234 Mr. PouLETT Scrope on the Geology of the Ponza Isles. 



that of Somma_, this vent appears first to have given rise to numerous currents 

 of feldspathose rocks, which were succeeded by leucitic lavas. 



The detached points on which eruptions appear to have taken place, though 

 in no great number, within the wide longitudinal valley of the Garigliano, viz. 

 at the Rocca d'Evandro, and in the vicinity of Pofi, Veroli, Frosiuone, and 

 Tichiena, connect the volcanic district of the Terra di Lavoro with that of the 

 Canipagna di Roma. The lavas produced by these mouths are almost solely 

 augitic. Those of the great volcanic group of the Monte Albano are on the 

 contrary leucitic, the leucite being frequently associated with and occasionally 

 replaced by melilite. North of Rome another series of volcanic vents com- 

 mences, in which a still closer association of the feldspathose and leucitic lavas 

 is observable. These fall without the limits of the map. 



The intervals that separate the volcanic groups of the Terra di Lavoro are 

 occupied by plains of stratified tufa, which appear to have been originally 

 redeemed from the sea, when at a higher comparative level than at present, 

 by the accumulation of the fragmentary matters thrown up by the neighbour- 

 ing vents, and distributed pretty evenly upon its shores and bottom. These 

 conglomerates, composed of comminuted pumice, seem to have been drifted 

 to a considerable distance up the valleys of the Apennine chain that open into 

 the plain of Campania, at the time when this massive limestone ridge formed 

 the coast of the Mediterranean. This circumstance confirms the opinion that 

 the earliest productions of the volcanos of this district were of feldspathose 

 nature. These stratified tufas must be supposed to extend beneath the space 

 actually occupied by the sea as far at least as the groups of Ischia, Ventotiene, 

 and Ponza, to which they were probably in part indebted for their origin. 

 Deposits of an analogous nature appear on various points of the valley of the 

 Garigliano and on the southern foot of the mountains of the Volsci, bordering 

 on the Pomptine marshes. Others again constitute the undulating plain of 

 the Campagna di Roma covering strata of tertiary marl and sandstone. In 

 all these localities they of course share in the mineral constitution of the lavas 

 produced contemporaneously with them from the neighbouring volcanos. 



The extensive tract of the Pomptine marshes themselves, has in all proba- 

 bility a general substratum of this nature, since it crops out superficially on 

 all the higher ground of this swampy expanse, and has been also observed in 

 the lower parts wherever excavations have been carried to any depth*. Here 

 it is covered by local formations of marl and peat, but chiefly by vast alluvial 

 deposits of sand thrown up by the action of the sea, Avhich have already con- 

 nected the once insular rock of Circe with the main land, and still continue to 



* Brocchi, Catalogo Ragionato, p. 78, 79. 



