54 Ascension. 



PAET I. 



pared with, the air-vesicles in the basaltic lava) and 

 other irregular hollows, apparently produced by decay, 

 were. filled with concentric layers of chalcedony ; in this 

 case, there can be little doubt that the same fluid 

 deposited the homogeneous base and the chaicedonic 

 layers. After these considerations, I cannot doubt but 

 that the jasper of Ascension may be viewed as a volcanic 

 rock silicified, in precisely the same sense as this term 

 is applied to wood, when silicified ; we are equally 

 ignorant of the means by which every atom of wood, 

 whilst in a perfect state, is removed and replaced by 

 atoms of silica, as we are of the means by which the 

 constituent parts of a volcanic rock could be thus acted 

 on. 1 I was led to the careful examination of these rocks, 

 and to the conclusion here given, from having heard 

 the Rev. Professor Henslow express a similar opinion, 

 regarding the origin in trap-rocks of many chalcedonies 

 and agates. Siliceous deposits seem to be very general, 

 if not of universal occurrence, in partially decomposed 

 trachytic tuffs ; 2 and as these hills, according to the 

 view above given, consist of trachyte softened and 

 altered in situ, the presence of free silica in this case 

 may be added as one more instance to the list. 



Concretions in pumiceous tuff. — The hill, marked 



1 Beudant ('Voyage en Hongrie,' torn. iii. pp. 502, 504) describes 

 kidney-shaped masses of jasper-opal, which either blend into the 

 surrounding trachytic conglomerate, or are embedded in it like chalk- 

 flints ; and he compares them with the fragments of opalised wood, 

 which are abundant in this same formation. Beudant, however, 

 appears to have viewed the process of their formation rather as one 

 of simple infiltration than of molecular exchange ; but the presence 

 of a concretion, wholly different from the surrounding matter, if not 

 formed in a pre-exisring hollow, clearly seems to me to require, either 

 a molecular or mechanical displacement of the atoms, which occu- 

 pied the space afterwards filled by it. The jasper-opal of Hungary 

 passes into chalcedony, and therefore in this case, as in that of Ascen- 

 sion, jasper seems to be intimately related in origin with chalcedony. 



2 Beudant (' Voyage Min.' torn. iii. p. 507) enumerates cases in 

 Hungary, Germany, Central France, Italy, Greece, and Mexico. 



