530 PLIOCENE VOLCANOS. [Ch. XXXI 



thgft similar appearances would be seen, if we could examine the floor 

 of tlie sea in that part of the Mediterranean where the waves have 

 recently washed away the new volcanic island ; for when a superincum- 

 bent mass of ejected fragments has been removed by denudation, we 

 may expect to see sections of dikes traversing tuff, or in other words, 

 sections of the channels of communication by which the subterranean 

 lavas reached the surface. 



CHAPTEH XXXI. 



ON THE DIFFERENT AGES OF THE VOLCANIC ROCKS — continued. 



Volcanic rocks of the Older Pliocene period — Tuscany — Rome — Volcanic region 

 of Olot in Catalonia — Cones and lava-currents — Ravines and ancient gravel- 

 beds — Jets of air called Bufadors — Age of the Catalonian volcanos — Miocene 

 period — Brown coal of the Eifel and contemporaneous trachytic breccias — 

 Age of the brown-coal — Peculiar characters of the volcanos of the upper and 

 lower Eifel — Lake craters — Trass — Hungarian volcanos. 



Older Pliocene Period — Italy. — In Tuscany, as at Radicofani, Viterbo, 

 and Aquapendente, and in the Campagna di Koma, submarine volcanic 

 tuffs are interstratified with the Older Pliocene strata of the Subapennine 

 hills, in such a manner as to leave no doubt that they were the products 

 of eruptions which occurred when the shelly marls and sands of the Sub- 

 apennine hills were in the course of deposition. This opinion I expressed* 

 after my visit to Italy in 1828, and it has recently (1850) been confirmed 

 by the arguments adduced by Sir R. Murchison in favor of the submarine 

 origin of the earlier volcanic rocks of Italy .f These rocks are well known 

 to rest conformably on the Subapennine marls, even as far south as Monte 

 Mario in the suburbs of Rome. On the exact age of the deposits of Monte 

 Mario new light has recently been thrown by a careful study of their 

 marine fossil shells, undertaken by MM. Rayneval, Vanden Hecke, and 

 Ponza. They have compared no less than 160 speciesj with the shells of 

 the Coralline Crag of Suffolk, so well described by Mr. Searles Wood ; 

 and the specific agreement between the British and Italian fossils is so 

 great, if we make due allowance for geographical distance and the differ- 

 ence of latitude, that we can have little hesitation in i-eferring both to the 

 same period or to the Older Pliocene of this work. It is highly probable 

 that, between the oldest trachytes of Tuscany and the newest rocks in the 



* See Ist edit, of Principles of Geology, vol. iii. chaps, xiii. and xiv. 1833; and 

 former edits, of this work, ch. xxxi. 

 f Geol. Quart. Journ. vol. vi. p. 281. 

 % Catalogue des Fossiles de Monte Mario, Rome, 1854. 



