50 Ascension. 



PAET I. 



into a pale greenish r gray variety, differing only in its 

 colour, and in not beiog so earthy ; the passage was in 

 one case effected insensibly ; in another, it was formed 

 by numerous, rounded and angular, masses of the green- 

 ish variety, being embedded in the white variety ; — in 

 this latter case, the appearance was very much like that 

 of a sedimentary deposit, torn up and abraded during 

 the deposition of a subsequent stratum. Both these 

 varieties are traversed by innumerable tortuous veins 

 (presently to be described), which are totally unlike 

 injected dikes, or indeed any other veins which I have 

 ever seen. Both varieties include a few scattered frag- 

 ments, large and small, of dark-coloured scoriaceous 

 rocks, the cells of some of which are partially filled with 

 the white earthy stone ; they likewise include some 

 huge blocks of a cellular porphyry. 1 These fragments 

 project from the weathered surface, and perfectly re- 

 semble fragments embedded in a true sedimentary tuff. 

 But as it is known that extraneous fragments of cellu- 

 lar rock are sometimes included in columnar trachyte, 

 in phonolite, 2 and in other compact lavas, this circum- 

 stance is not any real argument for the sedimentary 

 origin of the white earthy stone. 3 The insensible 

 passage of the greenish variety into the white one, 

 and likewise the more abrupt passage by fragments of 

 the former being embedded in the latter, might result 



1 The porphyry is dark coloured ; it contains numerous, often 

 fractured, crystals of white opaque feldspar, also decomposing 

 crystals of oxide of iron ; its vesicles include masses of delicate, 

 hair-like, crystals, apparently of analcime. 



2 D'Aubuisson, ' Traite de Geognosie,' torn. ii. p. 54S. 



3 Dr. Daubeny (on Volcanos, p. 180) seems to have bsen led to 

 believe that certain trachytic formations of Iscnia and of the Puy 

 de Dome, which closely resemble these of Ascension, were of sedi- 

 mentary origin, chiefly from the frequent presence in them ' of 

 scoriform portions, different in colour from the matrix.' Dr. Daubeny 

 adds, that on the other hand, Brocchi, and other eminent geologists, 

 have considered these beds as earthy varieties of trachyte ; he con- 

 siders the subject deserving of further attention. 



