Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/84

 imbedded in a thin stratum of pale red hornstone or chert with particles of calcareous spar. Vide Sowerby, tab. 120.

Near the steep acclivity which terminates Cronkley Fell, another range of basalt interrupts the course of the Tees, and causes the cataract called the High or Mickle force, where the water is precipitated from the height of 56 feet. The rock which here crosses the river is apparently an overlying mass of coarse-grained grey basalt, the hornblende and the felspar which compose it not being intimately blended. It rests upon the lead-mine sills, and shoots on the banks of the Tees into regular columns of considerable magnitude and elevation. A few miles below this cascade, and about three above Middleton, perpendicular basaltic rocks again form the banks of the river. To these, iron chains have been fastened for supporting Winch bridge. This remarkable structure (if it can be so called) is a plank 2 feet in breath with low hand rails, suspended 56 feet above the Tees, which is here 63 feet wide. Some miners contrived it for the purpose of passing from the county of Durham to Holwick in Yorkshire.

In the fragments of basalt which are found scattered over the surface in Teesdale, and in other parts of the district, small grains of yellow olivine and of greenish black augite are found imbedded.

Leaving the mining field at Temming on the borders of Cumberland, and at Stagshaw bank near to Fallowfield the basalt appears to fill dykes; but in the range of hills between these places it seems to form overlying masses.

In such overlying masses it bounds the lake of Shewingshields, and the rock on its northern acclivity which is nearly perpendicular assumes a columnar shape. The Romans constructed their wall for many miles close to the edge of this natural rampart; it may be seen at this day standing 4 feet high in many places. In the