372 * PROF. J. W. JTJDD ON THE TERTIAKT AND 



sometimes in considerable quantities, in the so-called trap-grannlites, 

 the diallage-grannlites, and other similar rocks all over the globe. 



The analyses which have been made of these excessively ferriferous 

 cnstatites are as follows : — 



I. II. III. 



Silica 49-80 48-2 51-348 



Alumina 505 



Ferrous oxide 25-60 28-4 33-924 



Manganous oxide ... 5-2 



Magnesia 17-70 16'7 11-092 



Lime 0-15 1-5 1-836 



Water 0*500 



98-30 100-0 98-700 



I. Is the analysis by Yom Rath of the original " amblystegite " of the Laacher 

 See. (Pogg. Ann. cxxxviii. (1869), p. 531.) i 



II. Is an analysis by Laurent of the dark-coloured enstatite found in cavities 

 of a rock enclosed in the trachyte of the Rocher du Capucin, Mont Dore, 

 quoted by Descloiseaux (Manuel de Mineralogie, tome ii. p. xviii.). 



III. Is the analysis of the altered form of the same mineral, which will be 

 referred to hereafter as occurring at Loch Ooruiskh, in Skye. The analysis was. 

 made by Muir and published by Thomson (' Outlines of Mineralogy,' 2nd ed 

 (1822). vol. i. p. 202. 



Although Prof. Yom Eath withdrew his name of " amblystegite '^ 

 when he found from Yon Leng's researches that all the planes of 

 its crystals were represented in the enstatite of the Breitenbach 

 meteorite *, yet if it be necessary to have a varietal name for these 

 excessively ferriferous and strongly pleochroic enstatites, it would 

 seem desirable to revive this term rather than to invent a new one. 



The amblystegites differ from the ordinary hypersthenes in their 

 unaltered state (proto-hypersthenes) quite as strikingly as do the 

 latter minerals from the unaltered bronzites (proto-bronzites), or as 

 these last do from the enstatites proper. In microscopic sections of 

 rocks the wonderfully vivid pleochroism, which gives rise to colours 

 varying from a rich garnet red to a vivid blue-green, is its most 

 striking characteristic, and serves to distinguish it from all other 

 rock-forming minerals. The only other rhombic mineral with 

 characters which at all approach it is andalusite ; but in practice the 

 distinction of these two minerals is perfectly easy. 



Among the remarkably fresh minerals of the Tert-iary gabbros and 

 peridotites it is possible to recognize all the varieties of the enstatite 

 group. 



In the island of Kum, we find a perfectly colourless rhombic 

 pyroxene, occurring in irregular grains with a colourless olivine 

 and the brilliant green diopside already described. In its perfectly 

 fresh condition this enstatite can only be discriminated from olivine, 

 also a rhombic mineral, with the greatest difficulty. The fine 

 striation relied upon for the distinction of the two species is a 



* Neues Jahrb. fiir Min. &c. 1871, p. 642. 



