Vol. 60.] OF THE EAST-CENTRAL HIGHLANDS. 407 



of the gneiss is mainly due to the parallel arrangement of the 

 micas : and microscopic sections show that, as a rule, these are so set 

 in the rock as to interfere only to a small extent with the granular 

 structure shown by the quartz and felspar. It is only when mica 

 is present in sufficiently-large quantity to impart an almost-fissile 

 character to the rock, that its influence appears in the rock- 

 structure. In that case, almost continuous films of biotite or biotite 

 and muscovite separate well-defined parallel strips of quartz-felspar 

 material. The edges of the grains in contact with the mica are now 

 distinctly flattened, and, moreover, the grains within the parallel 

 strips tend to assume a somewhat quadrangular form (Xos. 85, 89, 

 & 9U). 



In the Struan and adjacent areas, the biotite of the Moine Gneisses, 

 when fresh, is always of the normal haughtonite-type — that is, 

 when seen in cross-section and rotated under a single nicol, it 

 changes in colour from brown to a watery-black. Inclusions with 

 more or less pleochroic halos occur in the biotite, although they are 

 not a marked feature of the mineral. Chlorite is present in many 

 of the rocks, more particularly in the micaceous gneisses. It is very 

 difficult to say whether this is a replacement-product after brown 

 mica or not. It is too often forgotten that lime, though in small 

 quantity, is an essential constituent of normal biotite; and, in many 

 cases, the more or less chloritic original material from which these 

 rocks were produced did not contain sufficient lime to form even 

 biotite when metamorphosed, and then the chlorite is the direct 

 product of thermal metamorphism. Pleochroic spots are common 

 in this chlorite. The white mica presents no feature of importance, 

 except in its mode of occurrence. It does not conform to the 

 foliation so closely as the biotite, and in some of the rocks it is set 

 with the basal plane at right angles to the foliation. 



In addition to the minerals just enumerated, small crystals of 

 sphene are common in some of the specimens, and present in almost 

 all. They are often pointed, elliptical in shape, and are frequently 

 coated with a film of iron-oxide. They have usually the aspect of 

 metamorphic sphene, and are never strictly original. Apatite occurs 

 occasionally, as also zircon. The latter is not nearly so common 

 as might have been expected in such rocks, which clearly originated 

 from fine felspathic and micaceous sands. Small garnets occur in 

 certain dark blotches in one band of very pale gneiss, but the 

 mineral is not common in this area. 



Taking the grey gneisses as a whole, they are remarkable for the 

 amount of microcline present, and, in this respect, they differ from 

 the grey gneisses of the areas farther to the north-east (to be 

 described later), where microcline is less common, desrute the close 

 external resemblance between the rocks in the two areas. 



The Pink-edged Gneisses. — In addition to the dominant 

 grey-banded gneisses, there are also present some that weather with 

 a pink edge, even though they are grey on a freshly-fractured face. 

 These pink-edged varieties are especiallv interesting, because they 



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