IV. J. SOLLAS 



[JANUARY 



) SWORT w 



surrounds an island with an intervening saltwater channel, is illustrated 

 by another class, known as " encircling " or " barrier " reefs. In these, 

 as we might expect, the form of the reef is only remotely related to 

 the contour of the enclosed island, the valleys of which present that 

 fiord-like character so suggestive of sunken land. The last stage is 

 that of the atoll itself. 



The excellence of Darwin's theory lies in this, that it explains all 

 the essential features of an atoll on one simple assumption. It is 

 inconsistent with no known fact, and as additional discoveries have 

 been made it has not required to be supplemented by fresh hypotheses. 

 It is not like a Gothic structure, supported by flying buttresses and 

 other tours cle force, but rather resembles some noble Italian tower, 

 which rises from its base, straight, simple, and self-sufficing. It was no 

 sooner aiven to the world than it commanded almost universal assent. 



Nevertheless it has never been without a rival : even before 

 Darwin published his celebrated work, Ainsworth l had suggested a 

 different explanation. He rightly pointed out that Quoy and Gaimard 

 had not established a limit for all reef-building organisms, and that 



although certain corals, such 

 as they had observed, might 

 be restricted to shallow 

 waters, there might yet be 

 others capable of flourishing 

 at greater depths. If so, 

 these deep-water organisms 

 might be engaged in laying 

 the foundations of an atoll on which the shallower-water forms might 

 erect the superstructure (Fig. 3). This suggestion seems to have fallen 

 still-born, but the notion of " laying the foundation " of an atoll was 

 not destined to perish : it has been revived of late years by Sir 

 John Murray, who, guided by his observations made when on board 

 the " Challenger," was led to suppose that the submerged summits of 

 deeply-sunken islands 



*■ " . S'R John Muboav 



might be raised to within 

 the limit of 25 fathoms, 

 not by the upward growth 

 of corals but by the in- 

 cessant downward rain of 

 minute organisms from the 

 surface of the sea. The 



same agencies which were supposed to be spreading out a layer of chalky 

 mud or ooze over the abyssal floor of the ocean were also imagined as 

 engaged in piling a Pelion of mud on every submarine Ossa (Fig. 4). 



1 G. W. Ainsworth, " Analysis of a Voyage to the Pacific and Behring's Straits, to co- 

 operate with the Polar Expedition, performed in H.M. Ship Blossom, under command of 

 Capt. F. W. Beechey, R.N., in the years 1825-28," Gcog. Jour. vol. i. 1831. 



Fig. 3. 



