Sandstone in the County uf'Forfur. \9S 



two dikes of greenstone which cross the Erroch, both not very 

 far from the line which the serpentine, if continued from Bamff 

 to Clunie, would take. One of these dikes is rather more 

 than a mile above Blairgowrie. Its thickness is about twenty 

 feet, and the strata of sandstone and shale, in contact with it, 

 are much indurated. The other dike traverses the conglome- 

 rate several hundred yards below Craig-Hall. 



Whether there exists any connection between the serpentine 

 of Clunie and that on the Carity is uncertain ; but it can 

 scarcely be questioned bv any who consider the facts above 

 stated, that the same dike of serpentine is found recurring at 

 intervals for the space of at least fourteen miles or more, 

 stretching in a straight line from Cortachie to Bamff, its direc- 

 tion being parallel to the Grampians, and to the outcrop of 

 the several formations which succeed each other in regular or- 

 der in Forfarshire and Perthshire, from the granite to the old 

 red sandstone. But, notwithstanding this general conformity 

 of direction, the serpentine, wherever it is seen in contact with 

 the stratified rocks, intersects them, and is therefore not of 

 contemporaneous origin, but, like the greenstone with which 

 it is connected, of posterior date. 



Before concluding this communication, I shall take the op- 

 portunity of briefly mentioning an instance of the connection 

 of serpentine, greenstone, and sienite, which I examined in the 

 Lizard district in Cornwall, during the last summer, in company 

 with Monsieur Constandt Prevost, a gentleman well known to 

 geologists by his memoirs on several countries on the Continent. 

 This occurs at Cadgwith, near Coverack, and is slightly al- 

 luded to by Professor Sedgwick.* The serpentine there is 



the great conglomerate, as well as the superior red sandstone of Forfarshire 

 fragments of the older shale and micaceous sandstone. But although the 

 evidence of the partial destruction of those heds is thus decisive, yet, with 

 the 6ingle exception above stated, I have never found them entirely want» 

 ing to the eastward, along the foot of the Grampians, nor in the neigh- 

 bouring Seedlay Hills, where they are more fully developed, and where, 

 with their accompanying trap rocks, they almost exclusively compose that 

 chain. When the conglomerate consists exclusively, or nearly so, of trap 

 pebbles, as at Craig-Hall, and many other places, we must suppose that, 

 at the epoch of its formation, the inferior sandstone was, in those particu- 

 lar spots, entirely overlaid by trap. 



" Trans. Catnb. Phil. Society, vol. i. p. 3l<>. 



