286 me. w. e. watt on" the geologt [June 1QI4, 



West of the house at Ladysmith, a similar type occurs, where, 

 however, the biotite, scarcely represented in the previous section, 

 has entirely taken the place of the hypersthene. Dark-green 

 spinel-grains occur with felspar and cordierite, and likewise as the 

 core of a normal pink garnet. This would seem to be a further 

 development of the previous type, accompanied by the formation 

 of biotite. 



The cordierite-norites of the second type are found in the 

 neighbourhood of Drumdelgie, at Wells Cuternach, and between 

 that and ' The Gray Stane.' They mark the junction between 

 igneous rock and phyllite. In mineral composition they show little 

 difference from the previous group, and here again is observed a 

 gradation from a cordierite-hypersthene type to a garnet-biotite- 

 felspar type, accompanied by a rise in specific gravity from 2 '896 

 to 3*207. To a small extent are developed pleonaste, magnetite, 

 zircon, and quartz. 



The hypersthene usually is strongly pleochroic, and of a slightly 

 purplish hue. The cordierite in the rocks where it is abundant is 

 beautifully twinned, showing ' sector-polarization.' Usually, too, it 

 is full of dust-like inclusions, and occasionally is altered to pinite ; 

 halos are frequent. Biotite, colourless and pink garnet, and 

 pleonaste present the same features. The felspar is labradorite 

 (ju=: 1*559) or andesine (fi = 1*554), is quite fresh, and always 

 present in some amount. Quartz is sparingly represented, and 

 carries small needles of rutile. 



The finest example of cordierite-norite comes from the Wells at 

 Cuternach. In the hand- specimen, the cordierite is so plentiful as 

 to impart to the rock a distinctly greasy lustre. In section the 

 cordierite is seen to be beautifully twinned, possessing also halos 

 and clouds of minute inclusions. It occupies over 50 per cent, of 

 the rock, as is seen from the calculated mineral content. Here the 

 felspar is labradorite of the composition Ab„An u , approximately. 

 Biotite and hypersthene are the ferromagnesian minerals, the 

 latter predominating over the reddish-brown biotite. The spinel 

 is of a dark brownish-green colour, and seems to form skeleton- 

 crystals. No garnet is present in this rock. This type has been 

 analysed by Mr. Radley (see p. 289). 



In no succeeding case is there such a development of cordierite — 

 its commanding position being now taken by labradorite (/u = 1*559). 

 Hypersthene and biotite likewise continue, and with them occur 

 magnetite and the two garnets, which merge one into the other as 

 before. Here the colourless garnet shows vermicular markings 

 even towards the felspar, and in the pink garnet the same markings 

 are visible at the edges. Chlorite results from the alteration of the 

 biotite, which possesses its characteristic intergrowths. 



Very similar to this type is the rock north of the Gray Stane, 

 differing only in the smaller amount of cordierite. With others 

 at Cuternach this forms a link with those rocks that possess no 

 cordierite. In this final stage, hypersthene has also disappeared 



