76 



FORMATION OP LIMESTONE. 



calcareous remains, another succeeds, until the mass of coral is rais- 

 ed to the surface, when the formation ceases. Fragments of coral 

 are afterwards broken off by the waves, during storms, together with 

 shells, weeds, and sand, and are driven upon the other parts of the 

 island, and continue to elevate it until it is above the reach of their 

 action. From the accounts of the above naturalists, and the more 

 recent observations of Captain Beechy, it appears, that the species 

 of polypi that, chiefly, form coral islands, do not exist at greater 

 depths than a few fathoms below the surface ;^ therefore, the deep 

 soundings taken near these islands prove, that coral forms the crests 

 of steep submarine mountains, which were probably volcanic, as these 

 crests have frequently a circular shape, but are open on one side, 

 leaving a passage to a circular lagoon or lake within, which is shal- 

 low, and supposed to fill the crater of a submarine volcano. Though 

 the beds of coral that form islands are not of the vast thickness which 

 had been supposed, yet they rival, in extent and magnitude, some of 

 the large calcareous formations of our present continents.f Beds of 

 oyster shells, many miles in length, are also known to occur in Euro- 

 pean seas ; thus, millions of small marine animals are preparing fu- 

 ture abodes for other classes of animals of larger size, and living in 

 another element. Whence do these innumerable zoophytes and 

 molluscous animals procure the lime which, mixed with a small por- 

 tion of animal matter, forms the solid covering by which they are 

 protected ? Have they the power of separating it from other sub- 

 stances, or the still more extraordinary faculty of producing it, from 

 simple elements ? The latter I consider as more probable ; for, the 

 polypi which accumulate rocks of coral have no power of locomo- 

 tion; their growth is rapid, and the quantity of calcareous matter 

 they produce, in a short space of time, can scarcely be supposed to 

 exist in the waters of the ocean to which they have access, as sea 

 water contains but a minute portion of lime. 



It is now ascertained, that lime and the other earths are com- 

 pounds of oxygen united with metallic bases ; and the brilliant dis- 

 coveries of Sir H. Davy respecting the metallic nature of ammonia, 

 would lead to the conclusion, that the metallic bases of all the alka- 

 lies and alkaline earths, which have many properties in common, 

 may, like ammonia, be compounds of hydrogen and azote, but dif- 

 ferently combined. Now it is well known, that hydrogen and azote, 

 which exist as elementary constituent parts of almost all animal sub- 

 stances, may be derived from water and the atmosphere ; and should 

 the compound nature of the metallic bases of the earths be ascer- 

 tained, the formation of lime by animal secretion will admit of an 

 easy explanation. 



* Some species of coral were brought up by soundings, from the depth of one 

 hundred fathoms or more. 



+ Some of these islands are considerably elevated above the level of the sea ; in 

 all probability they have been upheaved by volcanic agency. 



