82 On the ConstUution of the Territory ofRome^ with 



and dose to the Basilica of St Lorenzo, outside the gate, it presents impet- 

 sions of the leaves of land plants ; and, in the hitter place, it is pierced by nume- 

 rous tubular canals, which indicate the previous existence of brunches and 

 stems of trees. Similar appearances are further seen in a hill at Monte Saero, 

 at the ancient Via Salara, near the Wine Mountain of the Jesuits ; and under 

 the city walls, between the Porta St Giovanni, and the Amphitheatrum 

 Castrense. 



With regard to the relations and positions of this species of tufa, the most 

 essential points to be observed are the following : It is, in general, much 

 more extensively diffused than the lithoidal tufa, and forms the principal 

 mass of the Piucio., of the Qutnnal, the Viminal, and the Palatine. In the en- 

 virons of Rome it is equally abundant ; and all the catacombs of Rome are 

 duo- in it, with the exception of those of St "Valentino." 



Many points, with regard to its relations, set in a clear light its position 

 with regard to the other formations of this district. Without doubt the 

 most important of these is its occurrence on the heights of the right bank of 

 the Tiber. Here the volcanic rock every where covers the above de- 

 scribed marine formation. Leopold von Buch first enumerates a stratum of 

 tufa, six feet thick, on the highest point of the Vatican, immediately over the 

 sandstone of the Osteria cruciano, at the Vigna of Guiseppe Frangioni. It 

 contains many small pieces of true peperino, round masses of a mixture of 

 uugite and leucite, similar to that of the Rocco dijxtpa in the Alban Hills ; and, 

 more rarelv still, small pieces of basalt. Above this, lies a remarkable stra- 

 tum of portions of an ash-grey pumice-stone, of the size of a walnut, and 

 which floats in water, which maybe shewn to extend to pretty considerable dis- 

 tances in this quarter. Just the same, or extremely similar, are its relations, 

 not only at the base of this hill, but also at the Janiculm. A greenish-grey 

 granular tufa is here exposed, among other places, at the Porta di Sto Spirito, 

 under the walls of the garden Barberini, and here it covers an aggregate of 

 pumice-stones, cemented by a basis of a whitish tufa. The ridge, which is 

 here separated from the rest of the hill by a little valley, as well as the op- 

 posite declivity in the court-room of the Papal court, is almost volcanic. Sucli 

 rocks also appear on the sunnnit of the Janiculus. Besides, where the different 

 tributary streams of the Tiber have furrowed this elevated jdain, a similar 

 succession of strata is seen, as below the Villa Frangioni. A granular tufa, or 

 T. terroso, of a brownish colour, is seen right opposite the Porta St Pancrazio, 

 at the upper margin of the hill, in which lie imbedded large pieces of pumice- 

 stone, in a state of good preservation ; and also before the gate to the left 

 in the city wall, accompanied by pumice-stone, and pieces of a yellow spongy 

 lava. These are the same strata which extend from here to the summit of 



• These catacombs are the Arenari<e of the aucients,which, according to 

 Brocchi, was the denomination, in former tunes, as weW as now, of the puz- 

 zolano pits at Frosinone and Segnile Arenare, for the puzzolano earth is no- 

 thinw else than a variety of this tufa, probably the^rcna nigra of Vitruvius, (ii. 

 p. 4, 6.) whilst the Arenartifa, which occurs in other parts of "Vitruvius, is, per- 

 haps, connected in reality with the red puzzolano, which is now esteemed 

 the best, and is found at St Paolo, near the Three Fountains. Both kinds 

 were used for cement in the ancient edifices. 



