122 TOWN GEOLOGY. [v. 



best at tlie very surface, and in the cut of the fiercest 

 surf. And so arose a puzzle as to how coral rock is 

 often found of vast thickness, which Mr. Darwin ex- 

 plained. His theory was, and there is no doubt now 

 that it is correct, that in these cases the sea-bottom is 

 sinking ; that as it sinks, carrying the coral beds down 

 with it, the coral dies, and a fresh live crop of polypes 

 builds on the top of the houses of their dead ancestors : 

 so that, as the depression goes on, generation after 

 generation builds upwards, the living on the dead, 

 keeping the upper surface of the reef at the same level, 

 while its base is sinking downward into the abyss. 



Applying this theory to the coral reef of the 

 Pacific Ocean, the following interesting facts were made 

 out: 



That where you find an Island rising out of deep 

 water, with a ring of coral round it, a little way from 

 the shore or, as in Eastern Australia, a coast with a 

 fringing reef (the Flinders reef of Australia is eleven 

 thousand miles long) that is a pretty sure sign that 

 that shore, or mountain, is sinking slowly beneath the 

 sea. That where you find, as you often do in the Pacific, 

 a mere atoll, or circular reef of coral, with a shallow 

 pond of smooth water in the centre, and deep sea round, 

 that is a pretty sure sign that the mountain-top has 

 sunk completely into the sea, and that the corals are 

 going on building where its peak once was. 



And more. On working out the geography of the 

 South Sea Islands by the light of this theory of 

 Mr. Darwin's, the following extraordinary fact has 

 been discovered : 



That over a great part of the Pacific Ocean sinking 

 is going on, and has been going on for ages ; and that 



