494 



ASCENSION. 



[chap. XXI. 



crust, and so produced the solid shell of stune ; 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 volcanic rocks, and which 

 has been incorrectly considered as the crater of a volcano, is re- 

 markable from its broad, slightly hollowed, and circular summit 

 having been filled up with many successive layers of ashes and 

 fine scoriae. 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 less than twenty-five 

 different kinds of the siliceous tissue of plants, chiefly of grasses. 



* Monats. der Koutg. Akad. d. Wiss, zu Berlin. Vom April, 1845 



