THE GEOLOGY OF FUNAFUTI. 65 



chiefly of "sand" composed almost entirely of joints of Halimeda, together with, to 

 a much less extent, Lithothamnion, foraminiferal shells and fragments of coral. The 

 two bores put down by Mr. G. H. Halligan in the floor of the lagoon showed this 

 Halimeda " sand " to be at least 80 feet thick in each place. Here and there clusters 

 of coral rise from the floor, in some cases to the level of low tide, making small shoals 

 like those of Te Akau Fasua, Te Akau Pusa, Te Akau Tuluaga, &c. Towards the 

 inner edge of the atoll rim the lagoon gradually shoals to a depth of 1 to 3 fathoms 

 at low tide ; the above-mentioned low cliff of 1 to 4 fathoms, succeeds, forming the 

 boundary of the lagoon platform. The latter varies in width from half a chain up 

 to 10 chains ; the average width being about 2 chains. Next follows a slope of sand or 

 shingle rising up to a few feet above high water, and next in many places, though 

 not everywhere developed, comes the Inner Hurricane Bank, of lagoon origin, its 

 summit on the eastern rim being 4 to 8 feet, usually about 5 feet, above the level of 

 high water. The Inner Hurricane Bank on that side of the atoll is thrown up by 

 the lagoon waves under the influence of the N.W. monsoons, and that on the west of 

 the atoU by the S.E. trade winds. Next follows the belt of vegetation supported 

 on calcareous sand and rubble, frequently sloping downwards to a corrosion hollow 

 at the centre of the islands. This hollow in many cases is inundated at high tide. 

 From here the ground rises seawards to the Outer Hurricane Bank, facing the ocean. 

 The top of this bank is usually G to 8 feet, occasionally 10 to 11 feet, above high 

 water, and at the S.W. end of Telele it rises to a height of IG feet above high water. 

 Next comes the portion of the reef platform covered by the ocean at high tide and 

 dry at low tide for a width of about 5 chains. From low tide the atoll slopes down- 

 wards gradually to a depth of 12-14 fathoms, steepening rapidly to an angle of about 

 30' down to 35 fathoms. Thence it falls almost precipitously, in places (as ofi" Tebuka, 

 Fuafatu and Funama.nu) as an actual precipice, to a total depth of 140 fathoms. The 

 existence of this persistent submarine clifi", or "To" (pronounced "Tow") as the 

 natives call it, is one of the most remarkable features in the relief of the atoll. 

 Professor Bonxey has likened it to a gigantic fortress wall crowning a conical 

 mountain.* In fact, if the whole atoll were elevated 140 fathoms, the portion then 

 above sea-level would everywhere present a clifi* face 300-GOO feet high with a slope 

 of from 70^ up to nearly 90°. (See Plate B, fig. 2.) 



From the 140-fathom level the atoll slopes downwards more gently, at first in 

 a convex curve, then in a more gradual slightly concave curve, to about 400 

 fathoms, the angle of slope being about 30° ; thence the slope diminishes down to 

 depths of nearly 3000 fathoms. 



The 140-fathom contour line is met with at about 200 fathoms from the edge of 

 the reef, the 500-fathom line at between 700 and 800 fathoms, the 1000-fathom line 

 at from 1| to 2 nautical miles, the 2000-fathom line at about 7 nautical miles, 



* ' Funafuti, ov Three Months on a Coral Island.' By Mrs, Edgeworth David, Mdth Scientific 

 Postscript by Professor Bonney, p. 307. 



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