Observations on the Geognostic Character of Ilaltj. 93 



mealy spots. How can he reconcile the constantly stratified dis- 

 position of this tufa, — the occurrence of thin layers of alluvion, 



the evident intermixture with rolled pieces of volcanic and foreio-n 

 rocks, of which we gave many examples, — how can we reconcile 

 all this with the supposition of its once having been in the state 

 of a red hot stream ? These numerous relations rather lead us 

 to the view, that the volcanic constituents of the tufa have re- 

 ceived their present properties through the medium of water. 

 In fact this is the view embraced by both the above mentioned 

 philosophers. 



Was it, then, the oceanic water which produced the tufaceous 

 covering (fthe Roman soil, or did it arise from terrestrialji-esh 

 loater ? Von Buch seems inclined to the latter supposition ; 

 and, indeed, the grounds which he advances would be decisive, 

 if the formations which we are considering were confined to 

 Rome. Tufa and travertine, which are so undeniably fresh 

 water deposites, are here, as we have above seen, often irregularly 

 alternating with one another. Almost all the hills of Rome 

 show examples of tufa strata resting on regularly deposited 

 travertine, and what we admit of one of these deposites cannot 

 be refused to the other. " The formation of these two rocks, 

 so different in external aspect, composition, and structure, must, 

 notwithstanding, be viewed as contemporary." These are the 

 words of this gifted naturalist. The view, on the contrary, 

 which Brocchi has adopted to account for the manner of forma- 

 tion of this volcanic tufa, excludes entirely the operation of 

 fresh-water, and it is certainly deserving of strict examination 

 the grounds on which this talented observer rests his positions. 



Undoubtedly it is of importance, in the first place, to con- 

 sider that the tufa-covering of Rome is not entirelv isolated in 

 the district of the Italian volcanic zone, but is regularly ex- 

 tended from the mountains of St Flora, in Tuscany, through 

 the Romagna into the plains of Campania, into the vicinity of 

 Vesuvius, and the Phlegra>an fields. Such an uniformity in a 

 stratum formed through the medium of water, of such an ex- 

 tent, certainly requires as great an extension of the water which 

 produced its deposition and consolidation. Fresh waters could 

 not have easily produced such relations. This tufa is, further, 



