420 MINEKALOGICAL CHANGES OBSERVED IN COKES OF FUNAFUTI BORINGS. 



DESCRIPTION OF PLATE F. 



Stained Sections of the Funafuti Rocks. 



{Note. — In figs. 1 and 2 the mineral which has taken the stain is aragonite, that which has remained unstained 

 is calcite. In figs. 3, 4, 5 and 6 the coloured mineral is calcite, the uncoloured, dolomite.) 



Fig. I.— SoLLAS Boring No. 2. Core D. 9. Depth nearly 30 feet, x 100. 



To the right is a mass of coral with encrusting organisms. To the left and below is a part of a 

 IlaUmcda frond ; separating this from the encrusted coral is finely-divided calcareous detritus (" primary 

 mud "). Above is a large cavity lined with secondary calcite. The cavities inside the coral and Halimeda 

 frond are nearl}^ all filled with secondary aragonite ; a few contain secondary calcite, which together with 

 that in the cavity outside the organisms is of the fibrous encrusting type. 



Fig. 2.— SOLLAS Boring No. 1. Core C. 20. Depth between 50 and 80 feet, x 100. 

 "Coial-reef sand " with constituent fragments cemented together by secondary calcite of the fibrous 

 encrusting type. 



Fig. 3.— Main Boring. Core 314 (lowest i)art). Depth 638 feet, x 200. 



Partially dolomitised limestone. In some parts the organisms are seen to be coated \vith acute ciystals 

 of secondary calcite, which either project into empty spaces or are embedded in granular dolomite ; in 

 others these crystals have been dolomitised ; and in still others the organisms themselves have suffered the 

 change, with a resulting partial or complete obliteration. Some of the dolomite has possibly been 

 deposited from solution, but the greater part is probably a substitute for pre-existing structui'cs of calcite 

 oi' aragonite. 



Fig. 4.— JNIain Boring. Core 225 A. Depth 842 feet, x 200. 



The organisms are coated with secondary calcite in acute crystals. These project into a layer of clear 

 dolomite, the rhombohedral crystals of which are covered in turn liy secondaiy calcite of the fibrous 

 encrusting type. 



Fig. 5.— Main Boring. Core 237 A. Depth 850 feet, x 100. 



The initintcd part of the section represents a portion of a laige mass of dolomitised coral, the cavities 

 in and aiound which are lined with a thick deposit of fibrous encrusting calcite. 



Fig. 6.— Main Boring. Coi'e 270 A. Depth 868 feet, x 200 (slightly diagrammatic). 



The layer of clear and well-defined dolomite crystals rests upon and encloses a mass of coral, which has 

 been converted into calcite. This layer in turn is covered up by a thick deposit having a radiating 

 structure,^and consisting of bands of calcite and films of dolomite in regular alternation. 



