BY W. H. TWELVETREES AND W. F. PETTERD. ()3 



and sometimes porphyritic augite. It has a sp. gr. of 3*15, 

 and dissolves to a large extent in HCl. Microscopically, 

 it is seen to consist of crj^stals and grains of olivine in a 

 gronndmass of crystals of melilite, accompanied l)y 

 perofskite or picotite. It contains no felspar, neither do 

 we detect nepheline. Nepheline, however, occurs in rocks 

 in such a form as often to be only recognisalile by chemical 

 methods, and hence it \NOuld perhaps be unsafe to assert 

 its total absence here. The melitite is the most inter- 

 esting element, as we believe it has not been recorded 

 previously in Australasia. It seems to occur only in 

 one generation, and in thin section yields two forms — 

 the prismatic vertical and the transverse section of the 

 prism. The boundaries of the prism are imperfect, show- 

 ing crenulated contours, and the elongated sections show 

 a peculiar mid-rib or median line, often beaded, sometimes 

 repeated as several vertical lines when the crystal is broad 

 enough. According to Dana," the peg structure of meli- 

 lite, which consists of parallel peg-like inclusions passing 

 from the base inwards, is not always easily seen. We 

 have not seen it in the Hunterston rock, nor in our slices 

 of melilite basalt from the Capo di Bove, near Rome, and 

 from the Hochbohl, Wiirttemberg. The transverse sec- 

 tions of the mineral in our rock have the grey interference 

 colour of felspar, from which, however, they can easily be 

 distinguished by their crenulate contours and isotropism 

 in basal sections. They are mostly, l)ut not always, water- 

 clear, while ^he longitudinal sections show a prevalent 

 granulation of the substance of the mineral. There is 

 none of the blue interference colour, which is sometimes 

 seen, for instance in the Hochbohl rock. Dana regards 

 melilite as crystallising in lieu of plagioclase, but Rosen- 

 buschf mentions the fact that while augite and melilite 

 exist in the rock in varying proportions, their sum 

 remains constant, and that consequently melilite takes the 

 place of the augite, and not of felspar. He correlates 

 melilite-basalt with the trachydolerite-limburgite series. 

 Short prismatic and granular microlites are abundant ; 

 these are probably augite ; nevertheless, the structure is 

 holocrystalline. There are numerous minute octrahedi'a 

 and grains of a highly refractive dark or imperfectly-trans- 

 lucent min^eral, which may be spinel or perofskite. In 

 one section we have observed a j^ellow garnet. 



* A Text-book of Mineralogy, E. S. Dana, 1898, p. 427. 

 f Elemente der Gesteinslehre, H. Rosenbusch, 1898, p. 359. 



