THE VOYAGE OF THE BEAGLE 



519 



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 these 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 like- 

 wise run wild. Some cats, which were originally turned out 

 to destroy the rats and mice, have increased, so as to be- 

 come 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. extrem- 

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

 island, not smiling with beauty, but staring with naked hide- 

 ousness. The lava streams are covered with hummocks, and 

 are rugged 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 pass- 

 ing 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 re- 

 volved 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 



