MINERAL0C4ICAL CHANGES OBSERVED IN CORES OF FUNAFUTI BORINGS. 395 



thrown down from solution, and to cause it to be deposited in a molecular form 

 identical with their own. Moreover, it is only upon such surfaces of these organisms 

 as are clean that the deposition of aragonite takes place, the intervention of the 

 thinnest film of mud being sufficient to destroy the influence of the organic fibres, 

 and to cause the precipitation of the more readily formed calcite. This is probably 

 the reason why the secondary aragonite deposited from solution is practically confined 

 to the cavities within the organisms, and is rarely, if ever, found upon the limitino- 

 surfaces of the interspaces between them. 



Fig. 24.— Main Boring. Core 28. Depth 27 feet. 



X 90. 

 Transverse section of Heliopora, showing the 

 . deposition of secondary aragonite in continuity 

 with the primary aragonite of the coral. 



Fig. 25.— Main Boring. Core 83. Depth 68 feet. 

 X 150. 



Coral showing commencement of deposition of 

 aragonite in empty cavities. The " d^rk line " 

 in the coral substance is well seen, but its pre- 

 servation at this depth is somewhat unusual. 



As a rule, the deposits of secondary calcite and aragonite show a marked relation 

 to the structural elements of the organisms which they invest. The crystals of 

 aragonite are invariably deposited in continuity with the fibres of aragonite organisms 

 (figs. 24, 25, 26, 29, 52, &c.), while those of calcite are generally, but not always, 

 continuations of the elements of calcite organisms (figs. 30, 39, 40). When calcite is 

 deposited upon an aragonite organism, it need hardly be stated, no such crystalline 

 continuity is established (fig. 29). 



It will be gathered from what has just been said, that whereas aragonite is 

 deposited in one form only, calcite is not. The calcite deposits are, in fact, of two 

 markedly difierent types. The first of these, which will be referred to as the fibrous 

 encrusting deposit, consists of densely packed minute prismatic elements, which are so 

 arranged as to impart to it a fibrous structure (figs. 26, 27, 28, 53 ; Plate F, figs. 1, 2). 

 It often exhibits a concentric structure also, due to difierences in the nature of its 

 successive layers ; the darker layers owe their inferior transparency to the presence of 



3 E 2 



