256 THE OCEAN. 



themselves by sudden contraction into their holes 

 the moment they are molested. 



It was for a long time supposed that all the islands 

 of Coral formation were reared from their bases, 

 fathomless depths in the Ocean, by the unaided 

 efforts of these minute creatures ; and from ex- 

 aggerated notions of the rapidity with wliich the 

 process was going on, anticipations were frequently 

 uttered that a large portion of the Pacific might at no 

 very distant period be occupied by the spreading struc- 

 tures united into a vast coral continent. More accu- 

 rate observations have, however, satisfactorily proved 

 that the living animals cannot exist at a greater depth 

 than twenty or thirty fathoms, so that the whole of 

 these animal secretions must have been deposited 

 within that distance from the surface. At the same 

 time it is no less true that the water in the immediate 

 vicinity of the islands is fathomless, and that the 

 descent of their outer edge is remarkably abrupt 

 and precipitous. The only satisfactory explanation of 

 the phenomenon appears to be the one proposed and 

 ably supported by Mr. Darwin, in his elaborate 

 treatise on Coral reefs. Many islands of the com- 

 mon rock formation are found in the Pacific, on the 

 shelving sides of which, a few fathoms below water, 

 the coral animals have fixed their stony habitations, 

 forming what is called a fringing reef, distinguished 

 from others by being immediately attached to the 

 land, without the intei-vention of any lagoon or 

 channel of water. Mr. Darwin supposes that every 

 island in the Pacific originally presented this struc- 

 ture, but that wherever a variation at present exists, 



