496 H. S. JEVONS, H. I. JENSEN, T. G. TAYLOR AND C. A. SUSSMILCH. 



Other Minerals. 

 The olivine occurs usually in large nearly idiomorphic 

 individuals, but in the outer envelope (pallio-essexite) there 

 is a second generation. It alters to a serpentine which is 

 always deeply coloured, sometimes olive-green, sometimes 

 brown. In the pallio-essexite the olivine is sometimes 

 practically unaltered (e.g. east end of the Reservoir Quarry). 

 This suggests that it is a highly ferriferous variety, a fact 

 which is confirmed both by the composition it was found 

 necessary to assign it in calculating the modes of the rocks 

 in which it occurs, and by its high D.R. (see appendix). Its 

 average composition is found to be approximately as follows 



Si0 2 35% FeO 41% MgO 24% 



Examination between crossed nicols shews that it is not 

 homogeneous in composition. In the main mass it is much 

 decomposed, but in the pallio-essexite the phenocrysts are 

 seen to have an outer border showing higher colours than 

 the interior of the crystal, which is fairly uniform in colour* 

 This outer zone and the small crystals of the second gener- 

 ation therefore approach nearer to fayalite in composition. 



The biotite is intensely pleochroic, a a very pale straw 

 yellow, h and t very deep reddish-brown. The compos- 

 ition assumed for it for purposes of calculation will be 

 found in the appendix. The ilmenite occurs mostly in 

 hexagonal tables and occasionally in triangular skeletons. 

 Between the two great segregation veins in the Reservoir 

 Quarry (Plate XXXV) the rock is remarkably rich in 

 ilmenite, which has the form of hexagonal lamellae, often 

 20 or 30 mm. in diameter, though no more than 1 mm. thick. 

 As calculation of the mode shows that magnetite is present 

 in addition to ilmenite in every case, though not distinguish- 

 able microscopically, it must usually be intergrown with 

 the ilmenite. 



