228 ADMIEAL SIE W. J. WHAETON [May 1 898, 



17. Note on Clippeetoi^ Atoll (I*^oetheeis^ Pacieic). By Reae- 

 Admieal Sir William J. Whaeton, E.N., K.C.B., F.R.S.y. 

 Hydrographer to the Admiralty. (Communicated by Sir Aechi- 

 BALD Geizie, D.Sc, LL.D., F.R.S. Read March 9th, 1898.) 



[Plates XX-XXII.] 



In the eastern part of the J^orthern Pacific, in lat. 10° 17' 'N., long.- 

 109° 13' W., and 600 miles away from the nearest point of the 

 continent of North America, is a small, lonely atoll called Clipperton, 

 It has never been properly surveyed, but was in 1840 sketched by 

 Capt. Sir E. Belcher, R.N. 



Its lagoon, 2 miles in diameter, was then open to the sea in two 

 places, both shallow. The remainder of the ring surrounding it was 

 a few feet above water, forming two flat, bare, crescent-shaped 

 islands. It was the haunt of innumerable sea-birds, as might be 

 expected from its solitary position, and it is doubtless to these birds 

 that the fact that no scrap of vegetation exists is due. 



Since the date of Sir E. Belcher's visit the entrances have been 

 closed by piled-up coral-sand, the ring is complete, and the lagoon 

 is cut off from the influx of ocean-waters, except by infiltration. 



So far,. Clipperton Atoll only resembles many other islands of 

 similar type and position, but there is one point which distinguishes 

 it, so far as I know, from all other atolls, and that is the existence 

 on one part of the ring of a mass of rock about 60 feet in height. 

 Some atolls have an island somewhere near the centre of the lagoon, 

 but none, as this, on the ring itself. 



I have long wished to know the geological character of this 

 rock, but it is only lately, by the kindness of Mr. J. Arundel, who 

 visited the island in connexion with its guano-deposits, which have 

 been for some time worked, that I have obtained specimens of it. 

 Mr. Arundel, in sending me the hand-specimens, stated that the rock 

 was composed of volcanic material and coralline limestone mixed, 

 and on receiving them I imagined that he had, except in one case, 

 sent me specimens of the calcareous rock only. 



I sent the specimens to Sir A.Geikie,who showed them to Mr. Teall, 

 and I believe that their first impression was similar to my own. 

 They found, however, that the pale specimens did not effervesce with 

 acid, and could not therefore be unaltered coral-rock, while the only 

 dark specimen contained porphyritic felspars in a finely crystalline 

 base which they surmised to be some variety of trachyte or andesite. 

 Mr. Teall's subsequent investigation, submitted to the Society 

 together with the present Note, brings to light a remarkable change 

 in the structure and composition of the apparently calcareous speci- 

 mens, which have proved to be all of the same trachytic origin. 



The position of this undoubtedly volcanic mass is of great interest,. 

 By no theory of the formation of atolls can the survival of a portion 



