388 PKOFESSOR J. W. JUDD. 



If* this be the explanation of the doloniitisatioii of coral-reef rocks, there is still 

 much that demands investigation. As shown by the study of the Funafuti materials, 

 and l)v the various rocks collected from numerous upraised reefs by Sir John Murray, 

 Professor A. Agassiz, and Professor T. E. David in tlie Indian and Pacific Oceans, 

 the dolomitisation of rocks is by no means constant or uniform in its action. While 

 certain portions of a reef niay contain a high percentage of magnesium carbonate, 

 other parts of the same reef may be almost free from that salt, and the reasons why 

 some parts of the mass are almost completely dolomitised, while other parts remain 

 chemically unchanged, is by no means obvi(nis. 



We have seen that doAvn to the depth of (337 feet the degree of enrichment of the 

 rock bv magnesium carbonate may be probably ascribed to the leaching out of calcium 

 carbonate. In this part of the core we have no examples of the formation of crystals 

 of dolomite in the mass. But in the lower part of the boring the formation of 

 dolomite crystals thi-oughout the mass has gone on to a greater or less extent, either 

 at the expense of the calcite of the original organisms or of the deposits of that 

 substance as mud or crystals within and around the organisms. 



It bv no means follow^s tliat, because the dolomite crystals are only found at 

 considerable depth, tlie action to which the formation of those crystals was due 

 took place only at this depth. The action may possibly have taken place at or 

 near the surface, and the rock have subsided after its alteration. At the same time, 

 it mavbe lutted that all the rocks now at short distances from the surface in Funafuti 

 show no dolomite crystals, and contain only such an amount of magnesium carbonate 

 as may be accounted foi- l)y the leaching-out process. 



The dirt'ereiit j)rop(»rtions of magnesium carbonate which occur in this lower part 

 of the boring, may evidently be acctninted for in many cases by the varying amount 

 of calcite organisms (foraminifera, &c.) which do not lend themselves readily to the 

 leaching-out ])rocess, and of algse, corals, &c., which are probal)ly inuch more susceptible 

 to this action. It is doubtful, however, whether the very consideral)le falling-ofl' in 

 tlie percentage of magnesium carbonate between 819 and 875 feet, and between 1050 

 and 1097 feet, can be altogether accounted for in this way. 



It is very noteworthy that, high as the percentage of magnesium carbonate rises — 

 up to and e\"en beyond 40 per cent. — it never readies the limit of nearly 46 per cent, 

 required for the con sersion of the whole mass into the double carbonate. This limit is 

 rejiresented by the dotted line in the diagrammatic section (tig. 2'-i, pp. 3G4~5). The 

 highest percentage of magnesium carbonate (4o per cent.) was found at a depth of 

 950 feet. It has been suggested to me by Professor Armstrong that possibly there 

 may exist resting limits in this replacement of one base by another in the mixed 

 carbonates, more stable points being reached from time to time. On the other hand, 

 the resistance which some calcite organisms apjjear to ofller to chemical change may 

 account for the result — thei'e being, apparently, almost always some remains of 

 unaltered organisms. It is note\\(>rthv that, among the rocks of the earths crust, 



