Ch. XXX.] NEWER PLIOCENE VOLCANIC ROCKS. 



529 



sions which give rise to the prismatic structure are at right angles to the 

 cooling surfaces. 



Newer Pliocene Period — Yal di Nolo. — I have already alluded (see 

 p. 156) to the igneous rocks which are associated with a great marine 

 formation of limestone, sand, and marl, in the southern part of Sicily, 

 as at Vizzini and other places. In this formation, which was shown to 

 belong to the Newer Pliocene period, large beds of oysters and corals 

 repose upon lava, and are unaltered at the point of contact. In other 

 places we find dikes of igneous rock intersecting the fossiliferous beds, 

 and converting the clays into siliceous schist, the laminae being contorted 

 and shivered into innumerable fragments at the junction, as near the 

 town of Vizzini. 



The volcanic formations of the Yal di Noto usually consist of the 

 most ordinary variety of basalt, with or without olivine. The rock is 

 sometimes compact, often very vesicular. The vesicles are occasionally 

 empty, both in dikes and currents, and are in some localities filled with 

 calcareous spar, arragonite, and zeolites. The structure is, in some 

 places, spheroidal ; in others, though rarely, columnar. I found dikes 

 of amygdaloid, wacke, and prismatic basalt, intersecting the limestone 

 at the bottom of the hollow called Gozzo degli Martiri, below Melilli. 



Dikes. — Dikes of vesicular and amygdaloidal lava are also seen tra- 

 versing marine tuff or peperino, west of Palagonia, some of the pores 

 of the lava being empty, while others are filled with carbonate of lime. 



Fig. 664 



Fig. €65. 







0' .-O 



Ground-plan of dikes near Palagonia. 



a. Lava. 



t. Peperino, consisting of volcanic sand, mixed with 

 fragments of lava and limestone. 



In such cases, we may suppose the peperino to have resulted from 

 showers of volcanic sand and scoria, together with fragments of lime- 

 stone, thrown out by a submarine explosion, similar to that which gave 

 rise to Graham Island in 1831. When the mass was, to a certain 

 degree, consolidated, it may have been rent open, so that the lava 

 ascended through fissures, the walls of which were perfectly even and 

 parallel. After the melted matter that filled the rent in fig. 664, had 

 cooled down, it must have been fractured and sliifted horizontally by a 

 lateral movement. 



In the second figure (fig. 665), the lava has more the appearance of a 

 vein which forced its way through the peperino. It is highly probabl© 



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