Geology, 273 



commencement of the sea cliff which forms the southern boundary 

 of the bay. It is a very hard compact yellow limestone, showing 

 little or no traces of bedding or jointing. Its upper surface dips 

 at about 40° a little to the W. of JS"., and it disappears beneath 

 the beach of coral fragments that has been thrown up at this point 

 on the landward side of the narrow fringing reef. On the land 

 side it can be traced up the cliff for about sixty yards, when it 

 disappears under the talus which entirely conceals its base. This 

 limestone is described above (i^o. 2, p. 226), and is of Eocene or 

 Oligocene age ; it was probably deposited in comparatively shallow 

 water, and before the great accumulations of the Miocene and later 

 limestones of the higher parts of the island could have been 

 formed considerable subsidence must have taken place. In the 

 rocks of corresponding age in Java, several species of NummuUtes 

 are said to occur in abundance, but, strangely enough, according 

 to Messrs. Jones & Chapman this genus is entirely wanting in 

 Christmas Island ; its place seems to have been taken by large 

 Heterostegines. 



Above this limestone is a thick bed of compact black basalt 

 (N"o. 1 ; see Fig. 3 and G in Fig. 2, A). The junction is marked 

 by a bed of soft rock some five or six inches thick, in which harder 

 nodules are embedded. These nodules consist of limestone containing 

 foraminifera like those of the rock below, together with fragments 

 of much altered basic glass (No. 3, p. 23 1 ). The soft matrix in which 

 these nodules are embedded is a much decomposed rock consisting 

 mainly of fragments of basic glass. The basalt mass measui'ed 

 along the shore is about 40 feet thick; it is roughly jointed into 

 spheroidal masses, and seems to have been exposed on the sea 

 bottom, the joint cracks being filled with a hard yellow rock, 

 consisting of lime and fine detritus, derived from basic volcanic 

 rocks and including numbers of various species of Glohigerina. 

 This rock (No. 5) is described on p. 258, and is figured on PL XXI, 

 Fig. 17. 



The basalt^ itself is a compact black rock, very fresh-looking. 

 The ground-mass consists of microliths of plagioclase, many small 

 prisms of purple augite, and much magnetite. There are porphyritic 

 crystals of olivine, more or less altered into serpentine and viridite. 

 The skeleton crystals of magnetite are often arranged parallel to 

 one another, and at right angles to the axis of the olivines. There 

 are numerous rounded cavities, lined, and in some cases filled, with 

 a strongly pleochroic green mineral, forming radial aggregates 

 which show a black cross between crossed nicols. 



The upper surface of this basalt flow is covered with thick masses 

 of Miocene Orbitoidal limestone (C in Figs. 2 and 5), which, near 

 the junction, contains numerous fragments of the basalt overgrown 



^ In the description of the volcanic rocks I am indebted to Mr. G. T. Prior, 

 of the Department of Mineralogy, for much assistance. 



