230 Mr. PouLETT Scbope on the Geology of the Ponza Isles. 



2ndly. That class of changes which volcanic rocks are often observed to un- 

 dergo from exposure to the action of sulphureous vapours. I am inclined to 

 beheve that the first of these processes may have partially modified the local 

 disposition of the siliceous matter, taking it up from one point, and depositing 

 it perhaps in a more perfectly crystaUized and purer form on another ; and 

 this particularly in the case of the brecciated varieties. But the siliceous base 

 of the white porphyritic trachyte and its numerous imbedded crystals of quartz, 

 can never be accounted for by the supposition of infiltration : and the parts of 

 a different texture or structure must be presumed to have derived their abun- 

 dance of silex from the same source as this, in the absence of proofs to the 

 contrary. On the other hand, there are many circumstances which discoun- 

 tenance the notion that these rocks have been subject to decomposition by 

 acid vapours. Such are their general weight and density ; the uniformly sili- 

 ceous nature of the whole mass, (whereas in altered lavas this character is con- 

 fined to the immediate limits of the fwnarole or fissures by which the vapour 

 escapes) ; the dispersion of numerous grains of quartz through the base of the 

 rock wherever it puts on a porphyritic character; the non-existence of alumi- 

 nite, which must necessarily have been produced by the union of the sulphuric 

 acid with the alumine of the rock ; and lastly, the frequent brilliancy of the 

 inclosed crystals of felspar and mica. It is true that these minerals are occasion- 

 ally altered, the first into a kaolin, the last into an ochrey powder : but it is pro- 

 bable that this change is principally owing to meteoric influence ; and at all 

 events, since it is exactly in the compactest and most siliceous parts of the rock 

 that these imbedded crystals are the most brilliant, it is obvious that the pre- 

 dominance of silex cannot be referred to subsequent alteration. We must 

 therefore conclude that this character belonged to the original constitution of 

 the rock at the period of its production. 



6. The brecciated aspect of parts of this rock, as well as of the prismatic 

 trachyte, is important from the analogy it exhibits to the occasional pseudo- 

 brecciated structure of the serpentines, the older porphyries, and granite. 

 This appearance is here evidently owing to those parts of the mass which 

 consolidated first having been broken up by the continuance or renewal of its 

 motion, and their fragments enveloped in the part which still remained liquid. 

 Similar instances of this brecciform structure are to be seen on other points of 

 the volcanic district of Italy ; as for example, in a rock of graystone lava be- 

 tween Lacco and Casamicciole in the island of Ischia, and again on the S. E. 

 flank of the rock of Cumaj. A close inspection of these and other rocks of an 

 analogous character, leads to the conclusion that a concretionary separation 

 of the constituent minerals influenced by the forces of affinity, often assisted 



