-F. B. Coivper Reed — Ocean Island. 299' 



an irregular conglomerate of rounded or subangular fragments or 

 grains of various shapes and sizes. Recognisable remains of 

 organisms are very rare, but under the microscope a few frag- 

 mentary foraminiferal shells can be distinguished in a thin section. 

 The cementing material is a hard pale yellowish or brownish 

 substance of a semi-translucent and resinous appearance* which 

 encases the fragments and lines or more or less fills up the 

 interstices between them. In one specimen the grains are of fairly 

 regular size, and represent a coral sand, converted into a fine 

 oolite by each grain being surrounded by concentric coats of 

 the same cement, and the whole mass thus bound into a solid 

 and compact rock which rings under the hammer. Under crossed 

 nicols a thin section shows that the coats consist of a series of 

 regular concentric layers with a fibrous radial structure, like Eenard ^ 

 has described in the oolitic calcareous rocks of Ascension Island. In 

 these examples from Ocean Island we find all degrees of coarseness 

 in the oolitic structure — from a coarse pisolite to a very fine oolite ; 

 and in some specimens the cementing material is in excess of the 

 included grains and forms the mass of the rock, losing then its 

 radial structure and behaving under the microscope as an almost 

 isotropic groundmass. The cement in all cases effervesces scarcely 

 at all or but feebly with acid, and is of a phosphatic nature. A com- 

 pletely different type of phosphatic rock consists of regular narrow 

 bands of a yellowish or brownish substance of a semi-translucent 

 resinous appearance. This agate-like rock is very tough and heavy, 

 and has a sub-conchoidal fracture. It seems to consist entirely of 

 the cementing phosphatic material found in the above-mentioned 

 oolites, and behaves in a similar fashion with acids. 



One mass of coral (Mcsandrina) is encrusted with stalactitic 

 deposits of the brownish phosphate which surrounds the corallites, 

 and has more or less obliterated their structure round the margins. 

 Irregular mammillary or stalactitic forms of the phosphate are also 

 met with, and sometimes they enclose patches of fine soft calcareous 

 mud with a hard porcellanous fibrous coating of a bluish or buif 

 colour. One specimen seems to represent the soft calcareous mud 

 of the ancient lagoon, and it now forms a tough fine-grained chalky 

 limestone of a white or buff colour with a conchoidal fracture. 

 Under the microscope minute irregular fragments or broken prisms 

 of calcite are seen to be scattered thi'ough it. In another specimen 

 this consolidated mud is banded by brownish layers of a homogeneous 

 opaque substance, not effervescing in the least with acids, and 

 apparently representing original layers of deposition. A fragment 

 of calcareous tufa with the typical arborescent and cavernous 

 structure is present in the collection, but it has a hard brownish 

 phosphatic crust. 



The detailed distribution and relations of these various types of 

 rocks have not been studied, and I have no precise information as to 

 their mode of occurrence. The guano which has led to the formation 



1 Eenard : " Challenger " Eeports, Physics and Chemistry, vol. ii, Rep. Eocks, p. 71. 



