MIXERALOGICAL CHANGES OBSERVED IN CORES OF FUNAFUTI BORINGS. 401 



This loss of organic texture and conversion into calcite is a gradual process, the 

 various stages of which may be followed particularly well in thin sections of the corals. 

 The first indication of loss of organic texture in the coral is the disappearance of the 

 " dark line," a series of comparatively dense and opaque centres from which the 

 component fibres of the coi'al substance radiate. In a perfectly fresh coral these form 

 a very conspicuous dark moniliform line, occupying the middle of the coral wall, but, 

 as lower and lower cores are examined, this line becomes less and less distinct and 

 presently disappears. The next stage is the gradual obliteration of the fibres 



Fig. 35. — Maiii Boring. 

 100 feet. 



Core 105. Depth 90- 

 X 100. 



Coral showing cavities completely filled with 

 . secondary crystals deposited from solution, and 

 others containing calcareous detritus in process 

 of crystallisation. Of the former cavities, two 

 contain aragonite only, the third, calcite in 

 addition. 



Fig. 36.— Main Boring. Core 111. Depth 103 feet. 

 X 200. 



Coral surrounded by a fringe of secondary arago- 

 nite, which is in turn invested by a more or less 

 continuous layer of calcite crystals. The pris- 

 matic crystals of aragonite show no signs of 

 conversion into calcite. The calcite outside 

 them represents the latest product of deposition 

 from solution : the formation of aragonite has 

 been superseded by that of calcite. 



themselves. Yet later, the substance of the coral is seen to be broken up by a 

 number of sinuous lines into a granular mosaic ; but, for a while, both the granular 

 structure and something of the fibrous character may be discerned (figs. 37, 57). 

 Finally all traces of the fibres disappear, and the mass becomes an aggregate of 

 irregularly rounded and mutually interfering grains of calcite (figs. 37, 38, 58). 

 The secondary aragonite of the cavities has already undergone a similar change, so 

 that now the whole mass, coral substance and crystal-filled cavities alike, is composed 

 of granular calcite. The only respect in which the granular aggregate, resulting 

 from the conversion of the coral substance into calcite, may be distinguished from that 



3 F . 



