotland strata, might have burst the crust and hurled it partly into the depths of the sea, and partly upon the sides of the hills and their summits. The immense seawave which is known to accompany shocks of earthquakes, and commits great havoc along the coasts subjected to it, might in this instance have carried on retiring the greater portion of the coralline crust away, and left only a few as a remnant. It has been considered by others, that a similar agency as that which transported the gigantic boulders of the north of England and Germany has likewise operated here. I think it however probable, upon a reconsideration of my own supposition, that these masses have been detached from the cliffs which now border the Scotland district, previous to the upheaval of the marine bottom, and sunk on the submarine inequalities.
A DESCRIPTION OF FOSSILS FOUND IN BARBADOS. THE geologist is well-acquainted with the great value of the evidence, when judging of the age of rocks, derived from organic remains imbedded in the strata under his consideration. Neither bones of large terrestrial and marine mammalia, of birds or fishes, nor impressions or remains of plants, have been found in the rocks of Barbados; only the fossil remains
of the most minute forms of organic life, marine bivalve and univalve shells, most of them (with few exceptions) analogous to those of the present day, have been hitherto discovered in the rocks which compose the island. I shall attempt to give a general account of such fossils as I have become acquainted with during my researches in Barbados. As the animalcules constitute the lowest stage of animal life, they will form the first object of my description of the fossil remains in Barbados.
The Polycystina, or cellular animalcules of Barbados, which are quite distinct from true Infusoria, form an independent group of siliceousshielded animals. Previous to the discovery of the numerous species contained in the rocks of Barbados, Professor Ehrenberg classed the few he had previously described among siliceous Polygastrica (Infusoria). The examination of the Barbados rocks has made him acquainted with 361 species of animalcules, of which 282 belong to the group of Polycystina, which he divides into seven families, comprising forty-four genera1. It would be foreign to my object to give here a detailed description of these remarkable animalcules, or to enumerate them: I must refer the reader who is interested in such inquiries to a memoir which Professor Ehrenberg delivered before the Royal Academy of Sciences of Berlin, and a partial translation of which has been inserted in the Annals of Natural History?? Still, as far as such a description can be of general interest to the reader, I shall give it here. Infusory animalcules, or Infusoria, a denomination which is merely explanatory of their habitat but not of their structure, are organic beings so extremely minute as with few exceptions to be invisible to the naked eye. Their bodies are for the most part gelatinous, and they were formerly divided into two orders, Rotifera and Homogenea. Professor Grant, who has written on this subject, observes, that when we place a drop of any decayed infusion of animal or vegetable matter under a powerful microscope, and throw a light through that drop, and through the microscope to the eye, we discover in the drop of water various forms of living beings, some of a rounded, others of a lengthened form, and some exhibiting ramifications shooting in all directions, but all apparently of a soft transparent, gelatinous and almost homogeneous texture3.
These beings constitute the lowest forms of animals with which we are at present acquainted, and they were at first considered astomatous, that is, destitute of any mouth, and agastric, or possessing no stomach. Upon
Besides the above 282 species of Polycystina, he discovered 18 Polygastrica, 27 Phytolitharia, 27 Geolithia and 7 Polythalamia in the marls, sandstones and limestones of Barbados.
2 See Ann. of Nat. Hist. vol. xx. p. 115.
3 The most remarkable species among infusoria is the Proteus, which changes its figure momentarily, sometimes rounded, sometimes divided, so that it is impossible to assign to it any determinate form.