62 NEPHELINE AND MELILITE ROCKS. 



by light brown and yellowish nepheline, which has often 

 decomposed and originated snow-white radiated aggregates 

 of the Zeolite natrolite. The proportions of augite and 

 nepheline vary greatly. Sometimes the augite is extremely 

 abundant, otherwise more sparingly distributed. 



The mineral constitution of the rock may be stated tvs 

 follows : — 



Essential minerals = Nepheline, augite. 



Accessory minerals = Olivine, sanidine, apatite, melanite- 



garnet, magnetite. 



Secondary minerals == Natrolite, serpentine. 



Microscopical cJiaractevH. — The structure is holocrys- 

 talline, hypidiomorphic. No groundmass is present. 

 Nepheline, generally, forms about one-half of the entire 

 rock, sometimes more. It gives large sections bounded 

 by rectangular contours, margined with iron oxide, and 

 sometimes penetrated by augite. Its substance is mostly 

 converted into radiating natrolite : some patches, however, 

 remain water-clear. The clear nepheline encloses slender 

 rods of apatite, as well as other needles, which, from their 

 oblique extinction, we surmise to be augite. The natrolite 

 gives beautiful fan-shaped aggregates, polarising in grey, 

 low^ yellow, and orange colours. The nepheline crystals 

 are often cut up by rectangular cracks. 



The augite is in large prisms of green to violet tints, 

 sometimes showing both colours in the same crystal. Its 

 maximum extinction angle measured from the fissure 

 lines is 45°. It encloses prisms of apatite. A prominent 

 element of the rock is apatite in long transversely-jointed 

 rods and prisms, some of which are large enough to be 

 visible to the unaided eye. Olivine is an infrequent 

 accessory. It has crystalline contours, the usual rough- 

 looking surface with irregular cracks, and is associated 

 with some serpentinous material. Some orthoclase felspar 

 is also present in small quantity. Its transparency indi- 

 cates the sanidine variety. 



No one who has seen the familiar slides of the nephelinite 

 (or nepheline-dolerite as it has been called), of Katzen- 

 buckel in the Odenwald, can fail to recognise the same 

 type in slices of this Hunterston rock. The latter is the 

 same rock reproduced in the Southern Hemisphere. The 

 specific gravity of an average specimen was ascertained to 

 be 2-66. 



Melilite-BciscUt. — A-ssociated with the nephelinite is the 

 basaltic rock of the small conical hills referred to above. 

 This is dark grey compact basalt, with porphyritic olivine 



