378 PEOr. J. W. JFDD OlS" THE TEETIARY AND 



adopted the term " Sehiller-spar " as the name of a mineral species ; 

 bnt Ereithaupt, Haidinger, and Haiiy, by dividing the Schiller-spars 

 into species like diallage, diaclase, bastite, and hypersthene, showed 

 that they had recognized the fact that many different minerals may 

 exhibit the peculiar reflection of Schiller-spar. 



It is now recognized that many varieties, both of the monoclinic 

 and of the rhombic pyroxenes, under certain circumstances, may 

 exhibit this peculiar appearance. The different kinds of pyroxene, 

 with the corresponding " Schiller " varieties, may be classed as 

 follows : — 



Unaltered forms. " SchiUer " varieties. More altered form, 



j Enstatite proper. Diaclasite ? Talc (?) 



Enstatites, I Proto-bronzite. Bronzite. "l -p -•, 



Ehombic. ^| Proto-hypersthene. Hypersthene. J ±5astite. 



i_ Amblystegite. Hypersthene. 



AuGiTE, JAig^tetroper.l Diallage and Green dialiage. 



Monoclmic. | Hedenbergite. J Pseudo-hypersthene. Smaragdite, &c. 



The " Schiller "-varieties of pyroxenes, when examined in thin 

 sections under the microscope, are seen to owe their peculiar appear- 

 ance to the presence in them of a great number of enclosures, in the 

 form of thin plates or delicate rods, arranged along one or more sets 

 of parallel planes within the crystal. When the crystals are held 

 in certain positions, the numerous enclosures, which exhibit various 

 grey, yellow, and brown tints, and possess a submetallic lustre, 

 reflect the light traversing the transparent portions of the crystal, 

 and by this reflection give rise to the " Schiller" phenomenon. 



The crystals of augite like those of felspar exhibit the first traces 

 of alteration along the incipient cracks, whether due to cleavage or 

 other causes, which traverse them. Along these incipient cracks 

 and in bands parallel with them, cavities make their appearance, 

 some of these cavities containing liquids with moving bubbles, while 

 others enclose solid materials (see PI. XI. fig. 1). While the 

 augites of the superficial rocks contain but few of these enclosures, 

 they become exceedingly numerous as we trace the augite crystals 

 to greater distances from the original surface. It is therefore 

 impossible to doubt that these cavities formed in networks along 

 the incipient cracks of the crystal are, like the similar ones de- 

 scribed in the felspars, of secondary origin. They have probably 

 been formed by the solvent action of the fluids which now fill them, 

 acting under the enormous pressures consequent on their original 

 depth from the surface. 



Both the green augites (diopside) and the black varieties (common 

 augite) of the Western Isles of Scotland are found, when traced into 

 the more deeply seated masses, to pass gradually into the " Schiller " 

 varieties known as diallage and pseudo-hypersthene. That this is 

 the result of a secondary modification is proved by the fact that the 

 alteration of the crystals is seen in many cases to be confined to 

 their outer portions, so that a nucleus of ordinary augite is sur- 



