REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 



35 



This mineral is concretionary, and is sometimes foliated into thin plates ; it is whitish, 

 yellowish, or yellowish brown. It scratches glass readily, and does not effervesce when 

 treated with acids. Slices, 2 to 3 millimetres thick, are translucent. When heated in 

 the lamp, it becomes white without melting, and the residue after this operation 

 crumbles between the fingers. In the closed tube it yields water with an alkaline 

 reaction, and gives off an empyreumatic smell. Qualitative analysis shows in it 

 phosphate of aluminium and of iron, silica, and sulphate of lime. 1 Analysis gives the 

 following composition : — 



100-36 



The explorers of the Challenger landed afterwards at Eat Island, the most 

 important islet of the group after Fernando Noronha. Mr. Buchanan observed on the 

 west of Rat Island a massive basaltic rock, which we shall describe, and on the east a 

 granular calcareous rock. "It is probable," he adds, " that this calcareous grit overlies 

 the basalt ; its structure seems to indicate that it has been laid down as drift. This 

 consolidated sand is calcareous, and contains a large number of shells. On our way to 

 Eat Island, in passing alongside of Booby Island, we saw that it also is almost entirely 

 formed of this calcareous grit. No old igneous rock is to be seen in it ; and seeing, 

 from the ripple marks, that the stratification may continue under the sea-level, there is 

 some reason to think that Booby Island is subsiding, or that it has subsided at some 

 previous time." We shall shortly return to the calcareous grit just mentioned, but 

 will first describe the basalt of Rat Island. 



Examined macroscopically this rock is black, massive, perfectly homogeneous, and 

 has a sub-conchoidal fracture. In the very fine-grained ground-mass some yellow 

 granules of olivine are visible, and some very small prisms, which ought to be identified 

 as nepheline. When reduced to powder, this rock gelatinises markedly with acids. Its 

 specific gravity is 2 - 957. Microscopical examination places it among the nepheline 

 basalts. At first sight, what strikes one is the absence of polysynthetic felspathic 

 lamellae. The very fine-grained ground-mass is seen under high powers to be composed 

 essentially of nepheline and augite, without interposition of vitreous matter. These 

 two minerals have in general vague outlines, still there can be distinguished some 

 colourless hexagonal sections that remain dark during a complete rotation between 



1 J. Y. Buchanan, he. cit., pp. 013, Gil. 



