408 



PROCEEDINGS OF SECTION C. 



It cannot be insisted upon too strongly that all over the reef, 

 wherever water moves, there we find a perpetual swirl of grit. This 

 rasps the negroheads, and planes down inequahties on the reef crest. 

 But any hollow calm acts as a trap to arrest and retain this grit. The 

 larger the area of ground between tide marks the more room there is 

 to produce foraminifera, corals, and shells, which in turn supply build- 

 ing and eroding material. 



It is a geological axiom that as the source of a stream is approached 

 its burden consists of larger fragments. So approaching the weather 

 edge of a reef we pass from finer to coarser debris, for here is the main 

 source of building material. Under the high pressure of the hurricane, 

 the factory turns out the huge masses we call negroheads. The output 

 of a lesser gale is coral clinkers ; while ordinary weather throws up 

 sand and broken shells. 



As soon as the reef reaches the dead crest stage, the matter trans- 

 ported by the water commences to be washed across the reef into the 

 area enclosed by the horseshoe, here called the lagoon. Everywhere in 



Diagram showing Progressive Stages in the Broadening of the Lagoon Walls and the 

 Infilling of the Lagoon Itself. 



the lagoon of Cairns Reef our dredge brought up a load of weed, shells, 

 foraminifera, sand, and broken coral. The borings of Mr. G. H. 

 HalUgan showed the floor of Funafuti Lagoon to be composed of like 

 material. (gr) From this stage onwards the lagoon is Hke a lake in process 

 of being choked with sediment by brooks. If there is a gap or gutter 

 in the reef, the stream which spurts from it into the lagoon tends to 

 build a jetty. Such are shown on the west side of Captain Here's 

 model of Funah\ti.{h) Here and there, at a distance fro.n the reef, there 

 may be an odd clump or two of coral, but such plays a sUght part in 

 the infilling of the lagoon. The sediment in the lagoon is apt to smother 



(<7) "Nature," lxx., 1904, p. 319. 

 (h) David and Sweet: "The Atoll of Funafuti," 1904, p. 62, plate B. 



