-J- 14 MR. G. BAEEOW OX THE MOINE GNEISSES [Nov. I9O4, 



aspect, and possesses the typical granular structure of a Moine 

 Gneiss ; it is composed of both potash- and plagioclase-felspar, which 

 together exceed the quartz in amount. Biotite is fairly abun- 

 dant, and ejndote occurs along certain lines. This is the typical 

 pink-edged epidotic gneiss, which occurs over a wide area. Still 

 farther west, both angular and round-weathering grey bands 

 appear ; but the pink-edged rocks still predominate, always con- 

 taining a considerable amount of potash-felspar, the latter often 

 fringed with vermicular pegmatite. Closer to the head of the 

 Sluggan Burn, infolds of more highly-quartzose rock are met with, 

 and increase in size, until the large mass separated-out on the map 

 is reached, which is once more, in the main, the Central-Highland 

 Quartzite, with the Moine-Gneiss structure superimposed on it. 



On the south side of the Dee, to the east of Braemar, the old 

 difficulty recurs in separating the true Quartzite from the highly- 

 quartzose Moine Gneiss; but a faint remnant of the typical, 

 parallel-banded, grey material is seen in the little quarry, close 

 to the gate of the footpath that passes on the south side of Creag 

 Choinnich. 



One of the few cases in which the Quartzite in a ' Moine-phase ' 

 can be separated from the quartzose gneiss is met with on the hill- 

 top above Balloch Farni, some 2 miles north-east of Invercauld ; 

 the latter being seen practically in contact with the white margin 

 of the former. The best locality for studying this is between the 

 limestone and the small mass of diorite and granite, 1 farther west. 



Summary. 



This account of the Moine Gneisses may be summed up brietiy as 

 follows : — 



1. These gneisses are a parallel-banded series of sedimentary 

 origin, usually rich in felspar (largely microcline), and con- 

 taining dark biotite in variable quantity. 



"2. The gneisses are thinly-bedded, as a whole ; and their structure 

 is essentially parallel, but not lenticular or phacoidal. This 

 parallel structure is in most cases shown by the arrangement 

 of the biotite. 



3. Certain types can be recognized again and again throughout 



the whole area ; and their repeated occurrence shows that 

 the whole series is really thin, although by intense folding 

 it simulates a succession of enormous thickness. 



4. Although the variation in the typical grey gneisses, as they 



are traced eastward, is not great, still it is important. 

 Biotite is, on the whole, more abundant ; and the highly- 

 micaceous partings become more aluminous, that is, were 

 more of the nature of fine mud originally. 



5. A striking feature of the grey gneisses is seen in the films 



of felted biotite, derived from original clastic chlorite, and 



1 The position of these rocks is shown on the Geological-Survey 1-inch map, 

 Sheet 65, to be published shortly. 



