Vol. 60. OF THE EAST-CENTRAL HIGHLANDS. 435 



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that it is the base of the Main Limestone, which has here under- 

 gone one of the startling changes in appearance so often noted ; 

 for quite close by, and over much of the Falar area, the Alain Lime- 

 stone has the appearance of the normal rock of Blair Atholl. 



This change of the Honestones into a series of quartzite-bands of 

 variable colour is highly important for two reasons. In the first 

 place, it is obvious that these are the bands that, repeated incessantly 

 bv folding, form the Moine Gneisses along the Sluggan Road in Inver- 

 cauld forest, already described (see p. 413): the highest white band, 

 in particular, is especially important, for it forms the top of the 

 Moine Gneisses below Gilbert's Bridge and in many other localities. 

 The second point is that a change in composition, similar to that 

 along the main line where the Moine Gneisses begin, is now taking 

 place in a south-easterly instead of a north-westerly direction ; in 

 other words, the change in composition of the rocks, due to powerful 

 current-actions accompanied by the deposition of more siliceous 

 material, is repeated to the south-east. The survey of the whole 

 area has shown that this tendency to revert to more sandy con- 

 ditions of deposition occurs again and again south-east of the Moine 

 Gneiss area and, though carried to a far smaller extent, it is almost 

 invariably accompanied by the silting-away or non-deposition of the 

 liner clastic material. 



Below the type-section in Glen Mohr, the Honestones are often 

 exposed in the bed and sides of the gorge. A little above Falar 

 Burn they have an almost flinty aspect, and are characterized by 

 even banding, recalling a very fine phase of the Moine Gneiss 

 (11.125). Structurally, the rock is a very fine quartz-biotite- 

 grauulite. but its most striking feature is the arrangement of 

 the crystals of brown mica. Though rigidly parallel, they arc- 

 oblique to the bedding, which is clearly seen under the microscope. 

 Just at the mouth of Falar Burn there is a distinct increase in the 

 amount of originally-softer material present, and a type-specimen 

 ( U453) could be matched from the mouth of Glen Callater. Near 

 the foot of Glen Mohr the whole of this softer material has dis- 

 appeared, and now only a thin film of the more quartzose pink-and- 

 grev material separates the Main Limestone from the Quartzite. It 

 is obvious that there is a slight line of erosion at the base of the 

 Limestone, showing that it must be above the Quartzite. The pink- 

 aud-grey material (9406) is singularly like a portion of the Moine 

 Gneiss, except that it is finer in grain : and the resemblance is 

 equally marked in a microscopic section. 



(/) The Aberdeenshire Area. 



Turning now to the Aberdeenshire area, and following these finer 

 Parallel-Banded Rocks in a direction parallel to that along which the 

 coarser Moine Gneisses have been traced, an interesting outcrop 

 occurs about half a mile up Allt-na-Bronn, to the cast of the Bynack. 

 Here the quartzite is succeeded by a thin series, composed of 



