OCR Text |
Show 8 ST. PAUL'S ROCKS. Feb. 1832. during the morning of February 16th, close to the island of St. Paul. This cluster of rocks is situated in 0° 58' n?rth latitude, and 29° 15' west longitude. It is 540 miles distant from the coast of America, and 350 from the island of Fernando Noronha. The highest point is only fifty feet above the level of the sea, and the entire circumference is under threequarters of a mile. This small point rises abruptly out of the depths of the ocean. Its mineralogical constitution is not simple; in some parts, the rock is of a cherty, in others, of a felspathic nature ; and in the latter case it contains thin veins-of serpentine, mingled with calcareous matter. The circumstance of these rocks not being of volcanic origin is of interest, because, with very few exceptions, the islands situated in the midst of the great oceans are thus constituted. As the highest pinnacles of the great mountain ranges probably once existed as islands distant from any continent, we are led to expect that they would frequently consist of volcanic rocks. It becomes, therefore, a curious point to speculate on the changes which many of the present islands would undergo, during the lapse of the countless ages, which would be required to elevate them into snowclad summits. If we take the case of Ascension, or St. Helena, both of which have long existed in an extinct condition, we may feel assured, before so vast a period could elapse, during the whole of which the surface would be exposed to constant wear and tear, that the mere nucleus or core of the island would remain ; perhaps, every fragment of cellular rock having been decomposed, a mass of some compact stone, as phonolite or greenstone, would crown our new Chimborazo. The rocks of St. Paul appear from a distance of a brilliantly white colour. This is partly owing to the dung of a vast multitude of seafowl, and partly to a coating of a glossy white substance, which is intimately united to the surface of the rocks. This, when examined with a lens, is found to consist of numerous exceedingly thin layers, its total thickness being about the tenth of an inch. The surface is smooth Feb. 1832. ST. PAUL'S ROCKS. 9 and glossy, and has a pearly lustre; it is considerably harder than calcareous spar, although it can be scratched by a knife: under the blowpipe it decrepitates, slightly blackens and emits a fetid odour. It consists of phosphate of lime,' mingled with some impurities; and its origin without doubt is due to the acti?n of the rain or spray on the bird's dung. I may here mentwn, that I found in some hollows in the lava rocks of Ascension considerable masses of the substance called guano, which on the west coast of the intertropical parts of Sou~h America occurs in great beds, some yards thick, on the Islets frequented by seafowl. According to the analysis of Fourcroy and Vauquelin, it consists of the urates, phosp~ ates, and oxalates of lime, ammonia, and potash, together with some other salts, and some fatty and earthy matter. I believe there is no doubt of its being the richest manure which has ev~~ been disco:ered. At Ascension, close to the guano, stalactltw or botryoidal masses of impure phosphate of lime adhered to the basalt. The basal part of these had an earthy texture, but the extremities were smooth and o-lossy and ffi . 0 ' su ciently hard to scratch common glass. These stalactites appeared to have shrunk, perhaps from the removal of some soluble .matter, in the act of consolidation ; and hence they had an Irregular form. Similar stalactitic masses,* though I am not aware that they have ever been noticed, are, I believe, by no means of uncommon occurrence. We only observed two kinds of birds-the booby and the noddy. The former is a species of gannet, and the latter a tern. Both are of a tame and stupid disposition, and are so unaccustomed to visiters, that I could have killed any number of them with my geological hammer. The booby lays her eggs on the bare rock; but the tern makes a very simple nest * I may mention that I was shown, at Ascension, some very fine stalactites, composed of sulphate of lime, which had been taken out of a cavern. From their external appearance they would generally be mistaken for the ordinary calcareous kind. It was interesting to observe, in a frac~ ured specimen, the double cleavage intersecting with its even planes, the uregular layers of successive deposition. |