144 THE WORLD'S LUMBER ROOM. 



lime dissolved is not re-deposited, gives rise to the wonderful 

 caverns already described, whose contents have of course 

 been carried back to the ocean. Thus the shells and skele- 

 tons of countless former generations are again converted 

 into shells and skeletons by their modern descendants and 

 representatives. 



On the west of Ascension Island, to take one of many 

 similar instances, the beaches are heaped with rounded 

 fragments of shells, corals, &c., which, though loose on the 

 surface, are, at the depth of a few feet, cemented into solid 

 stone, some of which is actually too hard for building pur- 

 poses and has the ring of flint. It contains very few perfect 

 shells, but each rounded fragment may be distinctly seen to 

 be surrounded by a husk of transparent carbonate of lime, 

 and the stone is nearly as compact as marble which has 

 been subjected to heat and pressure. 



For this. is the final stage in the process by which coral- 

 or shell-sand is crystallised at last into the sparkling marble, 

 much resembling loaf-sugar, which the sculptor uses for his 

 statues. Not a trace of shell or other organic remains is now 

 left, and the stone is so fine-grained and snowy-white that 

 we could never guess its origin, did we not in many instances 

 actually see what has taken place. For where ordinary 

 limestone, whose composition we can see, has come in con- 

 tact with a stream of lava, all traces of its original structure 

 have disappeared. It has, in fact, been melted, and on cooling 

 down has become perfectly crystalline ; a little farther off 

 the stone is hardened and partially crystallised, and farther 

 off still it remains unaltered, being beyond the reach of the 

 heat. 



