Mr. PouLETT ScROPE oTi thc Gcologt/ of the Ponza Isles. 235 



elevate and g-radually leave dry the shelving- shore of the coast between Ter- 

 racina and Ostia. This sand^ whether taken from the actual shore^ or from 

 the interior of the Pomptine marshes^ where it extends almost as far as Cis- 

 ternal is found to be composed of grains of reddish flinty of milk quartZj and of 

 amethystine quartz, with a few fragments of felspar crystals and grains of 

 magnetic iron. This composition shows it to proceed from the harder detritus 

 of the Ponza isles^ or at least of the submarine rocks of the same nature with 

 which they are doubtless connected. 



Thus while the violent sweep of the south-westerly swell is engaged conti- 

 nually in degrading and eating away the already skeleton forms of these ex- 

 posed rocks, their water-worn fragments are drifted onwards by the same 

 force, and thrown up on the shore of the neighbouring continent, which gains 

 in extent of surface even more rapidly than the islands lose. It is thus that 

 the great law of organic nature exhibits itself with equal force in the mineral 

 kingdom ; the destruction of one formation giving rise to the production of 

 another. 



It might appear like an omission were I to terminate this paper without a 

 word upon the ages of the formations which have been described in it. I am 

 of opinion that most of our notions on the absolute age of the pyrogenous 

 rocks must be necessarily of a vague, uncertain, and hypothetical character. 

 Whatever doubts may exist as to the exact manner in which many of these 

 rocks have been produced on the surface of the globe, still enough is known 

 and generally recognized concerning the phasnomena in which they originate, 

 to show that with them the collocation of similar beds is no proof of their con- 

 temporaneous production. In the sedimental and arenaceous rocks, we infer 

 the age of a whole stratum or series of strata from the observed geological 

 relations of one of their parts, notwithstanding the solutions of continuity or 

 the concealment of intervening portions, that may separate this from the re- 

 maining mass. But amongst volcanic rocks we know that by the side of a 

 mass of early date, one of recent formation may be produced tomorrow, ex- 

 actly similar to the former in mineral composition. And in reality the natural 

 tendency of the volcanic phaenomena is by no means to produce vast and ex- 

 tensive formations at a single epoch ; but on the contrary, to form partial 

 masses, one by one, at very distant intervals of time, which yet often agree 

 most closely in their mineralogical characters*. 



* This is a consideration too much overlooked by some late authors, who, assuming the con. 

 temporaneous formation of whole districts of trachyte or basalt, have inferred their place in the 

 geological series by the observed relations of one or two isolated members. 



