OLDER PERIDOTITES OF SCOTLAND. 383 



of the star (sec PL XII. figs. 5«, 56, 5c). As these inclusions 



multiply, the crystal loses its transluceucy and finally becomes 



opaque, and exhibits by reflected light the colour and lustre of 



magnetite. As the tachylytcs of the Western Isles of Scotland are 



rendered perfectly black and opaque by the quantity of magnetite - 



dust which they contain, so the olivines are completely obscured 



in their characters by the development in their midst of these 



magnetite-enclosures. What is taken for magnetite in many 



gabbros is nothing but this greatly altered olivine. That these 



stellate bodies in the substance of the olivine crystals are really 



inclusions formed within cavities having a rectilinear outline, is 



demonstrated when they are examined with high powers of the 



microscope. Each ray of the star is then seen to end abruptly 



along a right line, and the limits of the cavities within which they 



are formed are thus clearly indicated (see PI. XII. fig. 5c). 



The curiously varied forms represented by Zirkel are due to portions 



only of these stellate bodies being within the field of view of the 



microscope at the time and often being viewed obliquely ; but by 



carefully focussing up and down, their true nature can be readily 



made out*. In their absolute dimensions these enclosures of the 



olivine crystals vary between very wide limits, as is the case with the 



similar bodies in the felspars and pyroxenes ; while some of the 



enclosures can be seen and studied with quite low microscopic powers^ 



others are crowded into a nebulous haze which can only be resolved 



by the very highest powers. 



The Biotites, which are among the most frequent of the accessory 

 minerals in the gabbros and peridotites, exhibit a similar secondary 

 structure to that described in the felspars, the pyroxenes, and the 

 olivines. Tabular enclosures of a deep brown or black colour 

 are developed along the planes of easy cleavage of the mineral, and 

 are sometimes so abundant as to render the mineral almost opaque 

 (see PI. XII. figs. 8, 9). 



§ 5. JN'ature and Origin op the Changes which have taken 

 PLACE in the Minerals of deep-seated Plutonic Eocks (" Schtl- 



LERIZATION "). 



We have seen that alike in the felspars, the pyroxenes, the' 

 olivines, and the biotites of plutonic rocks, there is evidence of pro- 

 gressive change taking place at gradually increasing depths. This 

 change consists in the deveropmeiit along certain planes within the 

 crystals of tabular, bacillar, or stellar enclosures, which, reflecting 

 the light falling upon them at certain angles, give rise to the peculiar 

 phenomenon expressed by the term " Schiller." It will be con- 

 venient to have a general name for this kind of change, and I pro- 

 pose to employ the term " Schillerization "' to express it. Thus I 

 shall call diallage and pseudo-hypersthene " Schillerized augites," 



* Zeitschr. d. deutsch. ^eolog. Ges. vol. xxiii. (1871) pi. iv. fig. 11. Zirkel, 

 Mikroskop. Beschaff. der Min. und Gest. (1873) p. 214. 



