200 PBOCEEDINGS OF THE GEOLOGICAL SOCIETY. 



those then entertained by the highest authorities in Scottish geology, 

 I have as yet seen no reason to recede from them, and shall now 

 bring forward some of the facts on which they are based. I must, 

 however, pass over several sections which, though I regard them as 

 favouring my views, I have not yet been able to examine thoroughly. 



17. Tyndrum and Black Mount. — The relation of this gneiss to 

 the mica-slate is well seen between Tyndrum and the Black Mount. 

 Near Killin, at the head of Loch Tay, the strata, principally mica- 

 slate, mixed with hornblende-slate and limestone (the latter, as 

 already stated, probably the continuation of the beds seen on Loch 

 Earn), dip at low angles (30°-40°) to N. 25° W. The same dips, 

 though more irregular, continue along Glen Dochart to the limestone 

 near Crianlarich, where there is probably a synclinal fold. From 

 this place to Tyndrum the general dip of the strata is to the S.E. 

 Bound Tyndrum the rocks are true mica-slate, intersected near the 

 mines by a vein of quartzose hornstone or felspar-porphyry running 

 about E. 30° JN". Following the road to the north-west, the mica- 

 slate is well seen, intermixed with a few beds of quartzite, and still 

 dipping at 30° or 40° to S. 40° E. A little beyond the village there 

 is a dyke of greenstone running about E. 10° S. At the top of the 

 ridge the dip becomes first easterly, and then gradually folds over to 

 the north and north-west*. In passing down the glen to Urchay 

 Bridge the rock changes to gneiss, and is seen distinctly resting on 

 the mica-slate on both sides of the valley. No true quartzite inter- 

 mediate between the gneiss and the mica-slate, as represented by 

 Dr. Macculloch, was seen in this line of section. On the hills to 

 the south-west great protruding veins of quartz occur, which Mr. 

 Thost informed me range for great distances across the country. 

 Beyond the Urchay, the gneiss continues by Inveroran and King's 

 House to the top of Glencoe, and, though often concealed in the low 

 moors, has its position well marked in the mountains on the west. 

 This section left no doubt on my mind that the gneiss forming the 

 great central region of the Black Mount overlies the mica-slate of 

 Tyndrum and Loch Tay, and thus, as stated, is a newer formationf. 



18. Glen Shee and Braemar. — The same relation of these two 

 formations appears to me to exist in the valley of the Tay and its 

 tributaries ; but as I have not been able recently to re-examine some 

 of the sections there, I shall pass to the valley of Glen Shee (fig. 9), 

 where the connexion of the mica-slate of the southern Grampians 

 to the gneiss of the central mountains is well seen. 



The greater part of this glen lies in the mica-slate, the beds 

 showing a general inclination to the north, though much concealed 



* Thus, my observations at intervals to beyond Urchay Bridge gave 30° to 

 E. 17° N. ; 22° to E. 27° N ; 37° to E. 37° N. ; 10° to E. 52° N. ; 20° to E. 

 77° N. ; and 30° to N. 27° W. 



f It is right, however, to mention that, in their " Sketch of a New Geol. Map of 

 Scotland," Sir B,. I. Murchison and Mr. Geikie make it an older formation. It 

 will be seen that the tract of country coloured by them as b' 1 -)- in this place 

 nearly corresponds with the outline of the gneiss and quartzite as drawn by Dr. 

 Macculloch and myself. The continuous limestone-zone, laid down in their 

 map as overlying the gneiss, was not observed by me. 



