288 ' PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 6, 



But, if for a moment we were inclined to suppose tliat sucli rocks as 

 the tuffs of the Campagna might have been formed under the atmo- 

 sphere, all doubt would be dispelled by finding them associated with, 

 and covered by great thicknesses of water-worn, pebbly detritus. 

 In short, when we follow these dejections from Monte Fiascone 

 to Viterbo and the lakes of Vico, Baccano, and Bracciano, up to 

 Monte Mario and the gates of Rome, we see that their very upper- 

 most dejections are so intimately associated with the upper sub- 

 apennine strata charged with marine shells and water-worn rounded 

 pebbles of apennine limestone, that all doubt is dispelled, and we are 

 compelled to conclude that all, or nearly all, such earlier volcanic 

 rocks as I have hitherto alluded to, and which occupy the Campagna, 

 were formed either entirely under the sea, or in that condition of 

 things when the former bottoms of seas were emerging and were 

 covered with brackish or impure lacustrine waters. Many of these 

 rocks may be included in the peperino of Brongniart ; and although 

 Brocchi attempted to divide them into stony, as distinguished from 

 granular tuffs, and a third variety that may be called earthy tuff, I 

 quite agree with Pareto, that all such divisions depend on mere local 

 accidents and have no sort of geological bearing. All these rocks, 

 however varied, constitute in fact but one formation, from the fari- 

 naceous tuff of Monte Verde, near Rome, to the more solid tuffs and 

 leucite lavas of the Ciminian Hills. I have thus dwelt on the true 

 nature of these rocks, in order to attach what I consider to be the 

 right meaning to the words "craters" and '^volcanos," which have 

 found their way into the descriptive hand-books of Italy, and which 

 may lead tourists to fancy that there were formerly many volcanos 

 like Vesuvius and Etna in this region of the Papal States. 



Of the older class of volcanic rocks I will now cite a few other ex- 

 amples. The deep ravine near Monte Fiascone exposes on its sides fine 

 sections of rocks, the high antiquity of which in relation to the present 

 condition of things is further indicated by their having afforded the 

 materials for those huge rounded blocks of peperino, &c. which are 

 strewed over the slope extending from Monte Fiascone into the sterile 

 valley north of Viterbo, in which the well-known Bulicami, or hot 

 springs, have their issue. All these blocks were manifestly trans- 

 ported by great aqueous currents * . 



It is in the sandy varieties of this group of rocks called panchina, 

 puzzuolano, &c., that far the greater number of the Necropolises of 

 the Etruscans were excavated, as well as the crypts and subterranean 

 sanctuaries of the earlier Christian martyrs and refugees in the en- 

 virons of Rome. He who visits the site of the ancient Etruscan city 

 of Veii may at once verify this observation ; for whilst the fortress 

 stood on the harder rocks (leucite lava, &c.), the tombs of the Necro- 

 polis were burrowed out of the softer sandy tuff to the north of the 

 city, which is manifestly a finely laminated subaqueous deposit. In 

 the same way at Volterra, where the rocks are not of igneous origin, 



* The formation of the great globular concretions of peculiar travertine by these 

 " bulicami" of Viterbo are described in Lyell's 'Principles of Geology,' 7th edit., 

 p. 243. 



