24 
THE OCEAN WORLD. 
very deep water, and it would be impossible to admit that the bottom 
of the sea has everywhere risen to the same level. It is, therefore, 
more probable that the foundations of the Coral Islands are only 
natural elevations of the bottom of the sea, or submerged mountains, 
not far from the surface of which the polypi have taken possession, 
in order to raise their structures. It is a curious fact that the 
barriers of coral which run parallcd to the coast are always sepa- 
rated by a broad canal, analogous to the lagoons in the atolls, and of 
a breadth varying from one to twelve miles. One of these reefs in- 
closes at times a dozen rocky islands. In the island of Bolabola, the 
baniei is transformed into dry land ; but the white line of enormous 
reefs, with small islands crowned with cocoa-nut trees scattered here 
and there, separates the dark ocean from the placid surface of the 
interior canal, whose limpid water bathes an alluvial soil covered 
with a tropical vegetation. This many-coloured riband stretches out 
at the foot of the wild and abrupt mountains of the centre. 
In the year 1858, Mr. Darwin explored in particular the island of 
Keeling, or of cocoa-nuts, to the south of Sumatra. It is formed of a 
circle of reefs, crowned by a wreath of very narrow islets, which leave 
towards the north a passage for ships. In the interior of the anchor- 
age the water is a calm and transparent lagoon, so pure that the white 
and level bottom is clearly seen ; this lagoon is many miles broad. 
Mr. Darwin landed with Captain Fitzroy upon an islet at the farther 
end of the lagoon, in order to see the waves break on the reefs to 
windward. The cocoa-nut palms formed festoons of emerald-green, 
clearly defined, against the deep blue of the sky ; the flat, calcareous 
beach, with scattered blocks of white stone, was bathed by the foaming 
waves. 
Besides the substances named, sea water also contains, in infinitesi- 
mally small quantities, metals such as iron, copper, lead, and silver. 
Ihe old copper collecting round the keels of ships sometimes contains 
so much silver that it has been thought worth extracting ! A curious 
calculation has been attempted, based on the age of ships and the 
distance they have gone during all their voyages, to show that the sea 
contains in solution two million tons of silver.* 
* Sir J. Herschel's “ Physical Geography,” p. 22, gives the basis and details of this 
calculation. 
