434 



EECINT WORK OE THE GEOLO&ICAL SURVEY 



divisional planes sericite has been abundantly developed, so that 

 the strata lose all their normal characters and merge into quartz - 

 schists. As a result of these changes, the quartzites have been 

 reduced to a third of their usual thickness. At the base of the 

 Stack of Glencoul and near Loch Ailsh, underneath the outcrop of 

 the Moine Thrust-plane, these new structures are strikingly dis- 

 played. At the latter locality other members of the Silurian series 

 share in the metamorphism. Advancing outwards from the Loch- 

 Ailsh road to Cnoc Chaoruinn, the Silurian zones, from the false- 

 bedded quartzites to the basal limestones, are repeated by thrusts at 

 intervals of a few yards. At first the various zones are quite 

 recognizable, the pipes in the quartzite being slightly bent over 

 and the Serpulite-grit yielding Serpulites after a careful search ; 

 but with each successive displacement their characters are gradually 

 modified, till it is impossible to distinguish them from some of the 

 members of the eastern schists. The false-bedded quartzites merge 

 into quartzose sericite-schists ; the " pipe-rock " passes into a fine 

 quartz-schist, in which the pipes are flattened like stfips of paper, 

 parallel with the foliation-surfaces ; the original lines of bedding of 

 the " Pucoid-beds " wholly disappear and are replaced by divisional 

 planes, coated with white mica ; the Serpulite-grit, no longer 

 j'ielding Serpuhtes, becomes a quartz-schist, and, finally, the lime- 

 stone becomes crj^stalline. On the new divisional planes numerous 

 fine parallel lines are met with, indicating the direction of move- 

 ment, trending generally E.S.E. ; indeed this " striping " is equally 

 apparent in the quartzites at the base of the Stack of Glencoul, at 

 the head of Glendubh, and other localities. 



4. Metamorphism of tlie Igneous Rocks intrusive in 

 the Cambrian and Silurian Strata. 



The evidence relating to regional metamorphism furnished by 

 the great series of intrusive sheets in Assynt likewise indicates 

 progressive alteration as we pass eastward to the Moine Thrust- 

 plane. In the undisturbed area to the west of the Post-Lower- 

 Silurian movements the igneous rocks of a granitoid type never show 

 the slightest trace of a foliated or banded arrangement. The 

 felsites, on the other hand, frequently show fluxion and sphe- 

 rulitic structures, where they traverse the old Archaean platform, 

 along the margins of the dykes. Passing eastwards to the displaced 

 Silurian zones underlying the Glencoul Thrust-plane, hardly any 

 change is observable in the sheets, except in those instances where 

 they have been driven along the " sole " of a major thrust. In the 

 latter case the diorites in the limestones have been slightly cleaved 

 and rendered schistose. 



Crossing the outcrop of the Glencoul Thrust to the slopes of 

 Coinne-mheall, we observe that some of the porphyritic felsites 

 show a flow-structure like that of the rhyolites, and that in one case 

 the rock has been completely reconstructed so as to become a fine- 

 grained schist. On the crest of Coinne-mheaU, just above the Ben- 



