70 PROFESSOR T. W. EDGEWORTH DAVID AND M~R. G. SWEET. 



Atoll shows that the Helio2:)ora ccBrulea reef is very extensively develoj^ed, and in 

 view of the fact that a large portion of what is shown on the maps as the dead 

 Lithothamnion (0.3) is merely a thin crust overlying the Heliopora ccsrulea, the latter 

 must constitute more than one half of the whole area of the portion of the atoll, 

 shown on the geological maps, and perhaps underlies the whole of it. That Heliopora 

 ccerulea may underlie that portion of the reef platform seems likely from the fact that 

 there are large areas of it living and extending from Fuafatu northwards to Pava on 

 the lagoon side of the atoll rim, and this view is supported by the discovery of 

 fragments of that coral in the breccia of the western rim, as already stated. The 

 most conspicuous feature about the reef platform on the western rim of the atoll 

 is the imm.ense development in situ of dead Porites. The bosses of this coral, as 

 described by one of us (Mr. G. Sweet), are from 5 to 8 feet in diameter, and 

 5 to 6 feet high. They are mostly traversed by radial and concentric cracks. 



Although Porites is one of the hardiest types of coral at Funafuti, and some species 

 live in pools on the reef platform at a height of over 2 feet above low water, still 

 these are exclusively thin flattened forms, the shape of which has adapted itself 

 to that of the shallow pools, and they are by no means comparable to the immense 

 heads of this coral i/i situ on the reef platform, so strikingly numerous between Fuagea 

 and Avalau. Their occurrence at levels up to 4 feet above high water confirms the 

 view already inferred from other evidences that there has been a recent downward 

 movement of the shore line to an extent of certainly over 6 feet, and almost certainly 

 to 10 feet. This 10 feet is calculated by adding 4 feet (the extreme height of the 

 Porites above high water) to 6^ feet (the range of the tide). 



The Breccia.- — Next in age, and newer than the Heliopora ccerulea and Porites reefs, 

 is the well-defined breccia horizon denoted O.2.A., O.2.B., O.2.C., O.2.D., and O.2.E., 

 including also jDatches of very dense sandstone occasionally met with in the breccia. 

 It is formed largely in its lower portions of fragments of the older Heliopora and 

 Porites reefs cemented partly by Lithothamnion and partly by Polytrema planum. 

 The fonner of these two cementing agents has rendered the upper portion of the 

 breccia particularly dense. 



The breccia is extremely hard, so that it is a difiicult matter, where it is exposed 

 in the smooth surface of the reef platform, to detach 'chips of it even with 

 a hammer and chisel. It generally extends from about 3 feet below high water 

 up to from 1 to 2 feet above high water. If, however, the outliers observed 

 along many parts of the ocean beach, and especially at Motusanapa, be included 

 as portion of the breccia, then it must be conceded that in places it reaches a height 

 of about 10 feet above high-water level. The thickness thus varies from 4 feet up to 

 about 13 feet. Our general conclusion then is that this breccia has been formed 

 below water, mostly at or' below the level of low tide, probably the latter. It is 

 clear that in many of the cases where the breccia surface is from 1 to 2 feet above 

 high v/ater a downward movement of the shore line of 7 to 8 feet is implied. 



