jo Ascension. 



PAKT I. 



oxide of iron : so that the foregoing glassy concre- 

 tionary substances all contain a larger proportion of 

 silica than that occurring in ordinary feldspathic or 

 trackytic rocks. DAnbuisson, 1 also, has remarked on 

 the large proportion of silica compared with alumina, 

 in six analyses of obsidian and pearlstone given in 

 Brongniart's k ' Mineralogy. 3 Hence I conclude, that the 

 foregoing concretions have been formed by a process of 

 a go-re station, strictly analogous to that which takes 

 place in aqueous deposits, acting chiefly on the silica, 

 but likewise on some of the other elements of the sur- 

 rounding mass, and thus producing the different con- 

 cretionary varieties. From the well-known effects of 

 rapid cooling 2 in giving glassiness of texture, it is 

 probably necessary that the entire mass, in cases like 

 that of Ascension, should have cooled at a certain rate; 

 but considering the repeated and complicated alterna- 

 tions of nodules and thin layers of a glassy texture 

 with other layers quire 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 3 very closely 



1 ' Trait e de Geogn.' torn. ii. p. 535. 



2 This is seen in the manufacture of common glass, and in 

 Gregory Watts's experiments on molten trap; also on tne natural 

 surfaces of lava-streams, and on the side-walls of dikes. 



3 I do not know whether it is generally known, that bodies having 

 exactly the same appearance as sphserulites, 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, presented 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 occa-io rally 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 sphasrulite often lie in 

 different zones of colour, but they are not cut off by them, as in the 

 agate. 



