278 Christmas Island. 



apparently altered into a similar substance, while the glassy base 

 has become palagonitic. The higher parts of the bed also seem 

 to have been more glassy than the lower. 



The thickness of this basalt is, as already noticed, very variable, 

 and the tipper surface is nowhere clearly defined. It is covered 

 by beds of palagonite tuffs, Avhich in places attain a thickness of 

 at least 50 feet. The passage from the basalt to the ash beds 

 seems to be marked by an ill-defined band of a peculiar rock 

 consisting of angular fragments of basic glass, some an inch across, 

 embedded in a copious cement of crystalline calcite or, in places, 

 of non-crystalline lime, containing fragments of palagonite (^probably 

 merely the smaller fragments of glass completely altered), and 

 occasionally foraminifera ; the cementing substance sometimes 

 makes up a great portion of the rock. Seen on a fractured surface 

 the basic glass is black in colour, of a resinoid lustre, and has 

 an irregular splintery fracture. In thin sections it is seen to be 

 a yellowish brown, and the fragments are usually altered along 

 their edges to an orange-yellow palagonite ; embedded in the glass 

 are many small nodules and imperfect crystals of olivine and some 

 very small crystals of augite. This rock was not actually seen 

 in situ, but it invariably occurred in the talus immediately beneath 

 the beds of palagonite tuff, which it almost certainly separated from 

 the basalt below. 



The palagonite tuffs (H, Figs. 2 and 5), like the underlying basalt, 

 vary considerably in thickness in different parts of the section. 

 Between the faults X-X and Y-Y (see Fig. 2) they probably attain 

 a thickness of quite 50 feet. Ftxrther towards the middle of 

 Flying Fish Cove they are mostly concealed by talus, but here and 

 there fonn a low cliff of red or red and green mottled rock, which 

 crumbles beneath the feet. Under the microscope it is seen that 

 the rock consists of small fragments of a highly vesicular basic 

 glass, which has been completely converted into palagonite ; the 

 whole is cemented together into a compact mass by crystalline 

 calcite, and here and there the shells of foraminifera occur. The 

 palagonite forms bands of different colours parallel to the outlines 

 of the fragments or of the vesicles, and there are often zones 

 clouded with immense numbers of small granules, which under 

 a low power appear perfectly opaque. Sometimes these dark zones 

 are numerous, but usually there is one within a narrow band of 

 clear palagonite. In a few instances the whole of the interior 

 of the fragments is clouded with the opaque granules. According 

 to Mr. Chapman the foraminifera which occur scattered through 

 the mass are usually species of Pulvinulina. 



The glassy basalt, the rock composed of fragments of basic glass, 

 and the thick masses of palagonite tuff above described seem to 

 have been the products of a submarine eruption, the fragments 

 of basic glass being derived from the shattering of the chilled 

 upper surface of the basalt flow, and the palagonite tuffs being 

 composed of the finer fragments of vesicular glass derived from the 



