Geology. 279 



same source and mingled to some extent with sediment containing 

 marine organisms. These rocks are very similar to specimens 

 described by Murray and Renard in the "Challenger" E-eport on 

 Deep-sea Deposits as forming the nuclei of manganese nodules 

 dredged from depths of more than 2,000 fathoms in the Indian 

 and Pacific Oceans. Thus the more altered portion of the glassy 

 basalt is very similar to that figured in the volume quoted on 

 pi. xix, fig. 4 ; the palagonite tuif to those shown on pi. xviii, 

 figs. 1, 2, 3 ; and the basic glass (No. 400) to that figured on 

 pi. xvii, fig. 3 ; and no doubt they were produced under similar 

 conditions. The degree to which the palagonite tuffs are exposed 

 on the cliff is shown in Fig. 2, where also it will be seen that, like 

 the basalt, these ash beds are cut off by the fault Y-Y and have 

 Miocene Orbitoidal limestones resting on their southern extremity. 



Inland, about half a mile south of Flying Fish Cove (Fig. 6), we 

 come upon an extensive exposure of this upper basalt. The eastern 

 edge of the exposure runs a little to the west of south, approxi- 

 mately along the 500 feet contour-line ; to the westward (that is, 

 the seaward) side the basalt forms a series of steep rounded slopes 

 separated by narrow valleys and running down to about the 

 300 feet contour-line ; here it again disappears beneath limestones, 

 chiefly Miocene and containing Orlitoides, but in some places 

 of more recent origin. This belt of basalt is about a third of 

 a mile wide at its northern end, and can be followed south for about 

 a mile, beyond which it is completely covered with Miocene lime- 

 stones : this is the most extensive exposure of volcanic rock in the 

 island. As already mentioned, the upper surface of the exposure 

 roughly follows the 500 feet contour-line where it disappears 

 beneath the overlying Miocene limestones. The palagonite tuffs 

 found in the cliff section are here almost entirely absent and usually 

 represented only by a bed of impure limestone, often stained and 

 banded red and yellow by water percolating along the surface of 

 the volcanic rock, and containing fragments of palagonite and 

 basalt, sometimes in such quantities as to form a kind of breccia. 



Towards the southern end of this exposure of basalt there are 

 several patches of Orbitoidal limestone at from 500 to 550 feet 

 above the sea ; these rest directly on the basalt, many fragments of 

 which are included in their basal portion, which may present the 

 appearance of a breccia of basalt fragments. Some of these lime- 

 stones (ISTos. 835, 827, 581) are described on pp. 254 and 255. 



Hetuming to the cliff section, we find that the upper surface 

 of the palagonite tuff bed may be covered directly by Miocene 

 Orbitoidal limestone, or in a few places (e.g. at the points where 

 the sections in Fig. 2 were taken) a thin band of basalt, which is 

 often highly vesicular and contains large crystals of augite ; above 

 this may be a thin band of palagonite tuff, or in places a volcanic 

 breccia consisting of fragments of highly altered basic glass and 

 basalt embedded in a ground-mass of yellowish limestone which may 

 contain foraminifera. 



