CHAP. XXV.] GEOLOGICAL STRUCTURE. 105 



very lowest ebb-tides showing any rock above the surface. 

 There are several deep entrances through this reef, and 

 inside it there is good anchorage in all weathers. The land 

 rises gradually to a moderate height, and numerous small 

 streams descend on all sides. The mere existence of these 

 streams would prove that the island w^as not entirely coral- 

 line, as in that case all the water would sink through the 

 porous rock as it does at Manowolko and Matabello ; but 

 we have more positive proof in the pebbles and stones of 

 their beds, which exhibit a variety of stratified crystalline 

 rocks. About a hundred yards from the beach rises a wall 

 of coral rock, ten or twenty feet high, above which is an 

 undulating surface of rugged coral, which slopes doionward 

 towards the interior, and then after a slight ascent is 

 bounded by a second wall of coral. Similar walls occur 

 higher up, and coral is found on the highest part of the 

 island. 



This peculiar structure teaches us that before the 

 coral was formed land existed in this spot ; that this 

 land sunk gradually beneath the waters, but with in- 

 tervals of rest, during which encircling reefs were formed 

 around it at different elevations ; that it then rose to 

 above its present elevation, and is now again sinking. 

 We infer this, because encircling reefs are a proof of 

 subsidence ; and if the island were again elevated about 

 a hundred feet, what is now the reef and the shallow 



