4S4 GEOLOGY OF THE ISLAND, [chap. xxi. 



In several places I noticed volcanic bombs, that is, 

 masses of lava which have been shot through the air 

 whilst fluid, and iiave consequently assumed a spherical or 

 pear shape. Not only their external form, but, in several 

 cases, their internal structure shows in a very curious 

 manner* that they have revolved in their aerial course. 

 The central part is coarsely cellular, the cells decreasing 

 in size towards the exterior.; where there is a shell-like 

 case about the third of an inch in thickness, of compact 

 stone, which again is overlaid by the outside crust of finely 

 cellular lava. I think there can be little doubt, first, that 

 the external crust cooled rapidly in the state in which we 

 now see it; secondly, that the still fluid lava within, was 

 packed by the centrifugal force, generated by the revolving of 

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

 the solid shell of stone ; and lastly, that the centrifrugal 

 force, by relieving the pressure in 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 scoriae. 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 

 Riding 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 

 organised : 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 



* Monats. der Kbnig. Akad. d. Wi'ss. zu Berlin. Vom April, 1845. 



