THE GEOLOGY OF FUNAFUTI. 83 



It should be explained that the above areas refer only to that portion of the atoll 

 which was geologically mapped, which is about four-fifths of the whole atoll rim, as 

 shown on Plate 1. The one-fifth omitted is all reef platform, submerged at high 

 tide, and of this fifth, approximately one-half would be 0.3 ; one quarter, O.L.I ; 

 about one-sixth, O.L. 10; and about one-twelfth, outlying patches of various types 

 of 0.2 (breccia sheet). 



With regard to the above areas, it should be noted that the old Heliopora ccerulea 

 reef, O.L.I, the area of the exposed surface of which is given as 1002'104 acres, 

 certainly underlies the whole of the northern, eastern, and southern portions of the 

 atoll rim, with the exception, perhaps, of about half the area denoted by O.L. 10, 

 that is, the portion of O.L. 10 on the ocean side; and there can be very little 

 question that the old Heliopora reef underlies a large portion, perhaps, the whole, 

 of the western rim also, so that, in the area mapped, it is not unreasonable to assume 

 that the Heliopora ccerulea reef underlies the whole of it, with the exception, 

 perhaps, of the area of the outer reef occupied by O.L. 10, which amounts to about 

 273 acres. The total area of the Heliopora ccerulea reef, in the portion mapped, 

 which totals 3893 acres, would thus be 3893 — 273 = 3620 acres, instead of 

 1002 acres, the area of the surface actually exposed. Of the area of 545 acres, 

 accredited to the living Lithothaninion reef O.L. 10, about one-half, viz., about 

 273 acres, may represent the area of the reef platform, on the ocean side, brought 

 above low water level during the latest downward vertical movement of the shore 

 line to the extent of about 6 to 7 inches. It may be noted that the total area of the 

 breccia now exposed is only about 596 acres. To this may be added the amount 

 hidden under a capping of newer deposits, which we estimate at about 704 acres. 

 This would approximately raise the total area now occupied by the breccia to about 

 1300 acres. 



The amount of material in the Hurricane Bank derived from the breaking up of 

 breccia sheet is not equal to the amount of breccia which can be proved to have been 

 eroded. The missing material, however, may be fairly accounted for by the amount 

 removed in solution, which must be considerable, and also by the amount of material 

 pounded up into fine sand at the foot of the Hurricane Bank. This material is partly 

 blown over the top of the bank on to the surface of the islets, and is partly sucked 

 back by the waves and tide into the channels on the outer slope of the reef. 



Summary of the Geology and Past Physical Geography of Funafuti. 



The facts already adduced show that the general shape of the original foundation 

 of this atoll has been determined by some cause, probably volcanic action, or folding 

 of the earth's crust, independent of the direction of the prevalent winds and ocean 



M 2 



