708 MESSES. F. CHAPMAN AND D. MAWSON ON [JN T OV. I906, 



The broken surface reveals numerously-scattered joints of Hali- 

 meda, 1 having a dull earthy appearance, white to buff-coloured. 

 The species of Halimeda commonly present in this rock is the 

 typical H. opuntia. A few of the joints, however, compare closely 

 with H. cuneata, Kuetzing. The calcareous fragments are seen to 

 include also Lithothamnion, gastropoda, and a fair number of the 

 discoidal tests of Orbitolites (PI. L, fig. 2). 



Microscopical details. — Under the microscope the larger 

 calcareous fragments in this rock are seen to consist mainly of 

 Halimeda, with a small proportion of Lithothamnion; there are also 

 numerous foraminifera, such as Miliolina, Orbitolites, Globigerina, 

 Polytrema planum, and Amphistegina-, also occasional fragments of 

 polyzoa, gastropoda, worm-tubes, echinoid-fragments, and tunicate- 

 spicules. These organic con stitu tents are compactly cemented by 

 a fine to coarsely-crystalline calcite-paste, in which are scattered 

 fragments of crystals and brown-glass particles derived from igneous 

 rocks. 



Although some examples of this rock show a considerable 

 admixture of foreign debris, other specimens contain a large pro- 

 portion of Halimeda, and furnish good microscope-slides, in which 

 the structure of this seaweed is seen to be excellently preserved 

 (PI. L, fig. 1). 



Specimen 264. 



Shepherd's Hill, Yila, Efate. 



This Halimeda- Limestone forms a band 15 inches thick, inter- 

 bedded in the reef-material at Shepherd's Hill. It crops out at an 

 elevation of 365 feet on the northern slope. Some of the coral- 

 limestone above this band looks fairly old, so that this Halimeda- 

 Limestone is probably older than the more recent raised reefs. 



Description of the Rock. — This rock consists almost entirely 

 of Halimeda-jomts. It is somewhat cavernous, and the surfaces of 

 the joints are coated thinly with a brown earthy encrustation, 

 probably introduced by percolating surface-waters. Thus, only the 

 freshly-fractured rock appears white. A rude parallelism of the 

 flat Hal imeda-f mgments is noticeable, especially when the rock is 

 machine-sliced (PI. LI, fig. 2). Several continuous fish-vertebroe 

 are embedded in one of the specimens. 



Of the three examples now described, the above is the richest 

 in Halimeda. 



Microscopical details. — The chief component of this rock is 

 Halimeda, which appears amber-brown in section, and still clearly 

 shows the internal canals. The rich brown colour of the Halimeda- 

 joints is suggestive of partial phosphatkation, but the analysis 

 discloses the presence of only an inappreciable amount of phosphoric 

 acid. The Halimeda -joints are unusually thick as compared with 

 77. opuntia, and they bear a strong resemblance to H. versatilis, 



1 The Hali?neda-vema,ms in this as well as in the following specimens were 

 first referred to II opuntia by Mr. T. Whitelegge, in the Appendix to the 

 ' Geology of the New Hebrides' Proc. Linn. Soc. N. S. W. vol. xxx (1905) 

 p. 479, 



