iMINERALOGICAL CHANGES OBSERVED IN CORES OF FUNAFUTI BORINGS. 418 



horizons the rocks show signs, not only under the microscope, but also with the 

 naked eye, of that solution which has produced such striking results in the soft 

 dolomite occurring higher up in the boring (fig. 45). In no case, however, are the 

 effects of solution here so marked as at that horizon, nor have the rocks affected so 

 great a vertical extent. That solvent action did not come into operation until after 

 the deposition of the fibrous encrusting material is proved by the fact that just 

 as parts of the organisms have been dissolved away, so, occasionally, have the more 

 soluble layers of this material itself been removed by solution — layers which are 

 now represented by a series of roughly concentric or parallel vacuities. 



The other characters of the dolomites at this depth, the state of preservation ol 

 the various organisms, their modes of 

 mineralisation and so forth, are in all essen- 

 tials identical with those of the rocks of the 

 same composition which occur higher up 

 in the boring. Indeed, the lower dolomites 

 differ from those above only in the presence 

 of the fibrous encrusting material and the 

 greater abundance of detrital mud. 



Fig. 47._Main Boring. Core 618a. Depth 1060- 

 1066 feet, x 100. 



Dolomitised " mud " containing well-formed dolo- 

 mite crystals. 



Between 1050 and 1070 feet dolomitic 

 limestones occur once more. At the former 

 depth, calcite, which in the dolomites imme- 

 diately above is of very rare occurrence 

 and constitutes, at naost, but an insig- 

 nificant proportion of the mass, begins to 

 increase in quantity. This increase con- 

 tinues until the depth of 1060 feet, or 

 thereabouts, is reached, from which point 

 a corresponding decrease ensues, with the 

 result that, finally, at or about 1070 feet, 



dolomite is once more established in overwhelming preponderance, and so continues 

 with little variation to the bottom of the boring. 



Starting from the level (1060 feet) at which the proportion of calcite is 

 greatest — rather more than two-fifths of the mass — and working both upwards 

 and downwards, the changes which present themselves are the same in kind and 

 order as were observed in the dolomitic limestones occurring between the depths of 

 820 and 875 feet. At first the calcite comprises not only the majority of the 

 organisms which are primarily composed of it, but also a certain proportion of 

 unaltered secondary calcite, whether it be that which in the form of scalenohedral 

 crystals invests the calcite organisms, or that which represents the products of the 

 paramorphism of the corals and other aragonite organisms, or that which as the 

 fibrous deposit lines or fills the cavities of the mass. The dolomite consists of 



