16 CHARLES DARWIN 



The geology of this island is the most interesting part of 

 its natural history. On entering the harbour, a perfectly 

 horizontal white band, in the face of the sea cliff, may be seen 

 running for some miles along the coast, and at the height of 

 about forty-five feet above the water. Upon examination, 

 this white stratum is found to consist of calcareous matter, 

 with numerous shells embedded, most or all of which now 

 exist on the neighbouring coast. It rests on ancient volcanic 

 rocks, and has been covered by a stream of basalt, which 

 must have entered the sea when the white shelly bed was 

 lying at the bottom. It is interesting to trace the changes, 

 produced by the heat of the overlying lava, on the friable 

 mass, which in parts has been converted into a crystalline 

 limestone, and in other parts into a compact spotted stone. 

 Where the lime has been caught up by the scoriaceous frag- 

 ments of the lower surface of the stream, it is converted into 

 groups of beautifully radiated fibres resembling arragonite. 

 The beds of lava rise in successive gently-sloping plains, 

 towards the interior, whence the deluges of melted stone 

 have originally proceeded. Within historical times, no signs 

 of volcanic activity have, I believe, been manifested in any 

 part of St. Jago. Even the form of a crater can but rarely 

 be discovered on the summits of the many red cindery hills; 

 yet the more recent streams can be distinguished on the 

 coast, forming lines of cliffs of less height, but stretching 

 out in advance of those belonging to an older series: the 

 height of the cliffs thus affording a rude measure of the age 

 of the streams. 



During our stay, I observed the habits of some marine 

 animals. A large Aplysia is very common. This sea-slug 

 is about five inches long; and is of a dirty yellowish colour, 

 veined with purple. On each side of the lower surface, or 

 foot, there is a broad membrane, which appears sometimes 

 to act as a ventilator, in causing a current of water to flow 

 over the dorsal branchiae or lungs. It feeds on the delicate 

 sea-weeds which grow among the stones in muddy and shal- 

 low water; and I found in its stomach several small pebbles, 

 as in the gizzard of a bird. This slug, when disturbed, emits 

 a very fine purplish-red fluid, which stains the water for the 

 space of a foot around. Besides this means of defence, an 



