CORAL ISLANDS. 21 



a subject of much astonishment, and ought, therefore, to find a place 

 in this chapter. 



The Pacific and Indian Oceans are studded with islands in a state 

 of formation, which owe their origin to the polypi and corallines. 

 These zoophytes extract from the sea water the lime and silicium 

 which are found there in the state of soluble salts. In order to grow 

 and develop, they must be continually under water. They are con- 

 stantly producing calcareous deposits; these deposits rise rapidly, 

 and at last reach the surface of the water. Then the seaweed and 

 rubbish of all kinds that the sea carries along with it, arrested by 

 these emerged masses, cover them with a layer of fertile soil, which is 

 soon covered with vegetation, as the birds and the waves bring seeds 

 thither. 



The Coral Islands of the Pacific, which are described in another 

 chapter, are formed in this manner. 



Besides the substances named, sea water also contains, in infinitesi- 

 mally small quantities, metals, such as iron, copper, lead and silver. 

 The old copper collecting ground the keels of ships sometimes 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.* 



The question has often been asked, whence comes the salt and other 

 substances held in solution in sea water ? If our readers will turn 

 back to the first few pages of " The World before the Deluge," they 

 will better understand the very simple geological explanation that we 

 are going to give of the origin of different substances dissolved in 

 sea water. 



In the first stage of our planet, before the watery vapours contained 

 in the primitive atmosphere were condensed, and before they had 

 begun to fall on the earth in the form of boiling rain, the shell of the 

 earth contained an infinite variety of heterogeneous mineral substances, 

 some soluble in water, others not. When rain fell on the burning sur- 

 face for the first time, the waters became charged with all the soluble 

 substances, which were reunited and afterwards deposited, accumu- 

 lating in the large depressions of the soil. The seas of the primitive 



* Sir J. Herschel'd " Physical Geography,' 5 p. 22, gives the basis and details of tins 

 calculation. 



