Vol. 60. J 0E THE EAST-CENTRA.L HIGHLANDS. 411 



visible, as compared with those already described ; and, in addition, 

 small outcrops of other members of the succession are met with, by 

 the aid of which the position of the parallel-banded gneisses can be 

 fixed. A distinct, though slight, change in composition is shown 

 by the increase of biotite in the rocks as a whole ; and this is 

 accompanied by the development of sillimanite in some of the more 

 micaceous thin partings, thereby fixing the phase of crystallization. 

 Obviously, the original material had become more muddy on the 

 whole, and less of a fine arkose or sand, though the latter character 

 was still retained in one part of the group. 



Two types of the gneiss in this area are worthy of special notice. 

 The first weathers with rounded outlines, is of somewhat massive 

 aspect, and resembles a fine granite. It forms the long, rounded 

 ridge stretching from An Sgarsoch, at the county-boundary, in a 

 southerly direction to Sron na Macranach, on the north side of the 

 Tarf. Considerable masses of similar rock occur on Cairn Fidhleir, 

 farther west. This tract lies in a line with the other outcrops of 

 the massive, round-weathering gneiss already mentioned, of which 

 it seems to be a slight modification. Sections of these rocks show 

 that they contain singularly-little microcline and an unusual amount 

 of plagioclase (mostly oligoclase), often fringed with vermicular 

 pegmatite, and at times partly idiomorphic with respect to the quartz 

 (11,059). J In both localities, the round-weathering type of gneiss 

 is succeeded by a singularly-flaggy phase, in which microcline is 

 abundant and parallel structures are well marked : thus strongly 

 suggesting that the rock so rich in plagioclase marks a distinct 

 horizon. As in the Struan area, thin bands of somewhat similar 

 material occur to the south-east : and in these the plagioclase has 

 frequentty much vermicular pegmatite on its margin, forming 

 clubbed ends to the narrow crystals (11,066). 



The second type of gneiss of special importance in this area is 

 sharply separated from the normal phases hitherto described by the 

 occurrence within the individual bands of a lenticular or ' thrust- 

 plane' structure, similar to that met with in the grits of the Southern- 

 Highland border, and due to mechanical deformation. A typical 

 specimen (11,076) is a grey gneiss with thrust-plane structure, in 

 which the movement-planes are coated with dark mica. It is 

 composed of abundant oligoclase and quartz occurring together in 

 lenticles, separated by films rich in reddish-brown mica. These 

 films alternately approach and recede from each other, and show 

 the typical undulatory parallelism of a true schist of the Southern- 

 Highland type. The occurrence of this structure is highly im- 

 portant, as it shows that when the material of the Moine Gneiss 

 was strongly affected by dynamic action, the rocks crystallized as 

 typical lenticular or phacoidal schists. It is a fair inference, that 

 the persistent absence of any such structure from the tj'pical grey 

 gneisses is conclusive evidence that they suffered practically no 

 mechanical deformation prior to crystallization within the individual 



1 See PI. XXXV, lig. :> (No. 107). 



