196 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



slate is a distinct formation from the clay- slate, separated from 

 it both by its lithological and physical characters and its geological 

 position and relations. 



Before leaving this vicinity, some notice must be taken of the 

 veins of trap which form a most striking feature in its geology. 

 Several of them are seen near Comrie, as in the bed of the Lednock 

 and on the road to the Melville Monument. In this line three, or, 

 probably, four of these dykes occur, generally consisting of black, 

 columnar basalt, decomposing in globular concretions. Their direc- 

 tion is E. and W. by compass, or a little to the south of east (E. 

 17° N". true), and they often show an inclination (60° to 90°) to the 

 north. Their breadth varies from 20 to 30 yards, but they run for 

 great distances nearly in the same direction ; some of those in the 

 Lednock, near Comrie, crossing the valley of the Earn two miles 

 further up, and then running into the Aberuchil Hills beyond. On 

 Loch Earn similar dykes, chiefly of grey greenstone, are common 

 with a parallel direction, and probably pass by Loch Rattachan 

 into the upper valley of the Lednoch, where similar E. and W. trap- 

 dykes again occur. We might be disposed to connect these dykes 

 with the earthquake-phenomena so common at one time near Com- 

 rie*, were it not that similar and nearly parallel dykes traverse the 

 whole chain of the southern Grampians from the east coast, near 

 Aberdeen, to the west coast of Bute and Can tyre. 



13. Dunkeld and Blairgowrie. — Further to the north-east, near 

 Birnam and Dunkeld, the clay- slate and greywacke also dip towards, 

 and apparently under, the mica-slate. It was this curious appearance 

 which induced me many years ago to state that "the strata seem 

 in some places to have been completely reversedf." I again drew 

 attention to this relation of the strata in 1858, observing, however, 

 that " The very distinct cleavage of the [clay-] slate, obscuring the 

 stratification and often mistaken for it, makes it very difficult in 

 many places to determine the true position of this rock"£. A similar 

 section is seen on the Ericht, above Blairgowrie, in the lower part 

 of Glen Shee. Here the Old Red Conglomerate, broken through by 

 trap-rocks, is beautifully exposed in the precipitous gorges in the 

 pleasure-grounds of Craighall. Further up, the river traverses the 

 clay-slate, quarried near Cally Bridge, and dipping north-west 

 towards the mica-slate. Probably a fault intervenes between the 

 two formations, but I have not examined the district so as to speak 

 with confidence. 



* A register of the time of occurrence and the direction of these earthquake- 

 shocks was kept by Mr. Patrick Macfarlane, of Comrie, whose kindness in show- 

 ing me the very ingenious apparatus for marking the intensity and direction of 

 their motion (some of it his own invention) I take this opportunity of acknow- 

 ledging. The fullest account of them is given in some valuable papers by Mr. 

 Milne-Home, in the Edin. New Phil. Journ. vol. xxxi., &c. Compare Greol. of 

 Scotland, pp.257, 258. They have recently become rare, or, rather, have ceased, to 

 the great relief of the natives, who found that they repelled strangers from visit- 

 ing one of the most picturesque localities on the outskirts of the Grampians. 



f Guide to Greol. of Scotland (1844), p. 169. 



X Note on Geol. Map of Scotland, p. 3. 



