Ohservat'mis on the Geognostic CJiaracier of Italy. 11 

 with steep declivities and level summits ; on the left, on the 

 contrary, a low broken hilly region, the diiferent eminences of 

 which are either completely isolated from one another by diffe- 

 rent intersecting prolongations of the valley, or constitute long 

 narrow ridges, which terminate in the valley by a soft and gentle 



declivity. 



It is extremely satisfactory to the geologist, that here, as in 

 so many other cases, the differences in the external physiogno- 

 mical characters of this district, stand in close and intimate con- 

 nexion with the nature of the rocks which constitute its interior. 



Three formations, formed at very different epochs, and un- 

 der very different circumstances, concur in the formation of the 

 district. Once covered by the sea to a considerable depth, the 

 fundamental rock was formed from the products of the univer- 

 sal ocean ; — pierced and ruptured by volcanoes, this received a 

 covering of matters taken from the interior of the earth's crust : 

 and, later still, it was overflowed to a surprising depth by fresh 

 water, which covered it with deposites, partly from a state of 

 chemical solution, and partly from a state of mechanical suspen- 

 sion. It appears, then, most proper to begin with the traces 

 left by the sea, the most general of all these forming powers, on 

 the surface of the district; then pass to the operation of volca- 

 noes ; and, finally, conclude with the most local and circum- 

 scribed phenomena, those referable to fresh water. 



I. Agency of' the Ocean. 



The chain of hills on the right bank of the Tiber, the lengthened ridges of 

 the Janiculus and the Vatican, both mere prolongations of Monte Mario, the 

 highest point of this part, belong, in the most essential part of their mass, to 

 the products of the ancient ocean. The uppermost stratum is a thick bed of 

 a peculiar sandstone. A yellow siliceo-calcareous sand is pretty plentiful at 

 the Vatican, in the garden of Belvidere, and before the Porta Angelica, to 

 the left behind the city wall. It uninterruptedly forms the whole declivity 

 of the Janiculus, on the side next the Tiber, as far as the exposed state of 

 the rock allows us to judge of its internal constitution ; and, on the opposite 

 brink, along the wall between the Porta St Spirito and the Port St Pan- 

 crazio, fully half the height of the precipice, eighty feet high, in the hollow of 

 the Valle d'Inferno. This sand is often a mere loose unconnected mass, 

 more or less evidently formed of fragments ; on the contrary, it is often ce- 

 mented by the intervention of a basis, into a regular horizontally stratified 

 conglomerate. Brocchi takes notice of a collection of fragments of limestone, 

 in front of the Porta Angelica. According to this author, fragments of lime- 

 stone and flint, mixed with loose sand, is seen behind the city-wall, between 



