THE CHEMICAL EXAI^HNATION OF THE MATERIALS FROM FUNAFUTL 3/3 



26*34 per cent, of calcium phosphate. The latter of these two rocks consisted of a 

 hi'own matrix with white limestone fragments scattered throng-h it. Enougli of the 

 hrown matrix was isolated hj careful picking hy Mr. J. Heniiy, in the Geological 

 Laboratory, for analysis, and this yielded Dr. Skeats 32 "5 per cent, oi' calcium 

 phosphate, while the white fragnients gave only 579 per cent, of the phosphate. 



As several islets on the western side of the atoll are haunted by sea-birds, it is not 

 impossible that other surface rocks of Funafuti may be found to be phosphatised in a 

 similar manner. 



The fact that no phosphatised rocks were found in the borings, probably points 

 to the conclusion that no part of the rocks traversed by the boring had been long 

 elevated and exposed at the surface, so as to become impregnated with solutions of 

 phosphates by the slow percolation of rain-water through the mass. It is in this 

 way that rocks like those of Christmas Island and Clipperton Atoll probably become 

 phosphatised, while rocks covered by guano, which are exposed to the action of the 

 sea, do not appear to undergo chemical changes of the same kind. 



7. Proportions of Calcium and Magnesium Carbonates in the Funafuti 

 Rocks as Bearing on Theories of Dolomitisation. 



From what has been stated in the preceding pages, it will be seen that the rocks 

 passed through in the borings at Funafuti are almost wholly composed of carbonates. 

 As we have already stated, the organic matter of the plants and animals, everywhere 

 small in amount, rapidly diminishes as we go downwards, and at the depth of 100 feet 

 has almost entirely disappeared. The proportion of insoluble inorganic materials — as 

 appears to be always the case with coral-reef rocks, when not in immediate proximity 

 to volcanic or other foreign rocks — is so small as to be also almost inappreciable. The 

 phosphates are present only in miinute proportion, and no othersubstances occur except 

 as the merest traces. Thus the great problem presented by these rocks is that which 

 relates to the changes in the relative proportions of the calcium and magnesium 

 carbonates and the causes to which these changes are to be ascribed. 



As has been already pointed out, the proportion of magnesium carbonate rises in 

 the first 50 feet of descent in all the borings, from the normal 1 to 5 per cent., up to 

 a maximum of nearly 16 per cent., which is attained at a depth of about 25 feet, and 

 then declines again to what may be considered the normal amount 1 to 5 per cent. 

 At 637 feet the percentage of magnesium carbonate again rises from this normal and 

 by 660 feet has reached nearly 40 per cent. This proportion with some small 

 exceptions is maintained to the bottom of the bore-hole at 1114 feet (see fig. 23, 

 pp. 364-5). 



A series of analyses have been made of the materials in and around the Funafuti 

 Atoll, with mineralogical studies of the rock at different depths from the bore-holes, 

 and the bearing of these upon the question of the causes of the changes in the 

 proportions of the two carbonates may now be discussed. 



