8 ST. PAUL'S EOCKS. [CHAP, j, 



abruptly out of the deptlis of the ocean. Its mineralogical consti- 

 tution is not simple ; in some parts the rock is of a cherty, in others 

 of a felspathic nature, including thin veins of serpentine. It is a 

 remarkable fact, that all the many small islands, lying far from any 

 continent, in the Pacific, Indian, and Atlantic Oceans, with the 

 exception of the Seychelles and this little point of rock, are, I be- 

 lieve, composed either of coral or of erupted matter. The volcanic 

 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 iinited 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 nulliporso (a family of hard calcareous sea- 

 plants), that in lately looking hastily over my collection I did not 



