106 



BULLETIN : MUSEUM OF COMPAKATIVE ZOOLOGY. 



reef, but also a barrier reef of a triangular shape surrounding it. It is 

 evident that this barrier has been formed upon the denuded and eroded 

 spurs of Totoya, which once extended seaward from the outer rim of the 

 volcano. 



Supposing, now, that the erosion of both Thombia and Totoya had 

 continued for a period of time long enough to have reduced the rims of 

 these volcanoes to the level of the sea, we should have, as soon as corals 

 had covered the flats thus formed indicating the former existence of the 

 rim, atolls of nearly circular form ; the one with a circumference of only 

 two miles, and a depth of 24 fathoms, without patches in the central 

 lagoon; the other much larger, more tlian 25 miles in circumference, 

 having a depth of 34 fathoms inside the lagoon, which would be dotted 

 with patches, some of them forming part of the rim, others being the 

 remains of eroded spurs extending towards the centre of the extinct 

 crater. 



Admiral Wharton ^ has given a most interesting sketch of Clipperton 

 Atoll, in which he confirms the trachytic nature of the " Eock " of Clip- 



CLIPPERTON ROCK. 

 (From a Photograph by J. T. Arundel.) 



perton, as determined by Professor Wolff ^ from specimens collected by 

 Mr. Jensen and kindly sent me by Professor Davidson. The photo- 

 graphs and specimens collected by Mr. J. T. Arundel, on which Admiral 

 Wharton's notice is based, have enabled him to give what seems to me 

 a natural explanation of the character of the atoll. As he says, it is 



1 Quart. Journ. Geol. Soc. of London, May, 1898, p. 228. 



2 Bull. Mus. Comp. Zool., Vol. XXVL No. 1, p. 174, 1894. 



