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PROCEEDINGS OE THE GEOLOGICAL SOCIETY. 



[Feb. 6, 



of the quartzose rocks, with here and there perhaps, where the un- 

 dulations are broader and deeper, an outlier or basin of the schistose 

 series. At the Bridge of Bah, between King's House and Inverouran, 

 the quartzose flagstones have a N.W. dip, and rise from under the 

 highly altered schists of Glen Coe already referred to. The dip va- 

 ries greatly even at short distances, but from this point towards the 

 south-east the north-westerly direction is on the whole maintained. 

 It is well seen at Loch Tulla, near the Marquis of Breadalbane's 

 Shooting Lodge of the Black Mount, and on the opposite mountains 

 of Ben Or, Ben Doran, Ben Do, Ben-na-chalader, and Ben Chreehan. 

 The south-easterly dip is resumed at the watershed, and eventually 

 towards Tyndrum we return again upon the schists. 



Before entering into the details of the eastern side of this great 

 anticlinal arch, we here give the results of an excursion by Glen 

 Orchy to Dalmally, and thence to Cruachan, whereby one of us 

 denned the southward limit of the arch, whilst the other explored, 

 for the third time, from the hospitable centre of the Black Mount. 



Black Mount by Glen Orchy to Loch Aive. — The quartzose flag- 

 stones of Loch Tulla are well seen in the channel of the stream 

 where the road crosses the Orchy. They dip N. 36° W. at 15°-25°. 

 They are micaceous, the mica being especially abundant along the 

 planes of bedding, wherelry a fissility is given to the rocks. About 

 a quarter of a mile below the bridge the same quartzose micaceous 

 strata turn round to due !N., but a short way down they resume their 

 dip to N. 26° W. at 20°-30°. Here they are well-marked flag- 

 stones, each flag being separated from its neighbours by plates of 

 silvery mica. As we descend the stream the character of the rock 

 continues the same for several miles, but the dip gradually veers 

 round to W., W. by S., and S.W., at the same gentle inclination : by 

 this change the series is slowly repeated ; and the beds eventually 

 becoming more and more micaceous, are, in fact, in many places true 

 mica-schist. At Achenafanich, near the bottom of Glen Orchy, a band 

 of white quartz-rock is seen descending obliquely the west side of 

 the valley with a south-westerly dip. It is clearly underlain by 

 some darker micaceous beds and covered by others of a similar 

 kind. At the bottom of the glen, where the two roads meat, schistose 

 micaceous strata dip W. by S. at 25° or 30°. They are traversed by 

 a dyke of augitic greenstone, which runs W. 10° N., rises along the 

 hillside to the north of the Orchy, and crosses the valley at the head 

 of Loch Awe near Stronmelch. In Professor Mcol's Map this dyke 

 is marked as having a N.ET.E. strike. A parallel ridgo to the south 

 may possibly mark the line of another dyke, but we did not examine 

 it. About a mile and a half before reaching Dalmally some flaggy 

 schistose beds are seen in a brook by the roadside, dipping S. by W. 



In the streamlet close to the inn of Dalmally, fissile mica-schist 

 dips S. 16° W. at 55°; and a short way further west, in an old quarry, 



interesting districts in the who'e range of Scottish geology." This granite extends 

 south-westwards into the shoulders of the mountains to the south of the King's 

 House, where it was pointed out to us by Lord Breadalbane in the old military 

 road of Marshal Wade, and where it is not marked in previous maps. 



