1838.] VOLCANIC BOMBS. 359 



varieties as described by Mr. Waterhouse ; one is of a black colour, 

 with fine glossy fur, and lives on the grassy summit ; the other is 

 brown-coloured and less glossy, with longer hairs, and lives near the 

 settlement on the coast. Both these varieties are one-third smaller 

 than the common black rat (M. rattus) ; and they differ from it both 

 in the colour and character of their fur, but in no other essential 

 respect. I can hardly doubt that thesei rats (like the common mouse, 

 which has also run wild) have been imported, 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 Verd 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, geo- 

 logically 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 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 wood- 

 cut on the next page. 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 centrifugal 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. 



