of the Buchan District. 13 



larger the porphyritic crystals are the less perfect is their 

 form; and this imperfection arises not from incomplete 

 crystallisation, but from the fracture of the crystals, or also 

 from their partial subsequent fusion. 



The inclusions are confined to exceedingly numerous 

 rounded or ragged portions of glass or slag, less numerous 

 colourless microliths, and apatite in prisms or needles. 

 This latter varies in amount in different samples examined, 

 as is also shown by the two analyses given in this paper. 



The alterations to which these felspars have been subjected 

 are of three kinds : — 



(a). To finely granular or scaly aggregates, which have 

 but a slight effect upon polarised light, and which may be 

 kaolin. 



(h). Rather minute aggregates of calcite. 



(c). Aggregates of plates and minute prisms of some 

 almost colourless or greenish minerals. 



It is frequently the case that the cleavages and flaws in 

 the felspars are lined with translucent red flakes of ferric 

 oxide (haematite). 



In order to determine, if possible,, the position of these 

 felspars in the series, I made observations on all the thin 

 slices, in order to determine the angle formed by the plane 



of vibration, and the twin plane (edge OP — go P go). The 

 measurements which I recorded were those which gave equal 

 or very nearly equal angles on each side of the twin plane, 



such measurements being in the zone OP — go P go. These 

 sections would, therefore, represent planes perpendicular to 



the edge OP — go P go, and having inclinations between two 

 limits — namely, the base (OP) and the macropinacoid 



(OP — go P go). The lowest observed angles should, there- 

 fore, represent more or less correctly the former, and the 

 highest angles the latter. Besides these sections of crystals 

 which were twinned, there were others which were simple, 

 and which, as they did not show either physical or optical 

 features referable to orthoclase, may reasonably be con- 



u ... 



sidered as representing the plane go P go of the same triclinic 

 felspar. 



I therefore obtained three groups of measurements, which 

 may be thus tabulated from six sets of observations : — 



