6 
ST. /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 lighter 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 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 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 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 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 seaweeds 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- 
