32(3 DE. F. DIXEY OX THE [vol. Ixxviil. 



It occurs in various tints of brown : the pleocliroism, Avliich is 

 genei'ally strong, lias been observed as follows : — 



X. liglit-red to bro"\vaiisli red ; Y, reddisii yellow ; Z. pale green. 



-Bastite is o£ rare occurrence only ; sometimes, however, the hyper- 

 sthene contains vellow-o-reen fibrous veins of a mineral which 

 resembles delessite : this mmeral has been noted in the h^^persthene 

 of the Charnockite Series^ and in the ' pyroxene-leptynite ' (Lacroix) 

 •of (Ceylon. Hypersthene is practically the only pyroxene of the 

 beerbachite, in which it is more or less granular, with a tendenc}^ 

 to idiomorphism. In the coarser rocks, on the other hand, it 

 sometimes forms large irregular plates enclosing all the other 

 minerals, after the manner of the big ragged augites in the 

 eucrites of Rum.^ Schillerization is developed much as in the 

 diallage, except that the inclusions are larger and consist of a 

 brown birefringent mineral. The hypersthene sometimes shows a 

 fine multiple twinning : this is probably secondary, and analogous 

 to that sometimes produced in felspars by local pressure. Rarely, 

 two sets of twinning are seen, crossing at right angles. 



The enstatite of the aplite-veins is often idiomorphic, and 

 not infrequently it takes the form of bladed skeleton-crystals ; 

 in the dolerite it generally occurs as anhedral grains. It is 

 colourless to pale green, optically positive, and the ])leochroism, 

 when visible, is X and Y yellowish or yellow-green and Z green ; 

 this pleochroism is rarely detected, except in those forms that are 

 crowded with minute rod-like inclusions. 



The order of crystallization of the various pyroxenes is dis- 

 tinctly variable. The pale augite usually appears as rounded 

 patches in the diallage ; small and fairly idiomorphic hypersthenes 

 are sometimes enclosed in augite ; but, on the other hand, plates of 

 hypersthene may enclose augite-cr3'stals.3 At other times, the 

 monoclinic and rhombic pyroxenes are intergrown ; they then show 

 the customary interpositions, and occasionally have a common 

 crystallogmphic orientation. Several examples were seen, similar 

 to those observed by Sir Jethro Teall in the dolerite of the Whin 

 Sill,^ and by Dr. J. Y. Elsden in the norite of St. David's.^ The 

 latter observer has already shown (ojj. cif. p. 286) that 



^ the simiiltaneons separation of these minerals, as sho^vn by intergrowths 

 and compound twins, might represent the eutectic composition,' 



in accordance with Prof. J. H. L. Yogt's view that in certain cases, 

 the sequence of these minerals should be dependent upon the 



1 Mem. Geol. Stirv. India, vol. xxviii (1900) p. 141. 



^ ' The Geology of the Small Isles of Inverness ' Mem. Geol. Surv. Scot. 

 1908, p. 100. 



^ Similar relations are noted in the norite occurring near Hiintly : see 

 W. E. Watt. ' The Geology of the Country around Huntly (Aberdeenshire) ' 

 ■Q.J.G.S. vol. Ixx (1914) p. 266. 



-• Ihid. vol. xl (1884) p. 649. 



^ Ihid. vol. Ixiv (1908) pi. xxxii. 



