8G 



M:\VI: R PLIOCENE pi: RIO D. 



[Ch. VII. 



No. 13. 



A, Zocolaro. B, Monte di Calanna. 



C, Plain at the head of the Valley of Calanna. 



a, Lava of 1819 descending the precipice and flowing through the valley. 



b, Lavas of 1811 and 1819 flowing round the hill of Calanna. 



The flows of melted matter have been deflected from their 

 course by this projecting mass, just as a tidal current, after 

 setting against a line of sea-cliffs,, is often thrown off into a 

 new direction by some rocky headland. 



Lava-streams, it is well known, become solid externally, even 

 while yet in motion, and their sides may he compared to two 

 rocky walls, which are sometimes inclined at an angle of forty- 

 five degrees. When such streams descend a considerable slope 

 at the base of a line of precipices,, and are turned from their 

 course by a projecting rock, they move right onwards in a new 

 direction., so as to leave a considerable space (as in the Valley 

 of Calanna) between them and the cliffs which may be con- 

 tinuous below the point of deflection. 



It happened in 1811 and 1819, that the flows of lava over- 

 topped the ridge intervening between the hills of Zocolaro and 

 Calanna, so that they fell in a cascade over a lofty precipice, 

 and began to fill up the valley. (Sec letter a, diagram No. 18.) 



The narrow cavity of St. Giucomo will admit of an explana- 

 tion precisely similar to that already offered for Calanna. 



