GEOLOGY. VOLCANIC BOMBS. 295 



ged to a degree which, geologically speaking, is 

 not of easy 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 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 conse- 

 quently 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 bro- 

 ken, is represented very accurately in the woodcut 

 on the next page. The central part is coarsely cel- 

 lular, the cells decreasing in size Rewards the exte- 

 rior, where there is a sheU-like case about the third 

 of an inch in thickness, of compact stone, which 

 again is overlaid by the outside crust of finely cel- 

 lular 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 centi-if- 

 ugal force, by refieving the pressure in the more 

 central parts of the bomb, allowed the heated va- 



