CORAL REEFS. 145 



the high volcanic land from a ring of coral 125 feet above sea level. 

 This ring entirely surrounds the island. It is 200 or 300 yards from 

 the volcanic land, though here and there united by limestone, and 

 in my opinion it represents an old barrier raised by movements 

 of elevation that succeeded the gradual depression to which the 

 formation of the barrier was due. There is no sign of marine 

 erosion on the floor of the alluvial land, and no cliffs fringe 

 the volcanic hill. The outer surface of the coral ring shows three 

 terraces, and the present reef platform is 200 yards wide. 



In conclusion, the author is compelled to say that after 

 observing the reefs of the Society and Cook Islands with con- 

 siderable care and with a knowledge of the statements and observa- 

 tions of previous visitors, he found no reasons to consider Darwin's 

 theory as in any way unsatisfactory, and many reasons to consider 

 it entirely satisfactory. On the other hand, many observations 

 were directly opposed to the statements of Agassiz, and he could 

 find no facts that supported Murray's theory. 



Wood-Jones' theory, which depends on the effect of sedimenta- 

 tion, could not be tested during such a short visit. It must be 

 remembered that wherever there is a check to the movement of 

 water the limiting level of sedimentation rises. Such check would 

 be experienced wherever coral structures grow, and the sediment 

 would be deposited in the sheltered spots of all coral calices. This 

 appears to be an important consideration, for suspended matter is 

 present in greatest quantity when the wave breaks on the coral, 

 and whenever there is a small sheltered' spot some of it will be 

 deposited. 



Two of the latest opinions on this subject are those of Suess 

 and of Langenbeck. The former says : — " Yet notwithstanding 

 the valuable investigations quite recently made, and notwith- 

 standing the objections which such distinguished investigators 

 as Semper, Murray and Agassiz have made, it must still be admitted 

 that the depth of the enclosed lagoon has not yet been satisfactorily 

 explained." (Suess, E., " Face of the Earth," vol. iv., 1910, p. 327) ; 

 and the latter, after reviewing the whole question, states, p. 59 : — 

 " Dass Dagegen die Darwin-Danasche theorie and sie allein eine 

 befriedigende Erklarung fur dieselben giebt." (Langenbeck " Die 

 Theorieen fiber die Entstehung der Koralleninseln und Korallen- 

 riffe," Leipzig, 1890, p. 59.) 



EXPLANATION OF PLATES X. AND XL 



PLATE X. 



L — Barrier reef, lagoon, and fringing reef. Looking north-ea„st from Mt. 

 Tapioi, 960 feet, Raiatea Island, Society group. 



2. — Awapiti Pass, Raiatea Island, Society group. From top of Mt 

 Tapioi, looking south-east. 



PLATIC XI. 



3. — Fringing reef, Rarotonga, Cook Islands. From top of Mt. Mamgatea 

 860 feet, looking north. 



4. — Raised coral, Ngatangiia, Rarotonga Island. Looking south. 



K 



