INTRUSIVE BASALT OF TEESDALE. 



337 



its place in the section is constant, and it occupies generally the 

 same situation in Teesdale, though in Weardale another layer of 

 basalt occurs. We cannot doubt that its thickness at different places 

 was effected by their proximity to the eruptive channel. In the short 

 space of six miles, from Caldron Snout to Hilton Beck, its thickness 

 is diminished by 200 or 300 feet to 24 feet, and farther south it 

 disappears totally. But to the northward the range is interruptedly 

 continued to the sea-coast of Dunstanborough. 



No dykes pass from this mass in Teesdale into the rocks above 

 or below ; so that a first view of the case suggests the idea that it 

 was poured out as a mass of submarine lava upon the yet incomplete 

 deposit of the Carboniferous limestone. Professor Sedgwick, how- 

 ever, 1 maintained that it was injected from below amongst these 



Fiy. 70. High Force, Teesdale. A waterfall in columnar basalt of the " whin sill" 

 over limestone. 



strata, and that it penetrated between the planes of the strata by 

 violently separating them. 



The strata in contact are altered by the basalt in several ways, 

 as may be seen about the High Force. The subjacent shales have a 

 prismatic structure so as to be mistaken for basalt, are generally grey 

 or whitened, and rendered brittle by condensation, but not much 

 hardened. The sandstones are in several places highly hardened, 

 rendered brittle and full of fissures, and much whitened. The lime- 

 stones below the shale are remarkable for having their top bed 

 full of iron pyrites ; those above the basalt but not in contact with 

 it are frequently changed from a full blue, hard, rather crinoidal 

 limestone, first into a pale blue, crystallised, soft marble ; and finally 



1 Camb. Phil. Trans. 

 VOL. I. Y 



