Mr. PouLETT ScROPE OH the Geology of the Ponza Isles. 223 



compact felspar or clinkstone. I met also with some pieces of a rock com- 

 posed of mica and pyroxene alone^ identical with many which occur in the 

 conglomerates of Somma. In the lower parts of the island these fragments 

 are partially water-worn. 



Throughout the whole extent of Ventotiene the upper strata of tufa are 

 more or less penetrated to a short distance from the surface by a calcareous 

 cement, producing a sort of sandstone, which is disposed in irregular horizon- 

 tal layers connected by smaller vertical veins. These are often interlaced in 

 a reticulated manner, and occasionally have the figure and structure of stalac- 

 tites, with a fistular cavity in the centre. A few fragments of shells are 

 observable in this sandstone, but too much broken to allow of their being 

 recognized. Where the volcanic sand predominates over the cement, this 

 stone is hard and solid ; where the calcareous matter abounds it is friable. In 

 some parts the grain is fine, and the rock becomes compact, and shows a semi- 

 conchoidal fracture. 



These beds appear of very recent origin, and to have resulted from the 

 deposition of calcareous particles by water filtering through strata of loose 

 volcanic sand of different degrees of fineness. But, since the rest of the island 

 is entirely volcanic, from whence are we to suppose the calcareous matter 

 derived ? It is difficult to conceive it to originate any where but in the upper 

 bed of vegetable earth through which the rain water percolates, carrying off 

 the substance of the testacea that are undergoing decomposition there. And 

 this hypothesis is corroborated by the remarkable quantity of recent land-shells 

 observable in the stratum of soil wherever it is cut through. Lime is known 

 to exist in a state of combination with the other constituents of volcanic rocks, 

 and consequently of their triturated particles which compose the tufa. From 

 thence we may suppose it taken up by the superficial vegetation, and con- 

 verted into a carbonate, which is transferred to the moUusca that feed on these 

 plants, and require it for the construction of their shells. The ultimate de- 

 composition of these restores it to the earth, where it enters again into the 

 composition of a mineral mass. 



A similar process seems to take place on the sides of Vesuvius, where cal- 

 careous stalactites have been observed to form upon the roofs of caves in the 

 lava rocks occurring within the cultivated district of the mountain. The 

 volcanic island of St. Helena presents an analogous formation, which is pro- 

 bably by no means of rare occurrence amongst volcanic districts, though the 

 absence of other sources of calcareous matter is not in all so incontestable. 



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