OCEAN-CURRENTS CAUSED BY MOLLUSCS. 377 



matter such as would impede its flow, were there no 

 provision ready to restore the equilibrium. For ob- 

 serve, the rain-cloud, as it rose by evaporation from 

 the sea, left behind it all the salts which it contained, 

 and these would make the rest of the water 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 

 IMolluscs, 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 ar- 

 ranged in beautiful forms, and subserves a great 

 organic end. Not only are animals thus furnished 

 with houses and solid structures, but the water, re- 

 lieved 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 equili- 

 brium, the animals would, by their secretion of salts 

 from it, produce currents : " The Mollusc, abstracting 

 the solid matters, has by that act destroyed the equili- 

 brium 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 



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



