THE VOYAGE OF THE BEAGLE 19 



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 believe, 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 bril- 

 liantly white colour. This is partly owing to the dung of a 

 vast multitude of seafowl, 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 nullipora? (a family 

 of hard calcareous sea-plants), that in lately looking hastily 

 over my collection I did not perceive the difference. 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, as represented in the woodcut, cer- 

 tain cryptogamic plants (Marchantiae) often seen on damp 

 walls. The surface of the fronds is beautifully glossy; and 

 tnose parts formed where fully exposed to the light are of a 

 jet black colour, but those shaded under ledges are only grey. 

 I have shown specimens of this incrustation to several 

 geologists, and they all thought that they were of volcanic 

 or igneous origin ! In its hardness and translucency in 



