265 



NOTE ON THE 



COMPOSITION OF SOME DOLOMITIC AND OTHER 



LIMESTONES FEOM CHRISTMAS ISLAND. 



By E. W. Skeats, B.Sc, F.G.S. 



No. 134. Summit of upper cliff on East Coast. 800 feet. 



Composition : Ca C O3 = 97-56 7^, Mg C O3 = 2-44 %, Caj P^ 0^ = 

 •15%. Total =100-15. 



A compact cream-coloured limestone, with no evident crystalline 

 structure in the hand specimen. The rock originally consisted of 

 organisms and small lumps of undifferentiated material set in 

 a cement of calcareous silt. Subsequent alteration consists in the 

 change of a small quantity of the fine silt into calcite crystals. 

 Some of the crystals occur in the form of definite rhombohedra, 

 with angles of 105° approximately. Empty lozenge-shaped spaces 

 in the rock possibly represent the positions of rhombohedra which 

 fell out probably during the grinding of the slice. 



The calcite can be recognized by its cleavage and polarization 

 colours. Sections of some of the organisms show long fibrous 

 crystals which may be fibrous calcite, but are possibly the rhombic 

 form aragonite. Minute scalenohedral (?) crystals of calcite 

 occasionally project into cavities formed by contraction during 

 recrystallization. 



The organic remains are comparatively few. They include 

 more than one species of the Calcareous Alga Lithothamnion. 

 The only other organisms present are foraminifera, including 

 Glohigerina (?) and the Rotaliiform genera Truncatulina and Plan- 

 orhulina (?). The shallow- water affinities of the two latter organisms 

 suggest that this deposit may have been laid down in the lagoon. 



No. 963. Limestone (Miocene) from high cliflP over Flying 

 Fish Cove. 500 feet. 



Composition : Ca C O3 = 97-29 % , Mg C O3 = 3- 19 %. Total = 

 100-48. A white, very compact limestone. 



A piece of unaltered rock (consisting of organic fragments 

 embedded in silt) may possibly be a fallen fragment ; its structure 

 serves as a contrast to that of the main mass of the rock. The 

 original silty matrix has very largely been replaced (as a result 

 possibly of consolidation under pressure and percolation of water) 

 by a clear mosaic of calcite surrounding the still unaltered large 

 organic fragments. 



Some of these fragments are, however, losing their distinctive 

 boundaries as a result of the extension of this recrystallization 

 into the bodies of the organisms. In certain parts of the slides, 



