6 ST. J AGO— CAPE DE VERB ISLANDS chap. 



mixed with finer matter. After this fact one need not be sur- 

 prised at the diffusion of the far h'ghter and smaller sporules of 

 cryptogamic plants. 



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 fort}'- 

 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 neigh- 

 bouring 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 fragments of the lower surface of the stream, 

 it is converted into groups of beautifully radiated fibres resem- 

 bling arragonite. The beds of lava rise in successive gentl}-- 

 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 mani- 

 fested 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\\eeds which 

 grow among the stones in muddy and shallow 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- 



