Ch. XXX.] 



POST-PLIOCENE VOLCANIC EOCKS. 



523 



origin of such conglomerates is explained by observing the shingle 

 beaches composed of trap pebbles in modern volcanic islands, or at the 

 base of Etna. 



Post-Pliocene Period (including the Recent). — I shall now select 

 examples of contemporaneous volcanic rocks of successive geological 

 periods, to show that igneous causes have been in activity in all past 

 ages of the world, and that they have been ever shifting the places 

 where they have broken out at the earth's surface. 



One portion of the lavas, tuffs, and trap-dikes of Etna, Vesuvius, 

 and the Island of Ischia, has been produced within the historical era ; 

 another, and a far more considerable part, originated at times immedi- 

 ately antecedent, when the waters of the Mediterranean were already 

 inhabited by the existing species of testacea. The southern and eastern 

 flanks of Etna are skirted by a fringe of alternating sedimentary and 

 volcanic deposits, of submarine origin, as at Aderno, Trezza, and other 

 places. Of sixty-five species of fossil shells which I procured in 1828 

 from this formation, near Trezza, it was impossible to distinguish any 

 one from species now living in the neighbouring sea. 



The Cyclopian Islands, called by the Sicilians Dei Faraglioni, in the 

 sea-cliffs of which these beds of clay, tuff, and associated lava are laid 

 open to view, are situated in the Bay of Trezza, and may be regarded 



Fig. 660. 



View of the Isle of Cyclops, in the Bay of Trezza.* 



as the extremity of a promontory severed from the main land. Here 

 numerous proofs are seen of submarine eruptions, by which the argilla- 

 ceous and sandy strata were invaded and cut through, and tufaceous 

 breccias formed. Inclosed in these breccias are many angular and har- 

 dened fragments of laminated clay in different states of alteration by heat, 

 and intermixed with volcanic sands. 



The loftiest of the Cyclopian islets, or rather rocks, is about 200 feet 

 in height, the summit being formed of a mass of stratified clay, the 

 laminee of which are occasionally subdivided by thin arenaceous layers. 



* This view of the Isle of Cyclops is from an original drawing by my friend 

 the late Capt. Basil Hall, R. N. 



