m THE NORTH-WEST HIGHLANDS OP SCOTLAND. 435 



More Thrust-plane, the sheets of felsite injected along the bedding- 

 planes of the basal quartzites have been converted into soft sericite- 

 Bchists, which can be cut with a penknife ; and in Corrie Mhadaidh 

 a felsite dyke on the same horizon has been cleaved parallel with 

 the planes of schistosity in the Cambrian strata and with the plane 

 of the Ben-More Thrust. All these changes have been developed in 

 the dykes without much alteration in the quartzites in which they 

 occur. Again, on the north side of the Oykel valley, a dyke of 

 porphyritic felsite in the inverted Cambrian strata, above the Ben- 

 More Thrust-plane, has been converted into a mica-schist, showing 

 that peculiar "frilled" structure so marked in the green " frilled 

 schists " of Eriboll. 



Still further eastwards, in the belt of thrust and sheared Silurian 

 strata stretching southwards from the Stack of Glencoul by the 

 Gorm Lochs to Loch Ailsh and AUt Ealag, nearly all the dykes and 

 sheets are beautifully foliated, the planes of schistosity being parallel 

 with the planes of thrust. The fine-grained diorites in the lime- 

 stones are now represented by green hornblende-schists and chlorite- 

 schists ; the holocrystalline rocks with porphyritic felspars set in a 

 micro-crystalline base appear as bands of "augen-gneiss " and " au- 

 gen-schist " ; and finally, along a line of powerful thrust in the great 

 granitoid sheet east of Loch Borrolan there is a belt of" augen-gneiss " 

 with pyroxenes, which, existing originally as porphyritic crystals, 

 now appear as " eyes " in the foliated rock. Indeed, so striking 

 are the changes in these intrusive sheets close to the Moine Thrust- 

 plane, that it would be almost impossible to identify them, were 

 it not for the still recognizable zones in which they occur. Where 

 the latter lose their distinctive characters, bands of white quartz- 

 schist are then found, alternating with grey or green hornblende- 

 schist. 



Prom these various lines of evidence it is quite apparent that 

 there is progressive metamorphism on a grand scale as the observer 

 passes eastwards from the undisturbed western belt of ground to 

 the horizon of the Moine Thrust-plane. It is also obvious that the 

 crystalline rocks, where they occur in thin sheets, become schistose 

 much more readily than the clastic rocks, and that the Cambrian 

 sandstones and shales are more easily cleaved than the Silurian 

 quartzites. It is also probable that the great thickness of the slice 

 of Archaean rocks above the Glencoul thrust-plane, together with 

 the heterogeneous character of its materials, prevented the develop- 

 ment of new divisional planes in the thrust-gneiss, the deformation 

 showing itself mainly in the fracture and crushing of the crystals. 

 Not till we reach the point where powerful thrusts follow each 

 other in rapid succession, repeating thinner slices of the old Archaean 

 platform in the overlying quartzites, is the Post-Lower-Silurian 

 shearing strongly marked in the Archaean rocks. At length in the 

 zone of green schist and sheared gneiss underlying the Moine Thrust 

 each divisional plane or foliation-surface is a shear-plane developed 

 by these Post-Lower-Silurian movements. 



