Observations on the Geognostk Character of Italy. 81 



subterranean galleries, which were formerly used for quarries. On the Av- 

 entine hill it appears in the Vigna Lovati, opposite St Prisca, where a quarry 

 has been opened, from which, as Von Buch mentions, were extracted the 

 stones for the foundations of the palace Braschi. The stone here, from its 

 hardness and fracture, as well as colour, resembles bricks so much, that it 

 might easily be mistaken for them, did not we see it rising before us into a 

 precipice CO feet high. Count Dunin Borkowski has, in his description of this 

 place, compared the lithoidal tufa from the pits of the Monte Verde, before 

 the Porta Portese, with a claystone porphyry. In the Vigna d'Asti, as well 

 as on the Aventine, Flaminio Vacca has alread_y mentioned the occurrence of 

 this tufa, and also round St Saba. It further appears on the Caelian Hill, in 

 the subterranean passages to the east of the church of St Giovanno e Paolo, 

 where are found the remains of an ancient Roman edifice ; and not far from 

 thence, at St Giovanni in Laterano, in the vault No. 22. Brocchi, too, saw it 

 on the Esquiline Hill, in the section laid bare by the subterranean passages 

 of the church St Francesco di Paolo, full of fragments of lava, and traversed 

 by variously contorted veins of fatty clay. It is abundant outside Rome, 

 nearer it at Monte Verde than at Ponte Nomentron, Torre Pignatara, be- 

 fore the Porta Maggiore, and, finally, at Ardea, and along the Via Ardeatina. 



2d, Granular Tufa, Brockeltuf. It is very difierent from the preceding : 

 colour blackish brown, or yellow brown ; light ; very friable, consisting of 

 large loosely connected grains, with white particles of mealy leucite ; frag- 

 ments of augite ; scales of mica, and, at times, containuig blackish-grey 

 masses of lava. 



With regard to the degree of solidity, texture, and colour, it offers great 

 varieties, according as it is more or less decomposed. It has either entire- 

 ly the character of lapUlo, and is only not so dry and less meagre to the 

 feel than that which is now ejected from volcanoes, or it is extremely friable, 

 loses the porous texture, and crumbles down into an earthy mass. It is still 

 more affected by filtrating moisture, which changes it into a kind of clay, 

 which adheres to the tongue ; is viscid, and from which the leucite has van- 

 ished, whilst the augite and mica remain. It is this same earth which, near 

 VeUetri, at the foot of Monte Artemisio, is used for the construction of 

 bricks ; and at St Agata, in Campania, between Molo di Gaeta and Capua, 

 for potter's ware. The rude sepulchral urns, at the lake of Carnevoli, in the 

 Albano, are formed of the same volcanic clay. 



This tufa sometimes forms a peculiar variety, when it is very much de- 

 composed, which Brocchi calls earthy tutf. Tufa terroso. (It is worthy of at- 

 tention, that what Brocchi calls, in his catalogue raisottnce, Tufa lerroso, is al- 

 ways this, which has been since called Granular Tufa ; the lithoidal tufa, on 

 the other hand, corresponds to the jnetroso of his catalogue. It is of a yellow 

 colour, much lighter, and so friable as to crumble down into a fine powder, 

 which absorbs water with a hissing noise, giving out, at the same time, an 

 earthy smell. Of such a description is especially the tufa described by 

 Leopold von Buch, (ii. p. 31.) This tufa, as well as the former, consists of 

 distinctly separated beds, and appears, like it, intersected by large crevices, 

 which divide it into more or less regular parallelopipeds. At Monte Pindo, 



Of TOBKR DECEMBER 1829- F 



