116 Mr Lyell on a Dike of Serpentine cutting' through 



to S. S. E. at an angle of about thirty-five, and, after extending 

 thirty-six yards, is suddenly cut off by the dike of serpentine. 



The side of the dike is composed, on each bank of the ri- 

 ver, of a very hard compact rock, about three yards thick, 

 which stands vertically and parallel to the serpentine, forming 

 a parting wall between it and the sandstone. 



This rock consists of equal parts of green serpentine, and 

 an indurated brick-coloured rock, harder than serpentine, and 

 sometimes passing into jasper. It is often very siliceous, and 

 resembles red argillaceous shale, as it is sometimes seen alter- 

 ed by the contact of a trap dike. Small specks of magnetic 

 iron are dispersed through parts of this rock. On the inner 

 side of this rock is seen some layers of contorted sandstone and 

 shale, which must evidently be considered as entangled in the 

 dike. The rock, which is in contact with the shale on the 

 other side, is unseen, owing to an interval of four yards of 

 deep soil, nor is the interruption supplied by the opposite side 

 of the river. Upon causing the ground to be dug up, the 

 rock, which had decomposed, appeared to have been serpentine. 



Next follows an olive-green serpentine, traversed by innu- 

 merable veins of fibrous asbestus. The serpentine, more to- 

 wards the centre of the dike, becomes dark green or blue ; the 

 surface has generally a glazed appearance, much resembling 

 the ordinary green serpentine of the Lizard district in Corn- 

 wall, but without a similar admixture of red serpentine. It 

 extends thirty or forty yards. In some places, irregular 

 roundish crystals of serpentine are imbedded in a softer ma- 

 trix of serpentine. There is much greenish bronze-coloured 

 diallage in the middle of the dike, in which part it precisely 

 resembles the green serpentine with diallage near Coverock. 

 in the Lizard district in Cornwall. Towards the eastern side 

 of the dike, a projecting mass of hypersthene rock, closely 

 resembling some specimens of thatwhich Dr MacCulloch found 

 at Loch Scarvy in Skye, is seen ; it does not interrupt the 

 serpentine, but appears inclosed in it. It is occasionally ac- 

 companied by some small fragments of talc. Next to the hy- 

 persthene, is a light olive-green serpentine, with specks of 

 magnetic iron in parts, which extends four or five yards to the 



edge of the dike, where sandstone and shale again appear. 



11 





