IX THE NORTH-WEST HIGHLANDS OF SCOTLAND. 437 



were developed by movements after the Lower Silurian period. 

 They differ, however, from the mylonites underlying the Moine 

 Thrust-plane in one important feature, viz., that their matrix is 

 holocrystalliue. In other words, the formation of the Moine 

 schists, as Professor Lapworth has shown, has been attended by 

 greater molecular changes. Throughout the crystalline matrix 

 numerous " eyes " of felspars and quartz occur, belonging to the 

 original rock out of which the schists have been formed. The 

 holocrystalliue character of the Moine schists points in all proba- 

 bility to the conclusion that in their case the movements took place 

 at a more rapid rate, thus producing a higher temperature and 

 giving rise to greater chemical changes after the movements had 

 ceased. 



To the east of the Kyle of Tongue, the Moine flaggy schists 

 alternate with occasional bands of hornblende-schist as far as 

 Strathan, where they are succeeded by a belt of undoubted Archaean 

 rocks, two miles in width, stretching eastwards nearly to the river 

 Borgie. Consisting mainly of hornblendic gneiss with masses of 

 rudely foliated diorite and dykes of ultra-basic materials (perido- 

 tites, &c.), these rocks present many of the typical features of the 

 Archaean gneiss west of Durness. When followed southwards, this 

 belt thins away till it disappears to the north of Loch Creagach, 

 near Ben Loyal. 



To the south of Tongue another belt of Archaean rocks has under- 

 gone a great amount of deformation by the Post-Lower-Silurian 

 movements, the foliation-planes coinciding in direction with those 

 of the Moine schists. But here and there throughout the belt 

 patches of gneiss and pegmatite, showing the Pre-Cambrian foliation, 

 may be detected. 



Advancing eastwards to the Borgie Eiver, we find alternations of 

 flaggy Moine schist and gneiss, Archaean hornblendic gneiss striking 

 generally north and south and dipping to the east, overlain by a 

 peculiar type of gneiss in Strathnaver. Consisting mainly of 

 black micaceous gneiss in which the mica is very abundant, this 

 zone contains " eyes," " cores," or oval-shaped masses of diorite, 

 whereof the longer axes lie parallel with the strike of the foliation. 

 Round these " cores," both mica and hornblende curve in wavy 

 lines, the latter disappearing as they are followed outwards into 

 the well-foliated gneiss. There can be little doubt that these lenti- 

 cular masses, or " cores," are patches of a once continuous sheet of 

 igneous rock, out of which the Strathnaver gneiss has been formed. 

 A remarkable characteristic of this foliated mass is the development 

 in it of pegmatites, mainly along the lines of foliation. Beginning 

 as isolated knots of felspar, they gradually become continuous, 

 giving rise to thin strings or veins and eventually increasing in size 

 till they form bands a hundred yards across. The formation of 

 these pegmatites evidently formed a part of the process of metamor- 

 phism whereby the eruptive igneous mass was converted into a 

 micaceous gneiss. Prom the fact that bands of Moine schist or 

 flaggy micaceous gneiss are intercalated with the Naver gneiss, it is 



