1836.J VOLCANIC BOMBS. 473 



whole plain was mottled; I now found that they were seafowl, 

 sleeping in such full confidence, that even in midday 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. In 

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

 which have been shot through the air whilst fluid, and have con- 

 sequently assumed a spherical or pear-shape. Not only their ex- 

 ternal 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 woodcut. The central part 



is coarsely cellular, the cells decreasing in size towards the ex- 

 terior ; 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 



