70 NEWER PLIOCENE PERIOD. [Ch.Vl. 



The annexed diagrams (Nos. 6 and 7) represent a ground 

 plan of the rocks as they are exposed to view on a horizontal 

 surface. We think it highly probable that similar appear- 

 ances would be seen, if we could examine the floor of the 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 denuda- 

 tion, 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. 



On the summit of the limestone platform of the Val di Noto, 

 I more than once saw analogous dikes, not only of lava but of 

 volcanic tuff, rising vertically through the horizontal strata, 

 and having no connexion with any igneous masses now appa- 

 rent on the surface. In regard to the dikes of tuff or peperino, 

 we may suppose them to have been open fissures at the bottom 

 of the sea, into which volcanic sand and scoriae were drifted 

 by a current. 



Tuffs and Peperinos. In the hill of Novera, between Viz- 

 zini and Militelli, a mass of limestone, horizontally stratified, 

 conies in contact with inclined strata of tuff (see diagram No, 8), 

 No. 8. 



A, Limestone. 



aa, Calcareous breccia with fragments of 

 lava. 



b, Black tuff. 



c, Tuff. 



while a mixed calcareous and volcanic breccia, a a, supports 

 the inclined layers of tuff, c. The vertical fissure, 6 b, is filled 

 with volcanic sand of a different colour. An inspection of this 

 section will convince the reader that the limestone must have 

 been greatly dislocated during the time that the submarine 

 eruptions were taking place. 



At the town of Vizzini, a dike of lava intersects the argilla- 

 ceous strata, and converts them into siliceous schist, which has 



