NARRATIVE OF THE EXPEDITION IN 1896. 



17 



for their canoes when they wish to cross from the lagoon to the ocean ; it may there- 

 fore represent a channel only recently filled np. A similar explanation may be given 

 of the absence of hard deposits in the upper 10 or 20 feet of sand passed through 

 in the first boring, for much of the sand in the immediate vicinity of this is certainly 

 of recent accumulation. No evidence of consolidation is afforded by the walls of the 

 wells which the natives have sunk in the sand to obtain fresh water. 



When sea- water, under the influence of the tides, circulates freely through the 

 islands, it exerts a solvent action on the coral rock, as already pointed out in describing 

 the gullies in the central flat of Funafuti ; but when its movements are impeded, as in 

 passing through an accumulation of sand and coral fragments, conditions may occur 

 which will be favourable to the deposition of calcium carbonate. Darwin seems to 

 have assumed something of the kind, for when endeavouring to account for the 

 existence of his ledge D, which corresponds to our pinnacles and pinnacle ridges, he 

 says : " . . . the lower fragments [of the outer ridge] are firmly cemented together 

 by percolated calcareous matter" (p. 17). The level of saturation in a pile of sand 

 and coral fra.gments will vary Avith the tides and the rainfall ; the moist sand within 

 the limits of this varying level will lose water by evaporation, which may thus bring 

 about a deposition of dissolved calcium carbonate and a consequent consolidation of the 

 loose material. In this way the lower and inner region of the outer ridge may possibly, 

 as Darwin seems to have supposed, become converted into hard rock. We have no 

 direct evidence to show that this is the case ; but assuming for a moment it is so, we 

 may proceed to enquire whether the existence of the pinnacles may not possibly be 

 accounted for on the hypothesis of a positive movement of the sea-Jevel. 



Thus, let the condition of the atoll at some former period of its existence, when the 

 sea-level was-, by hypothesis, lower than it is now, be represented by the diagrammatic 

 section below (fig. 9) : — 



L<a.goon mound 



Ouber ridqe. 



Mean 

 sea level 



Fig. 9. — Hypothetical section through the summit of the Atoll, previous to an assumed subsidence. 



Fig. 10. — The same submerged, the line marked 1 represents the former sea-level, that marked 2 the 



existing level. 



New outer ridge. 



pinnacles. 



Fig. 11. — The same, showing the consolidated core of the original outer ridge, exposed by denudation. 



D 



