Vol. 60.] OF THE EAST- CENTRAL HIGHLANDS. 415 



indicating the former bedding-planes. Their presence is 

 highly important, especially when we consider the mode 

 in which the ^loine Gneisses end off when traced to the 

 south-east, as it will be seen that they link the gneisses 

 with the Dark Schist, of which the same material was an 

 abundant constituent. 

 6. Lastly, a considerable mass of highlv-quartzose material, which, 

 for purposes of mapping, must be included in the Moine 

 Gneiss, can in the eastern part of the area be shown to be 

 really the Highland Quartzite (in what may be conveniently 

 called a ' Moine-phase'"), and should be excluded from the 

 group in discussing the origin of the grey gneisses. 



III. Mode of Exdixg-off of the Motne Gneisses. 



Having shown that these gneisses extend in a south-easterly 

 direction to the Tilt Valley, the Geldie, and the Dee, almost to 

 Braemar, we may pass on to consider the question why they do not 

 appear in their typical phases to the south-east of this long line. 

 The simplest explanation would be that they have been faulted-out ; 

 and in the Glen-Tilt area this, at first, seems to be the true one. 

 That it is not sufficient, however, is clear from the fact that in some 

 cases the gneisses cease to be recognizable before the main fault is 

 reached, while in the district east of the Geldie they cross the fault 

 in mass. Two other causes may be suggested : first, that they become 

 less crystalline, and so cease to be recognizable as Moine Gneisses ; 

 or, secondly, that they thin away. It will be shown that both 

 causes co-operate to render the further tracing of them a matter of 

 difficulty. 



(a) The Belt of Decreasing Crystallization. 



The first of the causes above suggested is most important in the 

 Tilt Valley, where the decrease in crystallizition is unusually 

 rapid. It occurs along a belt that has been traced from the coast 

 north of Stonehaven to a point north-east of Blair Atholl, a distance 

 of about 100 miles. This belt passes in a somewhat curving line 

 from the eastern coast to the head of Glen Isla, where it sweeps 

 round in a north-easterly direction almost to Ballater ; thence it 

 turns westward, and crosses the Dee somewhere between Balmoral 

 and Braemar. From the latter point it coiucides roughly with 

 the Dee Valley as far as the Geldie Burn, after which it follows, 

 approximately, the belt of faulting in the Tilt Valley to within 

 3 miles of Blair Atholl. West of the Geldie, this area of decreasing 

 metamorphism corresponds approximately with the belt along which 

 the Moine Gneisses disappear ; but east of the Geldie the two are 

 less intimately connected. When this belt of decreasing metamor- 

 phism attains its full development, we pass from the ' sillimanite *- 

 aureole ' to that characterized by the presence of kyanite and 



1 See • On an Intrusion of Muscovite-Biotite G-neiss in the S.E. Highlands 

 of Scotland * Quart. Journ. Geol. Soc. vol. xlix (1893) p. 332. 



