MINERALOGICAL CHANGES OBSEKVP^D IN CORES OF FUNAFUTI BORINGS. 41 1 



occurs were at least partially, if not completely, dolomitised before its deposition in 

 their cavities. 



Below 820 feet and above 875 feet the rocks are dolomitic limestones containing a 

 variable amount of recognisable calcite. At the upper of these two limits they contain 

 39 per cent, of magnesium carbonate, and the proportion of calcite is very small indeed ; 

 at the lower limit they contain 40'25 per cent, of magnesium carbonate, and there is no 

 calcite at all, but as a descent is made from the upper limit, or an ascent from the 

 lower, the relative amount of calcite rapidly increases, and for a considerable distance 

 above and below a point situate mid- way between them, calcite constitutes not less 

 than half the mass. Such analyses as have been made from the cores of this part of 

 the boring show a maximum of calcite at 826 feet, where the proportion of magnesium 

 carbonate is less than f) per cent. (4 "8 3). If it be assumed that this amount of 

 magnesium carbonate is combined with calcium carbonate to form dolomite, and that 

 all the rest of the calcium carbonate exists as visible calcite, then approximately 

 nine-tenths of the mass must be composed of calcite and only one-tenth of 

 dolomite, a conclusion which is quite in accord with what is observed in stained 

 sections. 



Starting from this point, at which so large a proportion of the mass consists of 

 calcite, and working both upwards and downwards towards the condition of complete 

 dolomitisation which exists above and below, it is interesting to note just the same 

 series of changes as is to be observed in passing from the limestones at G37 feet into 

 the dolomites below. In the rocks which contain the smallest proportion of dolomite, 

 and which, except for the presence of the fibrous encrusting deposit, may be compared 

 with those at 638 feet, the calcite of the mass comprises the primary calcite of the 

 unaltered organisms, the secondary calcite of the organisms that were originally 

 composed of aragonite, and the secondary calcite deposited from solution, which last 

 is of two kinds — an earlier-formed deposit of acute scalenohedral crystals investing 

 the organisms, and the later-formed fibrous encrusting deposit. The dolomite of the 

 mass consists in the main of transparent crystals — there is also some dolomitised 

 " mud " — which cover up and enclose the scalenohedra of calcite or invest the 

 calcitised aragonite organisms (Plate F, fig. 4) in such a manner as to leave little 

 doubt that they are the direct product of deposition from solution. They are 

 themselves covered up by the fibrous encrusting deposit of calcite. 



A little higher up and a little lower down much of the secondary calcite gives 

 place to dolojmte, but the primary calcite of the organisms still remains unchanged, 

 and so, in great measure, does the fibrous encrusting deposit. 



Finally, as the distance from the starting point is gradually increased, and the 

 horizons of complete dolomitisation, above and below, are more and more nearly 

 approached, even the calcite organisms are attacked and pass over into dolomite ; the 

 fibrous deposit too, when followed downwards, is seen to be dolomitised ; traced 



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