47 i ASCENSION. [CHAP. xxi. 



bomb, against the external cooled crust, and so produced the solid 

 shell of stone; and lastly, that the centrifugal force, by relieving 

 the pressure iu the more central parts of the bomb, allowed the 

 heated vapours to expand 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 remarkable 

 from its broad, slightly hollowed, and circular summit having been 

 filled up with many successive layers of ashes and fine scoria;. 

 These saucer-shaped layers crop out on the margin, 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 Eiding School. I brought 

 away specimens of one of the tufaceous 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 siliceous-shielded, fresh-water infusoria, and 

 no less than twenty-five different kinds of the siliceous tissue of 

 plants, chiefly of grasses. From the absence of all carbonaceous 

 matter, Professor Ehrenberg believes that these organic bodies 

 have passed through the volcanic fire, and have been erupted in 

 the state in which we now see them. The appearance of the layers 

 induced me to believe that they had been deposited under water, 

 though from the extreme dryness of the climate I was forced to 

 imagine, that torrents of rain had probably fallen during some 

 great eruption, and that thus a temporary lake had been formed 

 into which the ashes fell. But it may now be suspected that the 

 lake was not a temporary one. Anyhow, we may feel sure, that at 

 some former epoch, the climate and productions of Ascension were 

 very different from what they now are. Where on the face of the 

 earth can we find a spot, on which close investigation will not dis- 

 cover signs of that endless cycle of change, to which this earth has 

 been, is, and will be subjected ? 



On leaving Ascension, we sailed for Bahia, on the coast of Brazil, 

 in order to complete the chronometrical measurement of the world. 

 We arrived there on August 1st, and stayed four days, during 

 which I took several long walks. I was glad to find my enjoyment 

 in tropical scenery had not decreased from the want of novelty, 



* Monats. der Konig. Akad. d. Wiss. zu Berlin. Voin April, 1845. 



