296 



ASCENSION. 



pours to expand tlieir cells, thus forming the coarse- 

 ly cellular mass of the centre. 



A hill, formed of the older series of volcanic 

 rocks, and which has been incorrectly considered as 

 the crater of a volcano, is remarkable from its broad, 

 slightly hollowed, and circular summit having been 

 filled up \vith many successive layers of ashes and 

 fine scoriae. These saucer-shaped layers crop out 

 on the margin, forming perfect rings of many dif- 

 ferent colours, giving to the summit a most fantas- 

 tic appearance ; one of these rings is white and 

 broad, and resembles a course round which horses 

 have been exercised ; hence the hill has been call- 

 ed the Devil's Riding School. I brought away 

 specimens of one of the tufaceous layers of a pink- 

 ish colour ; and it is a most extraordinary fact that 

 Professor Ehrenberg* finds it almost wholly com- 



■* Monats.derKonig. Akad.d. Wiss.zu Berlin. Vom April, 184.5, 



