Vol. 60.] OF THE EAST-CENTRAL HIGHLANDS. 439 



crystallization : and this materially increases the difficulty of ascer- 

 taining the mode of ending-off of the»recognizable gneisses. When 

 an area of more sandy deposition is approached, there is always a 

 tendency for a hiatus to occur in the sequence, the Little Limestone 

 and part of the Dark Schist being almost always missing. Moreover, 

 the Dark Schist itself tends to become more siliceous and to contain 

 less dark dust. A similar change undoubtedly occurs in the Little 

 Limestone before it disappears ; but the bed is so thin that it is often 

 difficult to rind, and its exact mode of ending-off has not been satis- 

 factorily determined. The hiatus is most frequently noticed at the 

 base of the Main Limestone, which is clearly above the Dark Schist 

 and the Moine Gneisses. But the missing beds were originally of 

 no great thickness, and are only those that lie between the base 

 of the Limestone and the Parallel-Banded rocks, except where the 

 latter were originally composed of comparatively-fine mud. Over 

 the whole belt of ground examined, more than 30 miles long, this 

 hiatus never exceeds these limits, clearly showing that it cannot 

 be claimed as an important stratigraphical break in the sequence 

 of which the Moine Gneisses form a part. Where no hiatus at all 

 occurs, the Parallel-Banded rocks are succeeded by the Little Lime- 

 stone ; and the true stratigraphical position of the Moine Gneisses 

 is thus defined, as lying between the Little Limestone and the 

 white edge of the Quartzite, of which, indeed, they are simply 

 the flaggy top. The succession in the group of rocks described 

 and their mutual relations are briefly expressed in the appended 

 diagram (fig. 7, p. 438). 



(k) Horizon of the Gneisses north-west of the Belt along 

 which they thin away. 



When well across the line of thinning-away, the upper limit 

 of these gneisses can often be fixed ; and a few type-localities may 

 be selected for this purpose. One of the best lies about Derry 

 Lodge, where both the Limestone and the Dark Schist are present. 

 Close to the Derry Falls the Moine Gneisses are succeeded by a 

 small portion of the Dark Schist ; but this is so much more 

 quartzose, and contains so little dark dust, that it is practically 

 impossible to fix its exact horizon (10,882), although the occurrence 

 of the Main Limestone next to it shows that this must be a repre- 

 sentative of part of the Dark Schist. A small quantit}* of the typical 

 felspar is present ; and the rock possesses the flaser-structure so 

 characteristic of the Highland metamorphism. Some distance to 

 the south-east of Derry Lodge, a rather similar section occurs ; but 

 here the Limestone is associated with the Pink Felspathic material 

 once more (8274). 



Even when no limestone is present, the upper limit of the 

 gneisses can be approximately fixed by the presence of identifiable 

 portions of the Dark Schist. Two good illustrations of this occur 

 in the Tarf Valley. On the south side of the stream the infold is 



2 h 2 



