MINERALOGICAL CHANGES OBSERVED IN CORES OF FUNAFUTI BORINGS. 40!) 



become more evident, and* of such material as remains a larger proportion consists 

 of the tiny zoned rhombohedra, and a smaller proportion of recognisable organisms 

 (fig. 42) ; but with the exception of these variations in degree of solution and 

 crystallisation, the rocks remain of the same essential nature through the whole 

 of the interval between 660 and 740 feet. At about the latter level the cores 

 begin to get heavier and harder once more, and within a very few feet they assume 

 a dense and solid character. This change is denoted in thin sections by the gradual, 

 though rapid disappearance of those evi- 

 dences of solution which are so abundant 

 in the rocks above. 



Between the base of this soft dolomite 

 and the point, another 80 feet lower down, 

 at which the proportion of magnesium car- 

 bonate begins once more to lessen, the cores 

 remain of the hard, dense and compact type 

 (fig. 43), and such sections as have been 

 cut from them reveal no important recur- 

 rence of those signs of excessive solution 

 which are so distinctive of the soft dolomite 

 above ; but the rocks exhibit all the various 

 types of dolomitisation of the organisms 

 which have abeady been described. 



It may be mentioned here, that the 

 phenomenon of reversal, already described 

 on p. 403, is very characteristic of these 



Fig. 43. — Main Boring. Core 51a. Depth about 



lower parts of the boring. It is particularly 

 well displayed by the corals, which are often 

 preserved in the form of casts, the solid 

 substance of the coral having been removed 

 in solution, while the " mud " of the cavities 

 has remained undissolved. The coral sub- 

 stance has not always been dissolved out, 



Dolomitised " coral-reef sand." The calcite organ- 

 isms, though dolomitised, show little sign of 

 crystallisation ; the aragonite organisms, on the 

 contrary, are almost completely recrystallised, 

 and have consequently lost all their more deli- 

 cate original characters. The granular cement 

 probably represents, in large part, crystallised 

 " mud," comparatively unaltered patches of 

 which are seen here and there in the section. 

 The dark line defining the original boundaries of 

 the recrystallised organisms is well seen. 



however ; sometimes it has been converted 

 into secondary calcite, and this in its turn, at the horizons of complete dolomitisation, 

 has been converted into dolomite. In such cases the corals are not preserved in the 

 form of casts, but have simply undergone one or more changes of mineralisation. The 

 mud of the cavities is usually dolomitised, but occasionally it still consists of calcite. 



b. The Occurrence of Fibrous Deposits in the Cavities. 

 At the depth of about 815 feet the fibrous encrusting deposit which has 

 already been stated to occur in the lower parts of the boring makes its appearance, 



3 G 



