86 
NEWER PLIOCENE PERIOD. 
[Ch. VII. 
No. 13. 
A 
A, Zocolaro. B, Monte di Calanna. 
C, Plain at the head of the Valley of Calanna. 
«, 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 be 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. (See letter a, diagram No. 18.) 
The narrow cavity of St. Giacorao will admit of an explana- 
tion precisely similar to that already offered for Calanna. 
