404 ASCENSION. [cHaP. xxI, 
crust, and so produced the solid shell of stoue; and lastly, that 
the centrifugal force, by relieving the pressure in the more 
central parts of the bomb, allowed the heated vapours to ex- 
pand their cells, thus forming the coarsely cellular mass of the 
centre. 
A hill, formed of the older series of voleanic rocks, and which 
has been incorrectly considered as the crater of a voleano, is re- 
markable from its broad, slightly hollowed, and circular summit 
having been filled up with many successive layers of ashes and 
fine scorie. These saucer-shaped layers crop out on the mar- 
gin, forming perfect rings of many different colours, giving to 
the summit a most fantastic appearance; one of these rings is 
white and broad, and resembles a course round which horses 
have been exercised ; hence the hill has been called the Devil’s 
Riding School. I brought away specimens of one of the tufa- 
ceous layers of a pinkish colour; and it is a most extraordinary 
fact, that Professor Ehrenberg * finds it almost wholly composed 
of matter which has been organized: he detects in it some silice- 
ous-shielded, fresh-water infusoria, and no Jess than twenty-five 
different kinds of the siliceous tissue of plants, chiefly of grasses. 
* Monats, der Konig. Akad. d. Wiss. zu Berlin. Vom April, 1845. 
