POLYPI. 143 



the fringing reef {fg. 63.) is converted into the barrier reef {^fig. 69.), 

 and this into the atoll (^AV* '''O.). 



If the movement of the land should now be reversed, and the level 

 of the sea be again brought back by elevation of the island, to the 

 line (s. \-Jifj. VO.), an island apparently composed exclusively of coral 

 rock, like Elizabeth Island, would be the result. 



The prodigious extent of the combined and unintermitting labours 

 of these little world-architects must be witnessed in order to be 

 adequately conceived. They have built up a barrier-reef along 

 the shores of New Caledonia for a length of 400 miles, and 

 another which runs alonsr the north-east coast of Australia 1000 miles 

 in extent. To take a small example, a single atoll may be 50 miles 

 in length by 20 in breadth; so that if the ledge of coral rock forming 

 the ring were extended in one line it would be 120 miles in length. 

 Assuming it to be a quarter of a mile in breadth, and 150 feet deep, 

 here is a mound, compared with which the walls of Babylon, the 

 great wall of China, or the pyramids of Egypt, are but children's 

 toys ; and built too amidst the waves of the ocean and in defiance of 

 its storms, which sweep away the most solid works of man. 



The geologist, in contemplating these stupendous operations, 

 appreciates the conditions and powers by which were deposited in 

 ancient times, and under other atmospheric influences than now 

 characterise our climate, those downs of chalk which give fertility to 

 the south coast and many other parts of our native island. The 

 remains of the corals in these masses, though similar in their general 

 nature, are specifically distinct from the living polypes which are 

 now actively engaged in forming similar fertile deposits on the 

 undulating and half submerged crust of the earth, washed by the 

 Indian and Pacific oceans.* Again, those masses of limestone rocks 

 which form a large part of the older secondary formations, give 

 evidence, by their organic remains, that they too are due to the" 

 secretions of gelatinous polypes, the species of which perished before 

 those that formed the cretaceous strata were created. As the polypes 

 of the secondary epochs have been superseded by the Porites, 

 Millepone, Madreporce, and other genera of calcareous Anthozoa of 

 the present day, so these, in all probability, are destined to give way 

 in their turn to new forms of essentially analogous Zoophytes, to 

 which, in time to come, the same great office will be assigned, to 

 clothe with fertile lime-stone future rising continents. 



* See the admirable description of the corals of the chalk by Mr. Lonsdale, in 

 CXXI. pp. 237—324, 



