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PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



[Feb. 6, 



to Tyndrum is very instructive. The dyke which has been already 

 noticed as trending in a W.NYW. direction from the head of Loch 

 Awe to the foot of Glen Orchy, runs along the northern side of 

 this glen for about five miles from Dalmally, beyond which we 

 failed to trace it. The schistose rocks, seen where the two roads join, 

 are succeeded by inferior beds of quartzose flagstones, often highly 

 micaceous. These gradually change their dip from S.W. to S., and 

 to S. by E. and S.E. The angle of inclination is gentle (15°-20°), 

 so that the same beds which occur at the top of the hillside on one 

 side of the glen are seen at a lower level on the other, the S.E. side 

 being steeper, since it is the escarpment, and the north-west side 

 more shelving, the slope often nearly coinciding with the dip of the 

 strata. As we ascend, the oblique direction of the glen brings us 

 across the strata in a gradually ascending progression until we arrive 

 again at the micaceous schists. These are seen in the streams around 

 Tyndrum, where they have a marked S.E. dip. 



From this section it is evident that the anticlinal arch of the 

 Breadalbane Forest sinks down towards the south below the schists, 

 which curve round the quartzose series in a wide semicircular sweep 

 that extends from the flanks of Cruachan by Dalmally to Tyndrum. 



The Black Mount to Tyndrum.- — The quartzose flagstones of Loch 

 Tulla ascending into the Ben-Do range have there a marked north- 

 westerly dip, with a gentle inclination of 10° or 15°. Ascending by 

 the road from Orchy Bridge we find the flaggy beds well exposed in 

 ravines and cuttings by the wayside. The north-westerly dip and 

 gentle angle continue until towards the watershed, when the beds 

 begin to incline to the north, then gradually to the north-east and 

 east, until, when the summit of the road is reached, they take a de- 

 cided south-easterly dip, and a higher angle than on the north- 

 western side. By this means the flaggy beds are repeated, and we 

 speedily pass into the schists that overlie them. 



The axis of the Ben-Do and Ben-na-chalader chain strikes to- 

 wards the north-east into the Brae Lyon mountains. It is, however, 

 only a minor anticlinal fold along the great arch of quartzose rocks, 

 which we have defined as ranging through the Breadalbane Forest 

 from Ben Cruachan to Glen Bannoch, beyond which it stretches 

 away into Atholl Forest and the heart of the Grampian Mountains. 



It remains now to describe the disposition of the schists as they 

 fold round the south-eastern side of this great arch. 



Tyndrum to Loch Tay. — From Tyndrum down Glen Dochart the 

 schists have a south-easterly dip at from 10° to 15°. They occa- 

 sionally become highly altered, however, with a more rapid dip. 

 About a mile N.W. from Crianlarich, limestone is quarried. It is a 

 hard, blue, ciystalline and fissile limestone with numerous green ser- 

 pentinous and talcose interlaminations, as at Loch Awe*. 



At Loch Dochart the schistose or gneissose strata are much gnarled 

 and tw isted, dipping in various directions from X.E. to S.E. The 

 whole of the valley of the Dochart is more or less obscured by drift, 



* See this limestone described by David Forbes, Quart. Journ. Gteol. Soc. 

 Tol. xi. p. 166. 



