160 GEOLOGICAL FORMATION. [1839. 



which are generally shelving, are likewise of the same for- 

 mation. 



(5.) Various theories have been advanced in relation to the 

 geology of this group. Some have supposed that the islands 

 were entirely the work of the lithophyte ; but the better opinion 

 seems to be, that they are the crests of submarine volcanoes, 

 the ruins and bottoms of whose craters are overgrown with 

 coral.* Captain Wilkes has based a very pretty theory on 

 the result of his examinations, which has certainly the merit 

 of originality, if not of ingenuity. He supposes that the coral 

 islands of the Pacific originally composed a vast continent, 

 the several portions of which have been separated from each 

 other ; and that the borders of the islands, being less compact 

 in some places than in others, have been torn asunder, the 

 underlying strata carried off by the influx and efflux of the 

 sea, and thus undermined, the central portions have caved in 

 and formed the lagoons.t In support of this view, he lays 

 great stress upon the facts, that the islands are evidently in 

 a state of dissolution, produced, in the main, by the constant 

 abrasion of the sea, and that there are comparatively few liv- 

 ing polyps to be found. t 



But assuming his own premises, and taking his own facts, 

 although they may tend strongly to show that the islands 

 could not be the work of zoophytes, they clearly do not prove 

 the existence of a continent ; on the contrary, the theory 

 which he advances, appears to be left very much in the situa- 

 tion of the central portions of the islands, without any under- 

 lying strata to support it. It requires far less stretch of the 

 imagination, to suppose these islets to have been thrown up 

 separately, by volcanic agency, than that a whole continent 

 was upheaved, with its superincumbent load of corallites. 

 The position of the Paumotu Group, also, with regard to the 

 currents of the Pacific, the conical form of the islands, and 

 the existence of coral, in a living, or decomposing state, all 



* Lyell's Geology, Vol. Ill, p. 220, ct seq. 



t Narrative of the Exploring Expedition, Vol. IV, p. 26'8, ct sea. 



$ Narrative ut supra. 





