MINERALOGICAL CHANGES OBSERVED IN CORES OF FUNAFUTI BORINGS. 407 



This fact is of interest, as an analogous difference in permanence may be observed in 

 those parts of the boring where aragonite is altering to calcite ; it is the secondary 

 aragonite which first undergoes the paramorphic change, the substance of the corals 

 and other aragonite organisms preserving its integrity after the secondary aragonite 

 of the cavities has been converted into calcite. 



As still deeper cores are studied, the process of invasion by which the secondary 

 calcite has been converted into dolomite is seen to attack the primary calcite of the 

 organisms, with the ultimate result that they also become completely dolomitised. 

 This change may or may not involve their re-crystallisation. When it does, the 



Fig. 41.— Main Boring. Core 342. Depth 660 feet. 

 X 10. 



Showing the characters of the " soft dolomite," as 

 seen with low powers of the microscope. In 

 many cases the solid substance of the organisms 

 has been entirely dissolved out, only the cells 

 and tubules, filled with dolomitised " mud," 

 remaining. In others a certain proportion of 

 the organic substance has withstood the solvent 

 action, but has been converted into dolomite. 



Fig. 42.— Main Boring. Core 366. Depth 698 feet. 

 X 200. 



A well-crystallised portion of the " soft dolomite," 

 highly magnified. The mass is mainly composed 

 of groups and strings of minute rhombohedra of 

 dolomite, which define in a general way the 

 outlines and major structural features of the 

 organisms which were the original constituents 

 of the I'ock. Here and there, in the midst of the 

 larger groups of crystals, fragments of imeffaced 

 organisms may still be recognised. 



result is occasionally the almost entire obliteration of them, but much more often it is 

 the destruction of their minor features of texture and structure only (fig. 43), their 

 major features of structure and form continuing to be more or less evident. When, 

 on the other hand, dolomitisation does not involve recrystallisation of the organisms, 

 as is occasionally the case, their detailed structure may be preserved, and sometimes 

 in a remarkably perfect manner. 



The increase of dolomite, at the expense of calcite, continues until none of the 

 latter mineral remains. At the depth of 650 feet, or thereabouts, practically all 



