Physical Structure of the Site of Home. 27 



ejected from volcanoes under the surface of the sea, at a time 

 when the whole region that is lower than the summit of Mons 

 Albanus was submerged. That these loose materials were de- 

 posited in the sea, and not in fresh water, is evident from the 

 tufa containing marine shells in many places, and from its being 

 found in the islands of Ischia, Procida, and in Lipari, where 

 fresh-water lakes of any extent could not have existed. 



If we look for the probable sources of the materials of the 

 tufa which covers the country around Rome, we find, on the 

 one side, the volcanoes of the Alban Hills ; on the other, those 

 of Mons Ciminus, and those which surround the Lacus Sabati- 

 nus. When the mineralogical characters of the tufa are exami- 

 ned, it is found that they do not resemble the lava of the Alban 

 Hills, even that tufa which covers the country immediately sur- 

 rounding those hills ; that the fragments of pumice cannot have 

 come from thence, as that substance is not found in any part of 

 the volcanic district of Mons Albanus. But the mineralogical 

 characters of the tufa coincide with the lava of the volcanoes 

 which surround the lake Sabatinus, and with those of the range 

 of Mons Ciminus ; and it is probable, therefore, that from these 

 craters all the vast mass of matter was poured forth. That 

 loose materials may be thrown out by submarine volcanoes, we 

 have had evidence at a very late period, in the formation of the 

 island of Sabrina, off St Michael, in the Azores, in 1811 *. 



After the deposition of these volcanic materials, the ground 

 upon which they rested must have become dry land, and that 

 land in process of time was covered with vegetation, and was 

 inhabited by graminivorous animals. How many ages elapsed 

 in this transition, and during how many the region continued to be 

 so inhabited, before the next great catastrophe, our imagination 

 alone can number ; and if we take the experience of human re- 

 cords of changes on the earth's surface as our measure, we shall 



• In 1814, a volcanic island rose in the sea off the coast of Kamtchatka, 

 which is said to be 3000 feet high, and four miles round. — Lyell^ 307. New- 

 islands have often been thrown up off the coast of Iceland, and on one occa- 

 sion the quantity of pumice ejected was so great, that the light spongy stone 

 covered the sea to the distance of 150 miles, and to such an extent, that ships 

 were impeded in their course. The island thrown up in 1831, off tlie coast 

 of Sicily, will be in the recollection of every one. 



