Observations on the Geognostic Character of Italy. 79 



Many clay-pits are now sunk for the same purpose, on the Monte delle 

 Crete, and at the Monte dei Fomaci, which discover to us the interior of 

 the mountain. Leojiold Von Buch gives us a particular description of it, 

 from which we gather, that the marl has a decidedly stratified disposition, 

 and separates into beds of even a foot and a-half in thickness, which are al- 

 ternately, dark and light coloured. In its upper strata, the marl regularly 

 alternates with the beds of the above described sandstone and its breccia, 

 which is a proof of its contemporaneous formation. Its interior contains a 

 much greater number of organic remains than the sandstone. Brocchi de- 

 scribes, behind the sacristy of St Peter, numerous fragments of shells, Den- 

 talia, Tellince, and pieces of the cover of the Lepas Balanns. There are, like- 

 wise, numerous remains of plants, which seem to have belonged to branched 

 Fuci; Brocchi also found in it bituminous wood, traversed b^^ slender veins 

 of iron-pyrites. Flaminus Vacca even mentions, that large pieces of it were 

 found in the clay, on digging the foundations of the church of St Peter. On 

 the Monte delle Crete, are also found numerous remains of marine shells, 

 even in the beds of clay which alternate with the sandstone. The same is 

 mentioned by Breislak at the Monte dei Fornaci. 



II. Volcanic Agency. 



If we have seen the heights on the right bank of the Tiber, formed 

 throughout the greatest part of their mass of a marine formation ; on the other 

 hand we find, in the hilly country on its opposite side, viz. the seven hills of 

 Rome, and the flat country partly connected with them to the south of the 

 city, the predominant rocks to be the products of volcanic operations. The 

 rock, which is here most abundant, and which forms the main substance of 

 these hills, is a large continuous deposite of volcanic hiff, tufa of the Italian 

 naturalists, and separated, by Brocchi, from tofa— the fresli water deposite. 

 This species of rock, which is so abundant in manj^ parts of Italy, and in the 

 vicinity of every volcano, is distinguished from proper lavas by its never hav- 

 ing been seen in the form of a coulee, or stream •. 



• The nearest point to Rome, at which true lava is met with, is in the hill 

 of Capo di Bove, two miles from the Porta St Sebastian, where it is quar- 

 ried, and under the name of Selcea, or Selce Romano, forms the paving 

 stone of the city. It is a true basaltic lava, with a blackish-grey colour 

 sharp-edged fracture, formed, according to Fleuriau's acute observations, of 

 an internally crystallized granular aggregate of augite, leucite, magnetic iron, 

 different zeolites, &c. (Journ. de Phys. 1795, ii- p- o9-) In its cavities are 

 contained many small cubical mellilites, with a white fossU, which appears 

 felspar ; and, lastly, zeolites. The whole mass, evidently rests on peperino. 

 Leopold Von Buch believed this hill to stand isolated and unconnected with 

 any active volcano. Breislak concludes from it, the existence of a hypothe- 

 tical crater, which he thought to have discovered in the midst of the seven 

 hills of Rome; and that its connection witli it was destroyed by the hands of 

 man. But the researches of Riccioli have shewn it to be the termination of 

 a long stream, the origin of which can be traced into the Alban Hills, along 

 the Via Appia, the pavement of which often rests on it. 



Within these some years, another locality of this rock has been obserred, on 



