62 Ascension. 



PAET I, 



crustation and the shells of living molluscous animals. 1 

 This appears to me to be an interesting physiological 

 fact, 2 



Singular laminated beds alternating with and 

 passing into obsidian. — These beds occur within the 

 trachvtic district, at the western base of Green Mountain, 

 under which they dip at a high inclination. They are 

 only partially exposed, being covered up by modern 

 ejections ; from this cause, I was unable to trace their 

 junction with the trachyte, or to discover whether they 

 had flowed as a stream of lava, or had been injected 

 amidst the overlying strata. There are three principal 

 beds of obsidian, of which the thickest forms the base 

 of the section. The alternating stony layers appear to 

 me eminently curious, and shall be first described, and 

 afterwards their passage into the obsidian. They have 

 an extremely diversified appearance ; five principal 

 varieties may be noticed, but these insensibly blend 

 into each other by endless gradations. 



First, — A pale gray, irregularly and coarsely lami- 

 nated, 3 harsh- feeling rock, resembling clay-slate which 



1 In the section descriptive of St. Paul's Rocks, I have described 

 a glossy, pearly substance, which coats the rocks, and an allied 

 stalactitical iL crustation from Ascension, the crust of which re- 

 sembles the enamel of teeth, but is hard enough to scratch plate 

 glass, Both these substances contain animal matter, and seem to 

 have been derived from water intiltering through birds' dung. 



2 Mr. Horner and Sir David Brewster have described (' Philo- 

 sophical Transactions,' 1836, p. 65) a singular 'artificial substance, 

 resembling shell.' It is deposited in fine, transparent, highly- 

 polished, brown-coloured lamina?, possessing peculiar optical pro- 

 perties, on the inside of a vessel, in which cloth, first prepared with 

 glue and then with lime, is made to revolve rapidly in water. It is 

 much softer, more transparent, and contains more animal matter, 

 than the natural incrustation at Ascension; but we here again see 

 the strong tendency which carbonate of lime and animal matter 

 evince to form a solid substance allied to shell. 



3 This term is open to some misinterpretation, as it may be applied 

 both to rocks divided into lamina? of exactly the same composition, 

 and to layers firmly attached to each other, with no fissile tendency, 

 but composed of different minerals, or of different shades of colour. 



