Volcanic Islands visited during the Voyage of the Beagle. 353 



tlie separation from the surrounding mass of the whole or part of some 

 mineral substance, and its aggregation round certain points of attraction. 

 Guided by this fact, I have endeavoured to discover whether obsidian and 

 the sphaTulites (to which may be added marekanite and pearlstone, both of 

 them occurring in nodular concretions in thetrachytic series) differ in their 

 constituent parts from the minerals generally composing trachytic rocks. 

 It appears from three analyses, that obsidian contains on an average 76 per 

 cent, of silica ; from one analysis, that sphaerulites contain 79*12 ; from two, 

 that marekanite contains 79'25 ; and from two other analyses, that pearl- 

 stone contains 75'62 of silica*. Now, the constituent parts of trachyte, as 

 far as they can be distinguished, consist of felspar, containing 65*21 of 

 silica; or of albite, containing 69*09; of hornblende, containing 55*27 f. 

 and of oxide of iron : so that the foregoing glassy concretionary substance 

 all contain a larger proportion of silica than that occurring in ordinary 

 felspathic or trachytic rocks. D'Aubuissoni, also, has remarked on the 

 large proportion of silica compared with alumina, in six analyses of obsi- 

 dian and pearlstone given in Brongniart's Mineralogy. Hence I conclude, 

 that the foregoing concretions have been formed by a process of aggrega- 

 tion, strictly analogous to that which takes place in aqueous deposits, act- 

 ing chiefly on the silica, but likewise on some of the other elements of the 

 surrounding mass, and thus producing the different concretionary varieties. 

 From the well-known effects of rapid cooling§ in giving glassiness of tex- 

 ture, it is probably necessary that the entire mass, in cases like that of As- 

 cension, should have cooled at a certain rate; but considering the repeated 

 and complicated alternations, of nodules and thin layers of a glassy texture 

 with other layers quite stony or crystalline, all within the space of a few feet 

 or even inches, it is hardly possible that they could have cooled at different 

 rates, and thus have acquired their different textures. 



"The natural sphaerulites in these rocks ||, very closely resemble those 

 produced in glass when slowly cooled. In some fine specimens of par- 

 tially devitrified glass, in the possession of Mr. Stokes, the sphaerulites are 

 united into straight layers with even sides, parallel to each other, and to 

 one of the outer surfaces, exactly as in the obsidian. These layers some- 

 times interbranch and form loops; but I did not see any case of actual in- 

 tersection. They form the passage from the perfectly glassy portions, to 

 those jiearly homogeneous and stony, with only an obscure concretionary 



* " The foregoing analyses are taken from Beudant, Traite de Mineralogie, 

 tom.ii. p. 113; and one analysis of obsidia/i, from Phillips's Mineralogy." 



t " These analyses are taken fromVon Kobell's Grundzuge der Mineralogie, 

 1838." 



t ** Traiti de G 'ogn. torn. ii. p. 535." 



(j " This is seen in the manufactory of common glass, and in Gregory 

 Watts's experiments on molten trap; also on the natural surfaces of lava- 

 streams, and on the side-walls of dikes." 



II " I do not know whether it is generally known, that bodies having ex- 

 actly the same appearance as sphaerulites sometimes occur in agates. Mr. 

 Robert Brown showed me in an agate, formed within a cavity in a piece of 

 silicified wood, some little specks, which were only just visible to the naked 

 eye : these specks, when placed by him under a lens of high power, pre- 

 sented a beautiful appearance: they were perfectly circular, and consisted 

 of the finest fibres of a brown colour, radiating with great exactness from 

 a common centre. These little radiating stars are occasionally intersected, 

 and portions are quite cut off by the fine, ribbon-like zones of colour in the 

 agate. In the obsidian of Ascension, the halves of a sphaerulite often lie 

 in different /Ones of colour, but they are not cut off by them as in the agate." 



Phil. Mag. S 3. Vol. 26. No. 173. April \84-5. 2 B 



