and Associated Rocks. 45 T 



described from a number of localities, 1 in no case has the exact nature 

 of the mineral been determined. It has generally been described as 

 a mica or chlorite-like mineral. The optical properties resemble those 

 of pennine, but the pleochroism is too intense and the double refraction 

 too high for that mineral. The green colour of the olivine indicates 

 a high content of fayalite, and it is probable that the pseudomorph is 

 some type of iron-rich chlorite. 



The olivine- crystals are often surrounded by an irregular border of 

 strongly pleochroic mica, while sporadic flakes of the latter also 

 occur throughout the groundmass. The mineral seems to have arisen 

 through the resorption of the olivine by the water-rich residual 

 magma which finally crystallized as analcite. 



The augite occurs in large idiomorphic crystals, up to a centimetre 

 in diameter, and has the purple colour characteristic of the so-called 

 titanaugite of alkali-rich rocks. The ciystals are often twinned on 

 a (100), and irregular intergrowths and crystal-groups commonly 

 occur. There are numerous inclusions of olivine, labradorite, apatite, 

 and orthite (?), these being sometimes arranged parallel to the 

 boundaries of the augite. Morphologically and optically, the mineral 

 resembles the augite of the Bail Hill tuffs, 2 the hour-glass structure 

 being very marked and a strongly-developed zonary banding being 

 visible even in ordinary light. As determined in a number of 

 twinned crystals, the extinction in the ' b-sector ' of the hour-glass 

 is 40° and in the ' s-sector ' 421°. The mean refractive index is 

 about 1 "70 and the double refraction "030, the optic axial angle, as 

 determined by means of the Federov stage, being 57£°. The dispersion 

 of the bisectrices is fairly high, so that sections in or near the 

 symmetry-plane do not extinguish in white light. The pleochroism 

 is relatively strong, the scheme being a and c yellowish-brown, 

 b reddish-purple (madder). 



Some of the mineral was separated from the rock and analysed. 

 This analysis, however, does not give the true composition of the 

 mineral, as it is impossible to get rid of the numerous inclusions of 

 olivine (sometimes occupying 20 per cent by volume of the augite). 

 Column 6, table ii, gives the figures of the analysis made. It is 

 certain that the ratio of lime to magnesia is too low on account of the 

 amount of olivine present. The figures, however, could be used to 

 determine the mode of the rock, as most of the inclusions are so 

 small that they would be reckoned with the augite. 



The plagioclase occurs as abundant lath-shaped crystals with the 

 optical properties of ' basic ' labradorite. These are sometimes sub- 

 ophitically enclosed by the augite and occasionally even penetrate the 

 olivine, but in general they are arranged in characteristic aggregates 

 surrounding these two minerals. Occasionally much larger equi- 

 dimensional crystals with well-developed zonal structure are found, 

 and these appear to be of early formation. The orthoclase, which is 



1 H. W. Monokton, ibid., 1, pp. 40-1, 1894 ; H. H. Bemrose, loc. cit. ; J. S. 

 Flett in Geology of East Fife (Mem. Geol. Surv.), 1902, p. 392 ; B. B. Young, 

 Trans. Edin. Geol. Soc, viii, p. 327, 1903 ; S. H. Eeynolds, Quart. Journ. 

 Geol. Soc, lxiv, p. 508, 1908. 



2 A. Scott, Min. Mag., xvii, pp. 100-10, 1914. 



