Ch. XIIL] 



OF SICILY. 



157 



down and spread by the waves and currents so as to form strata of tuff, 

 which are found intercalated between beds of limestone and clay contain- 

 ing marine shells, the thickness of the whole mass exceeding 2000 feet. 

 The fissures through which the lava rose may be seen in many places 

 forming what are called dikes. 



In part of the region above alluded to, as, for example, near Lentini, 

 a conglomerate occurs in which I observed many pebbles of volcanic 

 rocks covered by full grown serpulce. We may explain the origin of 

 these by supposing that there were some small volcanic islands w^hich 

 may have been destroyed from time to time by the waves, as Graham 

 Island has been swept away since 1831. The rounded blocks and 

 pebbles of solid volcanic matter, after being rolled for a time on the 

 beach of such temporary islands, were carried at length into some tran- 

 quil part of the sea, where they lay for years, while the marine serpulce 

 adhered to them, their shells growing and covering their surface, as they 

 are seen adhering to the shell figured in p. 22. Finally, the bed of peb- 

 bles was itself covered with strata of shelly limestone. At Vizzini, a 

 town not many miles distant to the S. W., I remarked another striking 

 proof of the gradual manner in which these modern rocks were formed, 

 and the long intervals of time which elapsed between the pouring out of 

 distinct sheets of lava. A bed of oysters no less than 20 feet in thick- 

 ness rests upon a current of basaltic lava. The oysters are perfectly iden- 

 tifiable with our common eatable species. Upon the oyster bed, again, 

 is superimposed a second mass of lava, together with tufi" or peperino. 

 In the midst of the same alternating igneous and aqueous formations is 

 seen near Galieri, not far from Vizzini, a horizontal bed, about a foot and 

 a half in thickness, composed entirely of a common Mediterranean coral 

 {^Caryophyllia ccespitosa, Lam.). These corals stand erect as they grew; 



Fig. 128. 



CaryophylUa ccespitosa, Lam. (Cladocora siellaria, Milne Ed\v. and Ilairae.) 

 a. Stem with young stem growing from its side. 

 a*. Yonng stem of same twice magnified. 



0. Portion of brancli. twice magnified, Avitli the base of a lateral branch ; the exterior 

 ridges of the main branch appearing through the lamella; of the lateral one. 



c. Transverse section of same, proving by the integrity of the main branch, that the 



lateral one did not originate in a subdivision of the' animal. 



d. A branch, having at its base another laterally united to it, and two young corals at 



its upper part. 



e. A main branch, with a full grown lateral one. 

 /. A perfect terminal star. 



