THE UEOLOGY OF FlTNAFHTr. 75 



The types of rock referred to under the head of L.5, L.G, L.7.A., L.7.B., L.8.A., 

 and L.8.B. need little comment. They are all of detrital origin, formed by the 

 action of waves and wind operating on the lagoon shores of the atoll. They differ 

 from one another partly in composition (in some cases being formed chiefly of coarse 

 gravel and shingle, composed of coral breccia, Lithothamnioii, &c., and in other cases 

 of foraminiferal sand), partly in age. The relation of these rocks to one another is 

 shown on Plate 6, Funafara, Plate 7, Falefatu, Plate 9, Sections 9 and 10, on 

 Plate 12, Plan and Sections of Mulitefala, Plate 14, Plan and Sections of Tebuka. 



These plates show that the above formations mostly form curved belts, or parallel 

 bands, on the lagoon side of the islets, being composed partly of material pushed around 

 from the ocean front of the islets inwards towards the lagoon, but chiefly of sand 

 driven landwards from the lagoon by waves and wind. The greatest elevation 

 attained by the sandy deposit marked L.8.B., is at the north end of Fatato Islet, 

 where it is 13 feet above high water. A feature specially deserving of notice in 

 connection with these strata is that they are terraced, the older strata being a little 

 higher than the newer, as shown in the Sections on Plates 9, 12, and 14. While in 

 some 'cases the greater elevation of these inner terraces may be clue to storms of 

 exceptional violence, we think that, on the whole, the evidence is in favour of 

 downward movement of the shore line. 



As regards the deposit marked L.9.A., this is a recent gravel of the lagoon beach, 

 composed of well-rolled and subangular fragments of coral and coral breccia, from 

 about 1 inch up to 3 inches in diameter. This also is slightly terraced, the upper 

 terrace being about 1 to 2 feet above the lower. Of an intermediate character 

 between L.9.A. and L.9.B. is the fine subangular reef-rock rubble, which finds a 

 temporary resting place on, or near the erosion zone O.2.C. of the reef platform on the 

 ocean side. Some of this becomes imbedded between the coarser material of the 

 Hurricane Bank, some is washed back into the channels on the reef-edge, O.L. 10, and 

 is swept over the reef-platform into the lagoon. We observed in the latter case that 

 such subangular material, even when bedded in water subject to swift and variable 

 currents, came to rest at angles at least as high as 40°, as at the channel into the 

 boat harbour on the lagoon side of Pava Islet. One of us (Mr. Sweet) has observed 

 that at the north end of Avalau the foraminiferal sand rests at an angle of 33°, 

 even where bedded in the moving tidal water. The sand was chiefly Tinoporus, 

 and, no doubt, the spines on the tests facilitate this sand coming to rest at such a 

 steep angle. 



The steepness of angle at which angular rubble from a reef will rest in water is 

 obviously of interest in relation to the angle of the steep submarine slope bounding 

 coral reefs. Such material, the natural angle of repose of which, in still sea water, 

 may be 40°, or upwards, may attain a dip of 60° or 70°, or even become vertical 

 through becoming rapidly cemented by Lithothamnion, Folytrema, Carpenteria, 

 Serpidce, Polyzoa, &c. 



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