64 PROFESSOR T. W. EDGEWORTII DAVID AND j\IR. G. SWEET. 



priiicijDal axis. A possible explanation of this elongation in an east and west 

 direction is suggested by the results of the Admiralty magnetic survey. 



Captain W. E. Creak's reductions of the magnetic observations by Captain Field 

 and the officers of H.M.S. " Penguin " locate an area of great disturbance of the 

 vertical magnetic force at a point bearing from the Mission Church on Funafuti main 

 island S. 87° W., 5004 3'ards distant, and distant about 1311 yards from the east 

 extremity of Te Akau Fasua. The long axis of this area runs east and west, and at 

 about 5 1 miles further west is a second area of magnetic disturbance, of which the 

 long axis trends about N.N.E. and S.S.W. A prolongation westwards of the long 

 axis of the first disturbance area intersects this second disturbance area about 

 midway between its two foci. A mass of magnetic, and probably therefore volcanic 

 rock, may underlie this spot. If so, the following may be a possible explanation of 

 the existence of this submarine promontory. 



It may be of volcanic origin. In support of this view we may observe that the atoll 

 is widest at this spot and has a corresponding promontory and shoal at its western- 

 most point (Fuafatu) wliicli lies due west of the eastern jiromontory. This suggests 

 a possible cross line of volcanic fissure trending east and west, parallel to the trend 

 of those upon which the numerous small basaltic craters of Rotuma (the nearest 

 volcanic island to Funafuti) have been built up. 



With reference to the general confio'uration of the floor of the lagoon, it will be 

 noticed, on glancing at the Admiralty chart, that the lagoon is most shallow in the most 

 sheltered areas. This is owing to the chief deposition of sediment taking place to 

 leeward of the chief land areas, and at the inner angles of the reef, as, for example, 

 is the case just west of Fongafale on the inner bend of the main island and along the 

 lagoon side of the western rim of the atoll from Fuagea to the "pocket " inclusive. 

 The effect of shelter in favouring sedimentation is well shown by the incurving of the 

 20-fathom contour line to the west of the islets of Funangonga and Funamanu. In 

 those parts of the lagoon where recent sedimentation has not been heavy enough to 

 efiace this feature a well-marked low submarine cliff may be noticed, bounding the 

 reef platform lagoonwards,* and from 1 fatliom to about 4 fathoms deep. One would 

 expect to find it the exception rather than the rule that a submarine cliff such as 

 this should be preserved at all under conditions of strong variable currents and heavy 

 silting such as obtain in the Funafuti Lagoon. 



As regards the contour of the island in cross-section, the general shape is shown in 

 the lower figure (2) on Plate B, also a photograph of Captain E. C. Hore's model. 

 The highest point of the atoll, situated near the extreme S.W. end of the islet of 

 Telele, is IG feet above higli water, as determined by one of us (Mr. Sweet). 

 The general depth of the lagoon is about 20 fatlionis, tlie two deepest spots 

 measured being 30 fiithoms. As mentioned in Section IV^ this lagoon floor is formed 



* Reference to this has akcinly been made Iiy Mr. Stanley Gakdixek. ' I'roc. Canib. i'hil. Sue.,' 

 vol y, p. 42 D. 



