Art. VIII. Notices of Geology in the West-Indies.
Art. VIII. Notices of Geology in the West-Indies.
REMARKS.
In the former Number of this work, a notice was published respecting siliceous petrifactions of wood, from Antigua. We now publish a geological sketch of the island, with notices of some other parts of the West Indies. This communication is made by a friend, with permission to publish it. It is a production of the pen of Dr. Nugent, of St. Johns, Antigua, a gentleman of eminent scientific acquirements, who, it is hoped, will continue his laudable and able efforts to illustrate the natural history of the West-Indies.
Memorandum concerning the Geology of Antigua, &c.
The southern and more mountainous part of the island consists of trap rocks; more particularly of trap breccia and wacké-porphyry. On these beds rests a series of very peculiar stratified conglomerate rocks. These strata vary exceedingly in colour and thickness, but all dip, at a considerable angle, to the northwest. The more usual character of this rock, is that of a clayey basis, with minute particles of felspar, and small spots of grünerde[19] (or chlorite Baldogée.) This latter is frequently diffused over the whole, and gives it a green tinge: the colour has been thought by some to proceed from the impregnation of copper, but I am rather of opinion that is owing to manganese and iron. The conglomerate character of this rock, is derived from its having imbedded in it, or incorporated with it, numerous fragments, of all sizes, of petrified wood, chert, with and without coralline impressions, agate, jasper, amygdaloid, greenstone, hornstone, porphyry, porphyry slate, and other substances.
On this singular class of strata, reposes an extensive calcareous formation, occupying the northern and eastern part of the island, having subordinate to it, and at its lowest part, where it is in contact with the conglomerate, large beds and patches of chert, which contains also a vast variety of petrified woods, several of which are of the palm tribe, with silicified shells, chiefly cerithea; though at the Church-hill, at St. Johns, formed of this chert, casts of bivalve and ramose madrepores are likewise found. The calcareous beds are principally of a friable marl, with blocks and layers of limestone irregularly included. In this formation[20] are many fossil shells, both in the calcareous and siliceous state; and there appear to be some beds, wherein is a mixture of shells of marine, and others of a fresh water, or at least a terrestrial origin. The coralline agates found in nodules and patches therein, and which may readily be distinguished from the coralline chert of the previous beds, are the most beautiful which have any where been yet noticed; and when well selected and polished, make very pleasing ornaments.
The island, as well as Barbuda, thirty miles to the northward, the Grande Terre part of Guadaloupe, at a similar distance to the southward and eastward, with several others of the West-India Islands, give proof of an extensive formation, more recent than those to which naturalists have heretofore principally confined their' attention; and which is, perhaps, contemporaneous with, if not later than, the Paris Basin, so well described by Cuvier and Brongniart.
April 10th, 1818. N. N.
N. B. A few specimens are sent.
REMARKS.
If the above paper be read attentively, in connexion with that in No. 1. on the petrified wood of Antigua, it will afford some very curious information to the geologist respecting these petrifactions, and must lead to interesting speculations respecting their origin, under circumstances so very peculiar, and to which we do not recollect to have heard of any parallel.