Vol. 56.] AND OTHER IGNEOUS ROCKS IN ARGYLLSHIRE. 535 



structure between crossed nicols. While the olivine- and augite- 

 crystals are of the same general size as those in the more central 

 portion of the rock-mass, the texture of the biotite and felspar of 

 the matrix has become much more fine-grained. The biotite occurs 

 freely in scattered flakes, groups of which, on close examination, 

 are seen to be in optical continuity, the appearance being similar to 

 that of the biotite in the coarser specimens, only on a much smaller 

 scale. Similarly, the biotite is seen to be of later formation than 

 the plagioclase, which is markedly idiomorphic, and to be moulded 

 upon it. 



Biotite moreover occasionally occurs as inclusions in the augites, 

 an occurrence which has also been noticed in the coarser rock both 

 from this and other localities. Presumably it represents an earlier 

 generation of the mineral, though there is no doubt that by far the 

 larger proportion of the biotite was one of the last minerals to 

 crystallize out. 



Orthoclase, judging from the coarser rock, is presumably present 

 as interstitial matter, but is difficult to identify with certainty, 

 owing to the fine-grained character of the groundmass. 



Passing now to the Ben-Bhuidhe area, we find that the olivine- 

 bearing rocks of this group vary somewhat from the commoner 

 type. They are rich in pyroxene, but do not contain quite so high 

 a proportion of olivine as the Kentallen rock ; while, on the other 

 hand, the proportion of felspar, and especially of orthoclase, to 

 olivine and pyroxene is decidedly higher. 



Of the two masses, exposed in the Brannie and An-Sithein 

 burns, that of Brannie Burn would seem to be slightly the more 

 basic. The augite, olivine, and biotite of these rocks do not 

 call for any further description, though it should be noted that the 

 augite does not exhibit crystal-form anything like so perfectly as in 

 the more typical rocks of this group, but tends to occur rather as 

 corroded grains and granular aggregates. The characteristic ophitic 

 structure of the biotite constitutes a pronounced feature in the 

 Brannie rock [7414]. In a section of the An-Sithein rock [7415] 

 (see PI. XXX, fig. 2) the proportion of olivine is relatively small. 

 The rock, however, is rich in pyroxene, and shows a slightly 

 pleochroic hypersthene in association with the augite. But the 

 most characteristic feature of both these rocks is the orthoclase- 

 felspar : in the Brannie rock it occurs in approximately equal pro- 

 portion with the plagioclase, but in the An-Sithein rock the 

 orthoclase is decidedly in excess. It is poecilitic in appearance, and 

 forms a matrix of comparatively large, clear, irregular patches, in 

 which small, lath-shaped, idiomorphic plagioclases lie embedded 

 unorientated. With respect to the olivine and pyroxene, the 

 orthoclase is interstitial. Individual patches occasionally show 

 twinning on the Carlsbad plan. 



We see, then, that in these Ben-Bhuidhe rocks the same peculiar 

 features as those which have been already noticed in the more typical 

 kentallenites are still found, and are represented in an even yet more 



