672 DR. F. WOOD JONES ON A [June 15, 



which does not reach 3000 fathoms — or ocean depths — till over 100 

 miles are traversed from the atoll. The summit of this long ridge 

 is composed of Globigerina ooze, and for ten miles from the reef 

 edge — that is to a depth of 2000 fathoms — the gradient of the 

 bank is 1 in 5. From this point on, to a distance of 50 miles 

 from the reef, the slope is about 1 in 80, and then for the next 

 50 miles or so, at 1 in 100 to the ocean depths of Radiolarian 

 ooze. The presence of this great bank of ooze demands an 

 explanation. Upon this submarine plateau there is a coral-reef, 

 and it is necessary to explain how the bank becomes a suitable 

 site for coral growth — since we know that the bathymetrical 

 range of reef -building corals is slight ; and also to account for this 

 limitation of the extension of reef-coral life in the depths of 

 the sea. 



"We know that on such banks such reefs exist below the surface 

 of the sea, and (from the soundings of Admiral Sir J. W. L. 

 Wharton) we also know that, before they reach the surface, 

 their margins are raised above the level of their central area. 

 These are Darwin's " drowned atolls," and if his theory of the 

 development of coral structures be not accepted, any new theoi-y 

 must take cognisance of these basin-shaped reefs. Another 

 typical feature is the level plateau of coral breccia that forms the 

 barrier reef and shore platform of the atoll, and that — although it 

 does not appear to be recognized — runs uninterrupted beneath the 

 surface of the islands, and outcrops upon the lagoon shore. To 

 this whole platform of consolidated coral conglomerate I have 

 given the name of the Breccia Platform, for its subdivision into 

 barrier reef, shore platform, lagoon breccia, &c. is artificial, and 

 ignores the fact that these parts are all in continuity, being 

 really one level stratum upon which the islands rest as mere 

 debris piles. The Breccia Platform of an atoll runs as a con- 

 tinuous structure round the whole extent of the ring, save where 

 the lagoon extrance exists. 



The origin of the Breccia Platform must be made clear, and 

 its presence as a continuous layer beneath the surface of the 

 islands needs explanation. It is the normal state of affairs for 

 the entrance to the lagoon to be situated upon the lee side of the 

 atoll and this requires explanation, as do also the facts that some 

 atolls are perfect circles with no entrance to the lagoon, while 

 some have a pseudo-entrance to the windward. 



In some atolls, each constituent island of the ring is itself an 

 atollon and encloses its own lagoonlet ; in others, certain of the 

 islands only become atollons, or some, or all, of the islands are 

 crescent-shaped, and are only approximations to the circular form. 

 These conditions are obviously the outcome of the actions of 

 foi-ces very similar to, or identical with, those that formed the 

 parent atoll, and the explanation of its formation should also be 

 applicable to the cases of the constituent islands. 



Observations extending over a long period of time may be 

 recorded of several atolls, and in these cases it is usual to find 



