OCEAN-CURRENTS CAUSED BY MOLLUSCS, 359 



denser ; but now the rain-cloud returns laden with as much 

 salt as it originally had, and the very fluidity of the sea is 

 in peril, for evaporation is incessantly going on, and rivers 

 are incessantly returning laden with lime. What becomes 

 of this excess of lime ? Polypes and Molluscs, Crustacea and 

 Fish, but mainly the two former, clutch hold of it, wring it 

 from the water, and mould it into habitations for themselves. 

 It is thus that vast coral islands and oyster-beds are formed. 

 The sea is a great lime-quarry ; but the lime is arranged in 

 beautiful forms, and subserves a great organic end. Not only 

 are animals thus furnished with houses and solid structures, 

 but the water, relieved of its excess, is enabled to flow in 

 mighty currents. This is the theory propounded by Lieu- 

 tenant Maury in his fascinating book.* Assuming the waters 

 of the sea to be in a state of perfect equilibrium, the animals 

 •would, by their secretion of salts from it, produce currents : 

 " The Mollusc, abstracting the solid matters, has by that act 

 destroyed the equilibrium of the whole ocean, for the specific 

 gravity of that portion of water from which this solid matter 

 has been abstracted is altered. Having lost a portion of its 

 solid contents, it has become specifically lighter than it was 

 before ; it must, therefore, give place to the pressure which 

 the heavier water exerts to push it aside, and occupy its 

 place ; and it must consequently travel about and miugle 

 with the waters of the other parts of the ocean, until its 



proportion of solid matter is returned to it The 



sea-breeze plays upon the surface ; it converts only fresh 

 water into vapour, and leaves the solid matter behind. The 



* Maurt : Physical Geography of the Sea, p. 167. 



