183G.] VOLCANIC BOMBS. 493 



and, as at the Galapagos, have varied from the effect of the new 

 conditions to which they have been exposed : hence the variety 

 on the summit of the island differs from that on the coast. Of 

 native birds there are none ; but the guinea-fowl, imported from 

 the Cape de Yerd Islands, is abundant, and the common fowl 

 has likewise run wild. Some cats, which were originally turned 

 out to destroy the rats and mice, have increased, so as to become 

 a great plague. The island is entirely without trees, in which, 

 and in every other respect, it is very far inferior to St. Helena. 



One of my excursions took me towards the S.W. extremity of 

 the island. The day was clear and hot, and I saw the island, 

 not smiling with beauty, but staring with naked hideousness. 

 The lava streams are covered with hummocks, and are rugged 

 to a degree which, geologically speaking, is not of easy expla- 

 nation. 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 

 witti which the whole plain was mottled ; I now found that they 

 were seafowl, 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. 

 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 on the next page. 

 The central part is coarsely cellular, the ^e\h 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, gene- 

 rated by the revolving of the bomb, against the external cooled 



