Volcanic Islands visited during the Voyage of the Beagle. SiS 



enamel. Several other varieties are chiefly characterized, by containing in- 

 numerable threadsof dark-green serpentine, and by having calcareous mat-; 

 ler in their interstices. These rocks have an obscure, concretionary struc- 

 ture, and are full of variously-coloured angular pseudo-fragments. These 

 angular pseudo-fragments consist of the first-described dark green rock, of 

 a brown softer kind, of serpentine, and of a yellowish harsh stone, which, 

 perhaps, is related to serpentine rock. There are other vesicular, calcareo- 

 ferruginous soft stones. There is no distinct stratification, but parts are 

 imperfectly laminated; and the whole abounds with innumerable veins, 

 and vein-like masses, both small and large. Of these vein-like masses, some 

 calcareous ones, which contain minute fragments of shells, are clearly of 

 subsequent origin to the others. 



" A glossy incrustation. — Extensive portions of these rocks are coated by 

 a layer of a glossy polished substance, with a pearly lustre and of a grayish- 

 white colour; it follows all the inequalities of the surface, to which it is 

 firmly attached. When examined with a lens, it is found to consist of nu- 

 merous exceedingly thin layers, their aggregate thickness being about the 

 tenth of an inch. It is considerably harder than calcareous spar, but can 

 be scratched with a knife ; under the blow-pipe it scales off, decrepitates, 

 slightly blackens, emits a fetid odour, and becomes strongly alkaline : it 

 does not effervesce in acids*. I presume this substance has been deposited 

 by water draining from the birds' dung, with which the rocks are covered. 

 At Ascension, near a cavity in the rocks, which was filled with a laminated 

 mass of infiltrated birds' dung, I found some irregularly-formed, stalacti- 

 tical masses of apparently the same nature. These masses when broken 

 had an earthy texture, but on their outsides, and especially at their extre- 

 mities, they were formed of a pearly substance, generally in little globules, 

 like the enamel of teeth, but more translucent, and so hard as just to scratch 

 plate-glass. This substance slightly blackens under the blow-pipe, emits a 

 bad smell, then becomes quite white', swelling a little, and fuses into a dull 

 white enamel ; it does not become alkaline; nor does it effervesce in acids. 

 The whole mass had a collapsed appearance, as if in the formation of the 

 hard glossy crust, the whole had shrunk much. At the Abrolhos Islands, 

 on the coast of Brazil, where also there is much birds' dung, I found a great 

 quantity of a brown, arborescent substance adhering to some trap- rock. In 

 its arborescent form, this substance singularly resembles some of the 

 branched species of Nullipora. Under the blow-pipe, it behaves like the 

 specimens from Ascension ; but it is less hard and glossy, and the surface 

 has not the shrunk appearance." 



In the third chapter, allotted to Ascension, we have an interesting 

 notice of the volcanic bombs which are so numerous in that island, 

 and which are probably connected in their origin with the explosions 

 of aeriform matter that have covered the flanks of Green Mountain 

 and the surrounding country with a mass some hundred feet in thick- 

 ness, of loose fragments, consisting chiefly of tuflfs and pumiceous 

 brecciaf. Here the author found bombs, the internal structure of 



* " In my Journal I have described this substance ; I then believed that it 

 was an impure phosphate of lime." 



t On the nortiiern side of Green Mountain a thin seam of compact 

 oxide of iron extends o\er a considerable area, lying in the lower part of 

 the mass of fragments. " This seam of compact stone," it is remarked, 

 p. 39, " by intercepting the little rain-water which falls on the island, gives 

 rise to a small dripping spring, first discovered by Dampier. It is the only 

 fresh-water on the island, so that the possibility of its being inhabited has 

 entirely depended on the occurrence of this ferruginous layer." 



