154 J. C. Branner — Geology of Fernando de Noronha. 



This locality is not included among those of the columnar 

 phonolite. Mr. Darwin, in his " Geological Observations," 

 remarks upon the occurrence near the base of the peak of slaty 

 phonolite with cleavage. This peculiarity is very marked at the 

 Pedra da Conceicao, the rocks dipping sharply to the south- 

 west, and having upon that side surfaces so flat and steep that 

 I was able to hold my place only by clambering along its ridge. 

 This is probably the locality to which Mr. Darwin refers. The 

 rock splits readily into comparatively smooth slabs. A some- 

 what similar structure was observed in loose fragments found 

 between the Morro do Meio and Atalaia Grande. (Nos. 40 and 

 42). 



Basalts. — Rocks of a basaltic type form the great body of 

 Fernando de Noronha. They occur in all portions of the 

 island, and in masses of all shapes and sizes from thin veins to 

 broad sheets. It was not observed, however, in any of the 

 prominent isolated peaks, like the phonolite, yet nepheline-ba- 

 salt was found on the summit of the Morro Francez, and augi- 

 tite in the tuffs upon the east side of that hill. It occurs about 

 the bases of the phonolite peaks, forming the body of Ilha 

 Rapta, Sao Jose, Morro Redondo, and the cape near the phono- 

 lite peak of the Sella a Gineta. Rocks of basaltic type (limbur- 

 gite) occur about the base of the Atalaia Grande and along the 

 shores west of the peak. The Dois Irmaos appear to be made 

 up of basalt, and so also the Laja Cape between the Atalainha 

 and Morro Branco. In none of these cases, however, was I 

 able to determine satisfactorily the relations of the basalt and 

 phonolite to each other. Perhaps the most striking exposures 

 of these rocks are to be seen upon the island of Sao Jose\ The 

 surf has here removed all the debris and uncovered columns of 

 extremely hard nepheline-basanite (No. 31), which are best 

 exposed upon the north side of the island, where a length of 

 about fifty feet may be seen. This rock forms the greater 

 part of Sao Jose and the two small adjoining islands, Pedra 

 Furada and Ilha Redonda. In each case the columnar basalt 

 forms the lower part of the island, and massive basalt the upper, 

 while Sao Jose" is further capped by a bed of the calcareous 

 sandstone like that of which Ilha Raza is made. The columns 

 of Sao Jose are usually bent. They vary in size and shape, as 

 well as in position, but are usually hexagonal and about one foot 

 in diameter, and break off in sections from one to four feet in 

 length. The best exposure of the columns is on the eastern 

 side of the island, where they are visible, however, only from 

 the water. Many of them contain irregular masses of perido- 

 tite (No. 34) almost as large as one's fist. The broken columns 

 are rolled by the surf upon the beach where they eventually 

 form great black cobbles. 



