72 Ascension. 



PAEI I. 



given by Beudant. 1 and that by Humboldt, of the same 

 formation in Mexico and Peru. 2 and likewise the descrip- 

 tions given by several authors 3 of the trachytic regions 

 in the Italian islands, agree with my observations at 

 Ascension. Many passages might have been transferred 

 without alteration from the works of the above authors, 

 and would have been applicable to this island. They 

 all agree in the laminated and stratified character of 

 the whole series : and Humboldt speaks of some of the 

 beds of obsidian being ribboned like jasper. 4 They all 

 agree in the nodular or concretionary character of the 

 obsidian, and of the passage of these nodules into layers. 

 They all refer to the repeated alternations, often in mo- 

 dulatory planes, of glassy, pearly, stony, and crystalline 

 layers : the crystalline layers, however, seem to be much 

 more perfectly developed at Ascension, than in the 

 above-named countries. Humboldt compares some of 

 the stony beds, when viewed from a distance, to strata 

 of a schistose sandstone. Sphaerulites are described as 

 occurring abundantly in all cases ; and they everywhere 



1 ' Voyage en Hongrie,' torn. i. p. 330 ; torn. ii. pp. 221 and 315 ; 

 torn. iii. pp. 36^, 371, 377, 381. 



2 'Essai Geognostique,' pp. 176, 326, 328. 



3 P. Scrope, in ' Geological Transactions,' vol. ii. (second series) 

 p. 195. Consult, alfo, Dolimieu's 'Voyage aux Isles Lipari/ and 

 D'Aubuisson, 'Traite de Geogn.' torn. ii. p. 534. 



4 In Mr. Stokes' fine collection of obsidians from Mexico, I 

 observe that the sphaernlites are generally much larger than those of 

 Ascension : they are generally white, opaque, and are united into 

 distinct layers : there are many singular varieties, different from 

 any at Ascension. The obsidians are finely zoned, in quite straight 

 or curved lines, with exceedingly slight differences of tint, of 

 cellularity, and of more cr less perfect degrees of glassiness. Tracing 

 some of the less perfectly glassy zones, they are seen to become 

 studded with minute white sphaerulites, which become more and 

 more numerous, until at last they unite and form a distinct layer : 

 on the other hand, at Ascension, only the brown sphasrulites unite 

 and form layers ; the white ones always being irregularly dis- 

 seminated. Some specimens at the Geological Society, said to 

 belong to an obsidian formation from Mexico, have an earthy frac- 

 ture, and are divided in the finest parallel laminae, by specks of a 

 black mineral, like the augitic or hornblendic specks in the rocks at 

 Asceusion. 



