620 Mr. J. Y. Buchanan on Geological Work 



50 feet high, and presents a perfectly smooth wall-face to the outside. 

 As it stretches down the hill-slope, which here is very steep, its height 

 diminishes irregularly until it is lost in the heap of loose stones which 

 covers the lower wall and the whole inside. 



The rock is hard and compact, of a light greenish grey colour, with 

 much of the appearance, though none of the ring, of phonolite. Near 

 the outside, or in the columnar part, the rock is closer-grained than in 

 the centre, and has a distinct cleavage in a plane perpendicular to the 

 length of the columns. It gelatinizes partially with hydrochloric acid, 

 and the solution contains much soda and some sulphuric acid. It is 

 therefore probable that both nepheline and nosean are present. 



Another prominence on this side of the harbour is formed of precisely 

 similar material. It is a round, greenish grey hill covered with phono- 

 litic rock lying about in angular fragments, generally of a size to be easily 

 lifted. The rock is very similar to that of the hills just described ; and 

 it seems to belong to the same class, differing from the others owing to 

 the complete disappearance of the outside wall, large pieces of which lie 

 scattered on the slope like portions of dislodged masonry. 



It is to be remarked that in neither of these cases was there any dis- 

 tortion in the beds in which the phonolite occurred. The line of junction 

 of the highest one with the augitic rock was very w T ell shown, and I was 

 enabled to get specimens from it. For some feet from the line of junc- 

 tion the basalt is considerably altered, the large crystals of augite and 

 olivine disappearing as the line of junction is approached. This line is 

 in general quite decided ; there are many angular particles of the phono- 

 lite completely surrounded by the basalt, whereas basalt imbedded in 

 phonolite was not observed. Further, the grain of this basalt, in im- 

 mediate proximity to the junction, is very fine, becomiug rapidly coarser 

 till the basalt at 10 feet from the junction has the porphyritic appearance 

 which it presents at other parts of the hill. These two facts appear to 

 point to the phonolite as being the more ancient of the two, and to the 

 basalt as having flowed round it. How the phonolite came to be sticking 

 up in the pillar-like form which it must have had is very difficult to say ; 

 but the peak of Fernando Noronha, however it may have been formed, 

 is always evidence of the possibility of such a thing. There is no neces- 

 sity for supposing that the portions of these phonolitic masses which we 

 see should be sections of cylinders : they may equally well be sections of 

 domes. The other view that the phonolite had burst through the lava 

 appears to me to be untenable in view of the facts above stated. 



Of the similar hills in Christmas Harbour, Table Mountain consists of 

 columnar basalt with large cavities filled with olivine. The columns 

 starting normally to the cylindrical surface of the enclosiug rock curve 

 upwards, and, unlike the phonolite, are continued well into the mass of 

 the hill. The top of this hill is covered with loose fragments of basaltic 

 columns. I did not succeed in obtaining specimens from the junction 



