CORALLINES. 
171 
which is imprisoned, and riveted to their person ; their stomachs in the 
bark, their arms on a branch, their movements perfect repose ! 
^.These minute silent workers are active and indefatigable; tlieir task 
is to separate the salt and other chemical particles from the waters of 
the ocean, and, while feeding themselves, secrete and organise the axis 
which bears their lodging. They love the warmer regions of the ocean ; 
in colder regions, the results of their labours are extremely limited : 
the one forms a sward of submarine life, which carpets the rocks ; the 
other produces animated stalactites, great shrubs, whole forests oi small 
trees. The electric cable, which unites Sardinia to the Genoese fort, 
Was so encrusted with polypiers and bryozoares, that certain portions 
taken from the water for repairs had attained the size of a small barrel. 
The atolls present three unfailing and constant peculiarities. 
Sometimes they constitute a great circular chain, the centre of which 
is occupied by a deep basin, in direct communication with the exterior 
s ea, through one or many breaches of great depth. These are the 
«*>&, described more than two centuries ago by Pyrard de Laval ; 
sometimes they surround, but at some distance, a small island, in such 
a manner as to constitute a sort of skeleton or girdle of reefs ; finally, 
ttiey may form the immediate edging or border of an island or continent. 
bR this last case, they are called fringing littorals, or edging reefs. At 
tlle distance of a few hundred yards only from the edge of some of 
these reefs, the sea is of such a depth that the sounding-lead has failed 
to reach the bottom. 
In order to give an idea of the general form oi these atolls, 
Although they are rarely so regular, the reader is referred to Pl. HI., 
which represents one of these islands of the. Pomotouan Archipelago, 
m the Indian Ocean. It represents the island of Clermont-Tonnerre, 
figured by Captain Wilkes in the American Exploring Expedition. 
Ibe exterior girdle of rocks here surrounds a basin nearly circular. 
S uoh is the general form— the typical form, so to speak— of the coral 
of which this is a fair representation. 
The zoophytes which form these mineral accumulations belong to 
Averse group's, and nowhere have the results of observations made 
u P°n these atolls been more minutely described than in Mr. Darwin’s 
r °marks on the grand Cocos Island situated to the south of Sumatra, in 
tll( 5 Indian Ocean. 
