





GEOLOGY OF FERNANDO NOBONHA. 



87 



are erect. Near Tangle Rock are horizontal columns so regular 

 that Mr. Ramage found in one spot a complete tunnel through 

 the mass formed by one or two having fallen out. 



Morro branco (" the white hill ") is a dome-shaped hill of a 

 white rock, very different in appearance to the normal phonolite 

 and somewhat resembling some specimens of the tuffs near Sponge 

 Bay. It appears to have been altered by contact with basalt. 

 A boulder of similar rock was cut through at Leao Bay by the con- 

 victs, who were making a road there. On Morro branco were 

 found several plants which grew only here and on the similar 

 rock of Look-out Hill, and one species of grass was peculiar to 

 the former spot. 



Not only at Morro branco but on the western cliffs beyond the 

 peak at Boldro we found phonolite rocks altered by contact with 

 basalt, evidencing the fact that the phonolite was anterior to the 

 basalt. It seems, in fact, to form a groundwork of the island, 

 covered in part with later deposits of basalt, through which pro- 

 ject the high hills and peaks. 



In Sella Giueta and Tangle Rock were strata of a muddy 

 chert passing into a form of semiopal. This appears to be the 

 residue of hot siliceous springs, and must have been deposited 

 between the pouring out of one layer of phonolite and the next. 

 In one specimen is the mould of a very large crystal, apparently 

 of calcite. From this rock, which appears to have been laid down 

 horizontally, it may be gathered that the phonolite was not 

 originally injected into older rocks and then elevated above the 

 surface, the older rocks being weathered away, but rather that 

 it was poured out at intervals between which the hot springs 

 deposited their silica on the cold or cooling phonolites, which was 

 afterwards covered with another layer of the phonolites. 



















Basaltic Socks. 



The larger part of the islands now consists of these rocks, which 

 occur in the forms of columnar or spheroidal basalt, in layers or 

 dykes, scorias, pumice, tuffs, volcanic agglomerates. In Eat 

 Island the basalt rises from spa-level at the western end, where it 



though chiefly 



with 



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consisting of the ordinary fine-grained olivine basalt of the island, 

 here and there, at the isthmus which connects the eastern promon- 

 tory with the main part of the island, vesicular and amygdaloidal 















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