Yol. 62.] HALIMEDA AS A REEF-FORMING ORGANISM. 707 



and in the nature of the cement. It seems of sufficient interest, 

 therefore, to describe the specimens separately, in both their macro- 

 and microscopic aspects. 



Specimen 109. 



Boulder projecting from the ground, near track, 100 feet above 

 Bartaleppe, Malekula. 



Description of the Hock. — This rock presents a striking 

 appearance, being a cream-coloured limestone, relieved with snow- 

 white patches due to fractured joints of Halimeda. This tendency 

 to fracture in Halimeda is caused by the imperfect mineralization of 

 the median series of tubes, which allows the joints to separate 

 along the median plane. In structure the rock is slightly cavernous, 

 and in hand-specimens is seen to consist almost wholly of Halimeda- 

 remains (Tl. XLIX, fig. 2). 



Microscopical details. — A microscopic examination shows this 

 rock to consist of numerous Halimeda-joints, the interspaces being 

 filled with a comparatively-coarse pavement of crystalline calcite. 

 Occasionally the Halimeda-jomts are surrounded by the encrusting 

 foraminifer, Polytrema planum. The Halimeda is not thoroughly 

 mineralized, although in some instances the tubular structure is 

 encrusted and infilled with deposited calcareous matter. On the 

 whole, the rock is very friable or pulverulent, so that sections have 

 to be prepared with extreme care. The species commonly present 

 is undoubtedly the typical form of H. opuntia, Lamx., and the 

 peripheral cells vary in diameter from 40 fx to 54 p. 



The other organisms taking part in the formation of this lime- 

 stone are, — a branching Lithothamnion, the foraminifera Alveolina 

 and Polytrema planum, and echinoids, represented by spines and 

 plates (PI. XLIX, fig. 1). 



The following bulk-analysis of this rock shows it to be a 

 remarkably-pure limestone : — 



H Q (below 110° C.) 0-31 



Hip (above i 10° C.) 069 



Insol. +Si0 2 0-05 



AL0 3 +Fe 2 3 (trace MnO) ... 027 



CaO 5414 



MgO 1-00 



CO., 43-24 



P 2 5 028 



Alkalies nil 



Sulphate nil 



Chloride nil 



Organic matter nil 



99-98 



H (below 110° C.) 0-31 



JELp (above 110° C.) 0-69 



Insol. 4-SiO, 0-05 



Al 2 3 -fFe 2 3 0-27 



Calcium-carbonate 95*89 



Magnesium-carbonate 2*07 



Tribasic calcium-phosphate . . - 61 

 Lime (probably mineral com- 

 bination) 0-09 



99-98 



Specimen 111. 



From boulders out of the bed of the Atsone River, S. Santo. 

 Description of the Rock. — A somewhat dark and impure 

 limestone, bluish when freshly fractured, weathering to brown. 



