412 



DR. C. GILBERT CULLIS. 



upwards it gradually diminishes in amount and finally disappears, so that at 820 feet 

 on the one hand and 875 feet on the other, practically all calcite has disappeared. 



Although the stages of this progressive dolomitisatiou, when studied in detail, are 

 neither quite so simple nor quite so regular as here described, owing to local disturbing 

 influences, it is believed that the general sequence of the changes here indicated is 

 substantially correct. 



From 875 to 1050 feet the rocks are again composed almost exclusively of dolomite, 



Fig. 45.— Main Boring. Core 389a. Depth 926- 

 9:5G feet, x 30. 



Dolomitised " coral-reef sand " cemented by fibrous 

 entrusting material. The fibrous deposit is in 

 places coated with well-defined water-clear crys- 

 tals of dolomite. The rock, except for changes 

 which have accompanied dolomitisation, is identi- 

 cal in charactei' with that represented in fig. 27. 



Fig. 46.— Main Boring. Core 503a. Depth 987- 

 991 feet, x 100. 



Dolomitised coral, with an irregular cavity con- 

 taining dense homogeneous detritus and fibrous 

 encrusting calcite. The fibrous deposit is the 

 oidy calcite in the section. Neither the "mud" 

 nor the fibrous material comes into direct con- 

 tact with the coral. Each is separated from it 

 by a layer of clear dolomite crystals. 



visible calcite in the form of undolomitised structures being of rare occurrence. They 

 contain, however, the same excess of uncombined calcium carbonate as the rocks 

 occurring between 650 and 820 feet, the proportion of magnesium carbonate being 

 approximately 40 per cent. 



The most variable constituent in them is the fibrous encrusting deposit. This is 

 sometimes entirely absent, sometimes scarce, and sometimes abundant ; it displays no 

 constancy in its mineralogical composition, consisting most commonly of dolomite, 

 but often of alternating layers of dolomite and calcite. Finely divided dolomitised 

 detritus is common, and forms conspicuous floors to the cavities (fig. 46). At several 



