370 PROF. J. W. JTJDD ON THE TEETIA.RT AND 



to a greater extent by ferrous oxide, the mineral becomes of a more 

 and more pronounced brown tint with marked pleocbroism. With 

 the highest percentage of iron the mineral assumes a very dark 

 colour, and when examined in thin sections exhibits the most 

 startling pleochroism {a, brilliant red; b, duller red; c, bright 

 green). 



There has arisen much confusion as to the names which should 

 be given to the different varieties of the rhombic pyroxenes. Most 

 authors call the non-ferriferous varieties " enstatite," the more 

 ferriferous kinds " bronzite," and still more highly ferriferous forms 

 " hypersthene." Tschermak has proposed that varieties with less 

 than 5 per cent, of ferrous oxide should be called enstatite, those with 

 between 5 and 15 per cent, bronzite, and those with more than 15 

 per cent, hj^persthene *. The strict application of this rule would, 

 however, necessitate a chemical analysis in each case before the 

 species or variety could be determined. 



As a matter of fact, no two petrographers are agreed as to the 



limits between enstatite and bronzite, and between the latter mineral 



and hypersthene. Thus the mineral of the andesite of the Cheviot 



HiUs, regarded by Mr. Teall as hypersthene, is, by Dr. Petersen, 



who carried on his investigations in Prof. Eosenbusch's laboratory, 



classed as bronzite t. The similar mineral found in the pumice of 



£rakatoa is called hypersthene by many authors and bronzite by 



others. The minerals classed as enstatite, bronzite, and hypersthene 



respectively by Eammelsberg J do not conform to the rule laid 



down by Tschermak as to the chemical composition of these varieties. 



The gabbros and peridotites of the Western Isles of Scotland 



contain all the varieties of the enstatite group, in both their altered 



and their unaltered forms ; and these permit a careful study of the 



whole series of minerals. Through the replacement, to a varying 



extent, of a part of the magnesia by ferrous oxide, very distinct 



varieties are produced ; but, inasmuch as the crystalline form remains 



the same, it wiU be convenient to treat the rhombic pyroxenes like 



the monoclinic pyroxenes (" augites "), and give them a common 



name. I think there can be no doubt that the common name for 



these rhombic pyroxenes should be '' enstatite," for this was the 



first unaltered rhombic pyroxene which was detected and described 



by mineralogists. I believe that this is also the view taken by 



Prof. Eosenbusch. 



As, however, the variation in the relative proportions of magnesia 

 and ferrous oxides in these minerals leads to such startHng differences 

 in their appearance and optical properties, it may be necessary to 

 use a number of varietal names — like those employed in the case 

 of the monoclinic pyroxenes — diopside, common augite, and heden- 

 bergite. 



As the enstatites were till quite recent years entirely unknown to 

 mineralogists in their unaltered forms, a difficulty arises in selecting 



* Tschermak, 'Lehrbuch der Mineralogie' (1884), p. 436. 



t See Geol. Mag. Dec. ill. vol. i. (1884) p. 227. 



J Handbuch der Mineralcbemie, 2rid ed. 1875, pp. 382-385. 



