266 Christmas Island. 



empty spaces (with scalenohedral ? calcite crystals projecting into 

 them) occur as a result of contraction during recrystallization. 

 The unaltered organisms are numerous. The Calcareous Alga 

 Lithothamnion is represented by several species. 



A section of a shell (lamellibranch ?) occurs in one part of the 

 slide. The remaining organisms are foraminifera belonging to the 

 genera BoUvina, Glohigerina {conglohata and hulloides)^ Pulytrema, 

 Gypsina, Carpenteria, and Amphistegina. 



From the presence of Carpenteria and some pelagic forms, this 

 deposit was probably formed in water of a moderate depth, and on 

 the seaward side of the reef. 



No. 179. Top of second inland cliff, North-East Point. 600 feet. 



Composition: CaC O3 = 97-72 %, MgC03= 1'72 %» CasPjOs 

 = •20°/q. Total = 99-64. A very compact limestone, which in 

 the hand specimen does not show signs of recrystallization. 



The original rock consisted of organisms, more or less fragmentary, 

 and a matrix of silt, the finely comminuted product of trituration 

 of these organisms. This structure is still seen in the unaltered 

 parts of the slide, but in places the more finely divided material 

 has been recrystallized. The contraction due to recrystallization 

 has caused empty spaces to occur. These spaces have been partially, 

 and in some cases wholly, filled by subsequently deposited scaleno- 

 hedral (?) crystals of calcite. Further evidence of the secondary 

 formation of calcite is afforded by the fact that cracks which 

 were formed in the rock have been subsequently filled with 

 a calcite mosaic. The partial recrystallization of the matrix has 

 in one or two instances invaded the substance of the contiguous 

 organisms, whose outlines have consequently been rendered indis- 

 tinct. The unaltered organic remains constitute a considerable 

 proportion of the rock. Several species of the Calcareous Alga 

 Lithothamnion occur in abundance. 



The Poraminifera present include Carpenteria, Truncatulina, 

 Polytrema, Amphistegina, and a doubtful specimen of Gaudryina. 



The presence of Carpenteria would seem to suggest that the 

 deposit was formed at a moderate depth, as this genus rarely 

 occurs within the limits of wave-action. 



Specimens: 804, 800, 811, 308, 658, 378, and 514. These are 

 all dolomitic limestones whose microscopical structure is very 

 similar. The rocks are more or less porous : empty spaces having 

 arisen as a result of recrystallization. The cavities are lined by 

 dolomite crystals, which are usually clear, and show as blunt 

 rhombohedra having angles of 105° approximately. 



These dolomitic limestones are described in a definite order, the 

 first (804) having the largest percentage of calcium carbonate and 

 the last (514) the largest percentage of magnesium carbonate. 

 Dolomite consists of 54 % Ca C O3 and 46 % Mg C O3, consequently 

 in the rocks first described there is an excess of unchanged calcite 



