22 WHITE ROCKS. [chap. 



island of Fernando Noronha. The highest point is onl 

 fifty feet above the level of the sea, and the entire circui 

 ference is under three-quarters of a mile. This small poii 

 rises abruptly out of the depths of the ocean. Its mineral 

 ogical constitution is not simple ; in some parts the roclj 

 is of a cherty, in others of a felspathic nature, includinij 

 thin veins of serpentine. It is a remarkable fact, that 

 the many small islands, lying far from any continent, in thij 

 Pacific, Indian, and Atlantic Oceans, with the exception 

 the Seychelles and this little point of rock, are, I believe 

 composed either of coral or of erupted matter. The volcanif 

 nature of these oceanic islands is evidently an extension of 

 that law, and the effect of those same causes, whether 

 chemical or mechanical, from which it results that a vast 

 majority of the volcanoes now in action stand either near 

 sea-coasts or as islands in the midst of the sea. 



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 sea-fowl, and partly to a coating of a hard 

 glossy substance with a pearly lustre, 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. It contains much animal matter, and its origin, no 

 doubt, is due to the action of the rain or spray on the birds' 

 dung. Below some small masses of guano at Ascension, 

 and on the Abrolhos Islets, I found certain stalactitic 

 branching bodies, formed apparently in the same manner 

 as the thin white coating on these rocks. The branching 

 bodies so closely resembled in general appearance certain 

 nulliporae (a family of hard calcareous sea-plants), that in 

 lately looking hastily over my collection I did not perceive the 

 diffet;ence. The globular extremities of the branches are of a 

 pearly texture> like the enamel of teeth, but so hard as just 

 to scratch plate-glass. I may here mention, that on a part of 

 the coast of Ascension, where there is a vast accumulation of 

 shelly sand, an incrustation is deposited on the tidal rocks, 

 by the water of the sea, resembling certain cryptogamic 

 plants {MarchanticB) often seen on damp walls. The surface 

 of the fronds is beautifully glossy ; and those parts formed 

 where fully exposed to the light, are of a jet black colour, 

 but those shaded under ledges are only gray. I have shown 

 specimens of this incrustation to several geologists, and 

 they all thought that they were of volcanic or igneous 



