Vol. 60.] OF THE EAST-CENTRAL HIGHLANDS. 409 



after the formation of the micas, there was not an excess of alumina 

 sufficient to form such minerals, and the microscope confirms the 

 inference that the shale was originally somewhat gritty and impure. 



Other Exposures in the Struan District. 



The flaggy gneisses just described cover a large area on both sides 

 of the Garry, above Struan. On the open ground, however, good 

 exposures of them are not numerous, because the flanks of the hills 

 are much obscured by Drift, and the crests of the hills are frequently 

 covered by a somewhat angular rubble, which is due to the dis- 

 integration of the rocks. About the Dalnacardoch area, and for 

 some distance eastward, the massive grey gneiss is especially 

 abundant, where it weathers in the form of rounded blocks. 

 Owing to its greater power of resisting decomposition, it is often 

 seen in situ. Excellent sections are exposed, however, in the 

 streams that drain into the Garry, and these are often continuous 

 for considerable distances. Microscopic examination shows little 

 variation in the type-rocks ; while the appearance of a continuous 

 dip is at times even more marked than in the Garry section. 



In the area between Struan and Blair Atholl, the gneisses undergo 

 a slight change, becoming, on the whole, more micaceous, and in 

 many cases rather more fissile. The latter character is shown by 

 microscopic sections to be due to the parallel arrangement of the 

 white mica, as well as of the brown, the two being often in contact. 

 Good examples of this type occur in the cutting near the Manse, 

 north-west of Blair Atholl, where the rocks appear to contain rather 

 less microcline than usual. Around the igneous complex of Glen 

 Banvie, and for some distance to the south-east, the gneisses are 

 rather more quartzose than usual, becoming at times almost quartzites. 

 A typical example of the latter is essentially a granular mosaic of 

 quartz and felspar, with a little brown and white mica, but it differs 

 from the Central-Highland Quartzite in the large amount of micro- 

 cline present. Farther down the Banvie Burn greyer-banded types 

 occur, well shown in the quarry under the road at the western end 

 of the Whim Plantation. One of these (10,422) contains many oval 

 crystals of microcline, within which are numerous poikilitic grains 

 of quartz, as well as minute flakes of mica and grains of garnet and 

 epidote. Most of the grey bands, with parallel biotite, contain 

 much microcline, but when the biotite is abundant it begins to show 

 a reddish-brown tint. A little band (10,424) having much the 

 appearance of the material that forms the more massive rocks about 

 Dalnacardoch, is also seen here ; and it is especially worthy of note 

 that this contains hardly any microcline, but much plagioclase in 

 sheets enveloping the quartz. 



Before leaving the Garry area, attention may be drawn to two 

 special rocks. The first occurs in the river-bed at Dalnacardoch, and 

 has already been described (p. 408) ; the second is associated with 

 the typical grey-banded gneiss, and is composed of white quartzose- 

 looking material, within which are set a number of dark blotches 



