384 



C. CALLAWAY ON THE NEWER GNEISSIC 



end to end of the Brebag ridge. The gneiss of the two localities is 

 also the same formation. The Sconnan (Bealach Choinnich) section 

 displays the fault, which in the Balloch (Coniveall) is only inferen- 

 tial ; and the Balloch section reveals more fully the structure of the 

 overthrow . 



Section up Glen Coul (fig. 7, p. 391). — This important sec- 

 tion, already noticed, must now be more fully described. Sailing 

 up the sea-loch from E^lesku ferry, we pass a considerable thick- 

 ness of grey Hebridean gneiss. At first the beds are nearly 

 horizontal ; then they begin to dip at a low angle to the N.W., 

 and maintain this inclination up to about the middle of the 

 loch, where they are overlain by the Assynt series, dipping at 8° to 

 E.S.E. About half a mile further, a fault, seen in the precipices on 

 both sides, throws up the Assynt rocks, which had dipped down to 

 the sea-level, in broken but generally horizontal masses. Towards 

 the head of the loch the inclination suddenly rises, and curves right over 

 to the vertical. The crush at this point is such as I have rarely seen 

 equalled. Masses of quartzite, flags, and dolomite are thrown together 

 in inextricable confusion. Some beds, even of quartzite, are a mere 

 mass of breccia. The rocks are exposed, as in a model, on the shore 

 and in the islands ; but the clearness of the ground only the more 

 distinctly reveals the disorder. This locality is indicated in the 

 section ; but it would be impossible to represent the facts without 

 mapping every yard of ground. Dolomite beds are thrown between 

 flags and quartzite in such a way as to suggest true interbedding ; but 

 that they are mere faulted wedges is seen by following them along 

 the strike. In one place two bands in actual contact displayed 

 clearly a discordant strike. In another, masses of the same kind of 

 rock had been brought together at different angles and recemented. 

 At the east end of this crush, where the Hebridean overlies, the rocks 

 are less disturbed, the ordinary divisions of the Assynt series follow- 

 ing each other in curved beds dipping on the average at 60°. The 

 direction of dip varies between E.S.E. and S.S.E., owing to a bendiug- 

 round of the strike. At the junction on the shore, dolomite, from 10 

 to 15 feet thick, passes with apparent conformity below the Hebridean 

 (Logan Bock). The gneiss is chloritic, and so much decomposed and 

 jointed that a clean fracture could not be procured. The curvature 

 of strike is also seen in the gneiss, a fact which at first sight might 

 suggest conformity. 



At the back of the house the dolomite is much thicker, occupying 

 the breadth of a small field. Thickening out to such a degree within 

 a hundred yards or so is out of the question. So abrupt a change 

 at once suggests unconformity. 



Ascending the precipices which overhang the north side of the loch, 

 the most striking facts are disclosed. Bight along the cliffs, for more 

 than a mile to the west, at a height of about 100 yards, the old 

 gneiss (No. 75, p. 415) rests upon the Assynt series. The younger 

 rocks, as just described, lie at various angles ; but the gneiss dips 

 steadily to the N.E. Taking the two groups as a whole, the strike 



