REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 133 



give rise to this greenish zone. There is no plagioclase to be seen except the microliths 

 of the ground-mass. 



Other rocks of a reddish colour and much altered, which were collected by Mr. 

 Buchanan at Greenland Harbour, are also basalts. Microscopic examination shows 

 them to be fine-grained, with a ground-mass of microliths of plagioclase, and grains 

 of augite and magnetic iron. In this are embedded large sections of triclinic 

 felspar, traversed by cracks and in part opalised, like those we shall describe in 

 detail in the notes on Kandavu Island. Augite and olivine also appear as micro- 

 porphyritic elements ; the latter mineral particularly is more or less penetrated with 

 oxide of iron. 



The horizontally-bedded basalts of which we have been speaking surround grey 

 masses of trachyte and phonolite, projecting above the basalt and having a columnar 

 structure. Their geological disposition and aspect have already been described from Mr. 

 Buchanan's data. These rocks are hard and compact, their colour is greyish green, 

 and although they present marked resemblances to many phonolites, they do not ring, 

 as rocks of this type generally do. Specimens broken off the prisms are finer grained 

 than those from the central mass, and have a distinct cleavage perpendicular to the 

 length of the columns. This rock partially gelatinises in hydrochloric acid ; the solution 

 contains much soda and traces of sulphuric acid. From this reaction Mr. Buchanan 

 concluded that these rocks contained at the same time nepheline and nosean. They 

 may be classed as augitic trachytes ; in some cases, by the addition of nepheline, they 

 pass into phonolites, and then, finally, when sanidine is absent, pass into nephelinic 

 rocks containing acmite. 



We shall first describe the specimens taken from the wall of rock on the summit of 

 the hills lying west of Greenland Harbour. These rocks, projecting above the masses 

 of basalt, are phonolites. They are greenish grey, compact, with a slightly shining 

 fracture and a rather indistinct schistosity. They are sometimes spotted with more or 

 less circular black markings ; large crystals of sanidine are to be seen, and sometimes 

 milk-white microscopic sections of nepheline. Microscopic examination shows that the 

 rock is essentially composed of numerous small crystals of nepheline closely packed 

 together, but still preserving the general sharpness of their outlines. This mineral is 

 sometimes seen in larger hexagonal or quadratic sections with zonary structure, 

 standing out from the ground-mass formed by microliths of the same species. Sani- 

 dine is comparatively rare, and occurs in elongated lamellae twinned according to the 

 Carlsbad law. The green mineral is of quite small dimensions, and its outlines are 

 vague ; the angle of extinction measured for a great many crystals hardly ever 

 exceeds 15° or 20°, hence the crystals are very probably hornblende. Titanite is 



