IN THE NORTH-WEST HIGHLANDS OF SCOTLAND. 391 



Fig. 4. — Overfolding of Micaceous Layers along Thrusi-planes in 

 Archcean Gneiss. 



The arrow indicates direction of movement. 



planes is usually of a yellow colour, probably from the weathering 

 of specks of iron-pyrites. The bands of partially reconstructed gneiss 

 sometimes possess the same tint, in particular those trending from 

 Loch na Seilge to the sea-coast near Tarbat (Loch Laxford), which 

 contain a large quantity of mica. The folia of the more micaceous 

 layers in these l9ands are most frequently arranged at an oblique 

 angle to the planes of schistosity, which seems to indicate that 

 the higher bands of gneiss had moved over the lower ones in a 

 westerly direction. A similar arrangement is met with near Loch 

 na Claise Pearn, where the more quartzose layers of gneiss have 

 been piled over each other ; but in this case the laminae are further 

 apart. These phenomena closely resemble the heaping up of the 

 bands of the eastern schists (Moine), to be referred to on a subse- 

 quent page. 



Another important structure resulting from these mechanical 

 movements is the development of incipient newer foliation at an 

 oblique angle to the older one, and rudely parallel with the adjacent 

 lines of movement. This newer schistosity occurs in various stages 

 of development, sometimes so indistinct that it is hardly observable 

 except when the rock-surface is carefully examined, sometimes so 

 well marked that it is quite as conspicuous as the older one ; while, 

 again, it may be carried a stage further, when the old foliation is 

 wholly destroyed. The effect of the attempt to establish a newer 

 foliation is to rearrange the constituents of the bands along new 

 planes more or less inclined to the old ones. 



Elack mica invariably accompanies this structure, and it occurs in 

 greater or less quantity according to the stage of development of 

 the second foliation. When the original folia form an alternation 

 of light- and dark-coloured layers, it is observable that the oblique 

 schistosity is most conspicuous in the dark bands; indeed, in some 

 cases the lighter quartzose parts seem hardly affected at all, even 

 where the adjoining layers, both above and below, show a marked 

 second foliation. 



This double foliation is frequently accompanied by intense 

 crumpling and rapid contortions of the bands of gneiss, representing 



