524 ASCENSION 



explanation. The intervening spaces are concealed with layers 

 of pumice, ashes, and volcanic tuff. Whilst passing this end of 

 the island at sea, I could not imagine what the white patches 

 were with which the whole plain was mottled ; I now found 

 that they were sea -fowl, sleeping in such full confidence, that 

 even in mid -day a man could walk up and seize hold of them. 

 These birds were the only living creatures I saw during the 

 whole day. On the beach a great surf, although the breeze was 

 light, came tumbling over the broken lava rocks. 



The geology of this island is in many respects interesting. 



G3^ 



^iii^k^ 



CELLULAR FORMATION OF VOLCANIC BOMB. 



In several places I noticed volcanic bombs, that is, masses of 

 lava which have been shot through the air whilst fluid, and have 

 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 internal structure of one of these bombs, 

 when broken, is represented very accurately in the accompanying 

 woodcut. 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 ; 



