254 E. B. Bailey 8f Q. W. Gmhliam — Plagioclme Felspars. 



We may now briefly notice some of the peculiarities which the 

 secondary albite of these rocks presents.^ In one specimen from 

 the Arthur's Seat volcano small patches and veins of anorthoclase 

 accompany the albite, where this replaces phenocrysts, but such an 

 occurrence is certainly rare in Scotland. A universal feature, however, 

 is the presence of minute flakes of white mica (of sericitic appearance) 

 scattered through the albite, wherever this latter has replaced basic 

 felspar; the mica is absent, however, from the albite of the veins and 

 vesicles, so that it has probably arisen independently in the pseudo- 

 morphs and is not a mere weathering product of the albite. Much 

 time was spent in confirming the identification of this mineral as white 

 mica. It occurs in such small flakes that its polarization tints are 

 often lower than those of augite in the same slide, while, where 

 closely packed, it gives, with the albite, a most misleading low 

 aggregate refractive index as determined by the Becke effect. In 

 some instances, however, patches can be found with a higher refractive 

 index than chlorite (13,080), while it resists the attacks of acid, and its 

 greatest axis of optical elasticity is normal to the cleavage. It thus 

 appears to be identical with the sericites of other l(x;alities whose 

 composition has been determined by analysis. It would be interesting 

 to know whether the mica here is muscovite or paragonite, i.e. 

 potash or soda bearing, but as a rule its bulk is too small to be 

 a matter of much importance as affecting the composition of the 

 pseudomorphs. Chlorite is commonly associated with the secondary 

 albite ; sometimes it fills the central portions of the pseudomorphs, 

 sometimes it occupies the site of some particular zone (13,119) 

 of the original plagioclase. It will be remembered that in the 

 vesicles chlorite crystallized immediately after albite, so that the 

 appearance of chlorite as a fairly frequent but subordinate con- 

 stituent of the pseudomorphs is not surprising. As one might 

 expect, the occurrence of calcite in this position is less frequent, but 

 occasional examples are found (13,115). In a unique slide from the 

 Edinburgh City cutting, at the back of the Engine Shed, St. Margaret's 

 Works, a group of phenocrysts occurs replaced by calcite containing 

 nnorientated crystals of water-clear albite. In the same slide a broken 

 albitized felspar has received a thin coat, apparently of chlorite, and 

 later an outgrowth of water-clear albite, in optical continuity with 

 itself; the outgrowth in this case projecting forward into a calcite 

 vein. It seems that in these instances the complete precipitation of 

 the secondary albite was delayed beyond the chloiite stage, and thus 

 it happens that the mineral, elsewhere clouded with minute inclusions, 

 is here water-clear. Analcite (12,307) is an occasional associate of the 

 albite in the pseudomorphs, and at least a couple of other undetermined 

 minerals have been noticed in one or two of the slides. Epidote 

 occurs not uncommonly in small amount in the vesicles and ground- 

 mass of these albitized rocks (1191), but not in the felspars themselves. 

 It doubtless contains part of the lime removed from the felspars, but 



1 The minerals mentioned in this paragraph have, in other cases, undoubtedly 

 arisen through weathering. It is probable that juvenile and weathering reactions 

 frequently follow the same course, but that the ionner, under suitable conditions, 

 proceed with a much greater velocity. 



