6 ST. JAGO—CAPE DE VERD ISLANDS. [ouar 1. 
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 em- 
bedded, 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 crys- 
talline 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 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 be- 
lieve, 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 dis- 
tinguished 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 branchize or 
lungs. It feeds on the delicate sea-weeds which grow among 
the stones in muddy and shallow water; and I found in its sto- 
mach 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 acrid secretion, which is spread over its 
body, causes a sharp, stinging sensation, similar to that produced 
by the Physalia, or Portuguese man-of-war. 
I was much interested, on several occasions, by watching the 
habits of an Octopus, or cuttle-fish. Although common in the 
