74 Mr. N. J. Winch on the Geology of 



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 pre- 

 cipitated 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 mag- 

 nitude 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 Jemming on the borders of Cumber- 

 land, 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 



