92 On the Constitution of' the Territory qf' Rome, icith 



always met with on the strata of the Sub- Appennine Hills, accord- 

 ing to the authority of accurate observers. We have already 

 shown, that at the Janiculus and Vatican, and probably also at 

 the foot of the Tarpeian Rock, and uniformly under the cover- 

 ing of the Seven Hills, every where undermost is the marine 

 formation, and sjyread over it the volcanic products. 



The opinions of geologists are not so unanimous on the pe- 

 culiar causes and relations of the formation of this rock within 

 the walls of Rome. Breislak has advanced upon this point a 

 very surprising hypothesis. He imagined, from the form of the 

 Seven Hills, it might be inferred, that, formerly within the walls 

 of Rome, and in the Forum itself, a crater existed, from which 

 were expelled the surrounding igneous products. He even be- 

 lieved he had discovered small lateral craters on the outer- 

 most hills of the Aventine, and in the Intermontium of the Ca- 

 pitoline ; and he saw in the tufa of these hills, which we have 

 just regarded as a mechanical aggregate of volcanic matters, 

 nothing else than a lava which had really once flowed. The 

 grounds on which its proposer endeavoured to support this sin- 

 gular view have been lately refuted, from a consideration of the 

 district itself, by Leopold von Buch, and Brocchi. A glance at 

 the improved chart of the city, and especially at the excellent 

 plan of Nolli, on which both these philosophers based their ob- 

 servations, compared with the chart which Breislak has append- 

 ed to his work, plainly shows how arbitrary and rash changes we 

 must admit, in all the details of this district, in order to give the 

 form of a serrated crater to its present appearances. Further, 

 it has been demonstratively shown, that the tiifa found here is 

 not a lava. 



Breislak views it as a crystallized granular aggregation of he- 

 terogeneous fossils ; but Von Buch has given it as his express opi- 

 nion, that its constituents are never so sharp and regularly con- 

 nected, as would happen from a crystallizing process on the spot. 

 They bear numerous traces of abrasion on the surface, which 

 they must have experienced on being brought from a distance. 

 For example, this is well seen in the numerous leucites which 

 have completely lost their fresh appearance, and, by a gi-adual 

 change from the exterior to the interior, have crumbled into dull, 



