OLDER PERIDOXITES OF SCOTLAND. 389 



Aetzjiguren, But the Schillerization of minerals consists in the pro- 

 duction by natural causes of Aetzfiguren in three dimensions. It 

 bears witness to the existence of certain planes within crystals along 

 which a greater susceptibility to chemical action exists than in others. 

 It proves that, while often independent of the dii-ections along which 

 mechanical force and the different kinds of radiant energy most 

 easily act, the planes along which the chemical forces operate are 

 modified and controlled by structures like twinning. All these con- 

 clusions are in complete harmony with the results which have been 

 obtained by the study of the Attzjiguren. The results attained 

 by the observations and reasonings which we have been describing 

 bear the same relations to those obtained by the study of the 

 Aetzjiguren, as the phenomena of cleavage in a crystal do to the 

 observations made on crystal faces by means of the sclerometer. 



All the changes of which we have hitherto spoken are such as 

 may be traced m their effects by the aid of the microscope ; but the 

 same forces probably lead to other modifications of the internal 

 structure of crystals, which are altogether ultra-microscopical and 

 quite incapable of being detected by direct vision. But as it is 

 well known that light-waves are capable of being interfered with by 

 structures too minute to be discerned by the human eye, so v^e can 

 readily understand how solution along certain parallel planes within 

 the felspar crystal may lead to the formation of thin x^lates or layers, 

 probably composed of chalcedonic material, which give rise to the 

 beautiful cliatoymit effects exhibited by these minerals when they 

 have been submitted to deep-seated action. We have already pointed 

 out that the alteration in the density and chemical composition of 

 these chatoyant varieties of felspar support the view that they have 

 undergone the kind of change which we have been describing, and 

 that, in the Western Isles of Scotland, this phenomenon is exhibited 

 only by the felspars of what have been the most deeply-seated rock- 

 masses. 



§ 7. Varieties oe the Tertiary Ultra-basic Eocks. 



The Tertiary peridotites and other ultra-basic rocks differ from 

 one another both in their mineralogical constitution and in their 

 structure. Varieties corresponding to each of the rock-species which 

 have been established for the different types of peridotite are easily 

 recognizable in the Western Isles of Scotland. Thus in the Shiant 

 Isles and in the central parts of Bum we find a rock almost wholly 

 made up of grains of olivine enclosing rounded particles of chromite 

 and picotite — a rock which must be classed with the "dunites." In 

 various localities in the island of Bum, and also in the Shiant Isles, 

 we find rocks consisting essentially of olivine and augite, and these 

 must be classed as "picrites." Occasionally we find a considerable 

 amount of a more or less ferriferous enstatite, with some picotite or 

 chromite superadded to the ingredients of the last-mentioned rock, 

 and we have then an analogue of " Iherzolite." Some of the veins 

 which intersect the gabbros and peridotites of Bum are wholly made 

 up of a felspar (which, by its extinction-angles, its specific gravity, 



Q.J. G. S. Xo. 163. " 2-K 



