This rock is very much like a fine-grained sandstone, and contains so large a portion of silex and clay as to be scarcely worth burning for manure. It would be called a bastard limestone. This bed appears to be one of the lowest in the series to which the metalliferous limestones belong; it occurs in the north-western part of the district, and is not uncommon in Roxburghshire, lying very near the red sandstone.
Porphyritic Formations of the Cheviot Hills.
A considerable tract of the north-west of Northumberland is occupied by the Cheviot hills, which rising from below the stratified country of the Lead-mine measures, stretch westward into Roxburghshire. The higher parts of these mountains being covered with peat moss, and their lower acclivities with alluvial soil, it is not easy to trace the exact line of separation between the porphyritic rocks, of which they consist, and the Lead-mine measures. It has been seen however that to the north the porphyritic rocks do not descend to the banks of the Tweed; to the south-west, limestone is quarried on the sides of Carter Fell, and a small colliery is worked at Kedderbum in the same neighbourhood. Towards the south, porphyry is seen on the banks of the Coquet at Linn-bridge, a mile and a half south of which, on the hill at Woolcoats, several coal pits are worked. For the other boundaries of this range I must refer to the map.
Cheviot, which gives its name to the whole group, is a huge round topped mountain, rising 2642[1] feet above the level of the
- ↑ Leslie's Elements of Geometry, 2d edit.
