and its Bearing on the Coral Reef Problem. 191 



under the microscope. Such dolomites clearly could not have 

 originated hy the process of differential leaching-. 



In the case of recent coral limestones the author* has shown 

 by chemical analysis that specimens from the elevated fringing 

 reefs at Christmas Island, jNTuie, and elsewhere contain from 

 8-13 per cent of MgC0 3 , but shown no sign of dolomite. 

 Further, the limestones appear fresh, one contains up to 1*5 

 per cent of organic matter still remaining and the organisms 

 are beautifully preserved. There is therefore no sign of differ- 

 ential leaching having taken place ; on the contrary, microscopic 

 examination shows that there has been notable deposition in 

 cavities of the rock of secondary carbonate, sometimes in the 

 form of aragonite, sometimes of calcite. Precisely the same 

 evidence is to be noted in that part of the Funafuti bore, 

 between 15 and 25 feet from the surface, where the MgC0 3 

 reaches 16 per cent, and for which high percentage of MgC0 3 

 Prof. Judd advanced the explanation of differential leaching. 



The writer is quite sure that if Prof. Judd had been as 

 familiar with the mineralogical evidence as was Dr. Cullis, 

 who described the mineralogical changes in the Funafuti bore, 

 he would not have put forward the leaching hypothesis. Dr. 

 Cnllis's descriptions and drawings from this part of the bore 

 shows clearly that, however the MgC0 3 may be present, no 

 dolomite occurs, the organisms are wonderfully fresh, over 1 

 per cent of organic matter is present, and a considerable 

 amount of secondary carbonate having the form of calcite and 

 aragonite has been deposited in the interstices of the coral 

 limestone. 



It is clear, therefore, whatever be the explanation for the 

 presence of 10-16 per cent of MgC0 3 in certain recent coral 

 limestones, it is not due to the process of selective leaching by 

 carbonate water and that a general theory of dolomitization 

 based on the process of differential leaching is opposed to the 

 known facts. 



Dolomitization by Metasomatic Replacement. 



The presence of corals and other organisms replaced by 

 dolomite in many dolomitized limestones has for long made it 

 clear to field geologists that replacement of calcium carbonate 

 Hby magnesium carbonate has occurred in many limestones. 

 That this is the dominant process in regional dolomitization 

 has been made clearer by the detailed microscopic examination 

 of limestones, especially coral limestones, in recent years. Var- 

 ious stages in the process have been described by the author f 

 and by Dr. Cullis4 The Funafuti evidence, apart from the 



* Skeats, Bull. Mus. Comp. Zool. Harvard, 1903; Q. J. G. S , lxi, 132, 1905. 

 f Skeats, op. cit. % Cullis, op. cit. 



