336 DYKES IN OLDER PRIMARY ROCKS. 



High Pike, and a solitary red dyke ranges east and west of the granite 

 of Shap Fells. No porphyry occurs very far from the granites. 



North Wales. In North Wales felspathic porphyries and dolerites 

 are so connected by alternate bedding with the slates as to have been 

 subjected to the same elevations and undulations of dip ; and thus not 

 only prove their high antiquity, but also suggest views as to the fre- 

 quent recurrence of igneous action at the same points of the ancient 

 bed of the sea during the Cambrian period. Dykes are even more 

 abundant than interstratified igneous rock. For details reference may 

 be made to Sir A. Ramsay's " North Wales." 



Cornwall. We may believe that all the complicated rocks, wholly 

 or partially crystallised, composed of felspar, quartz, and mica, which 

 are included between the slaty rocks of Cornwall, and which traverse 

 them, are either the result of submarine eruptions during the forma 

 tion of the Devonian slate ; of the subsequent action of the heated 

 granitic masses upon the killas ; or are subsequent eruptions of melted 

 rock into fissures caused by convulsion, or result from some gradual 

 conversion and transfer of mineral ingredients. It is difficult to reason 

 on dyke phenomena so remarkable as those of Cornwall without refer- 

 ence to other districts. The student may consult the writings of J. 

 A. Phillips, F.R.S., on Cornwall, in the "Journal of the Geological 

 Society," already quoted, for details of their distribution. 



Fig. 69. Caldron Suout, Teesdade. A waterfall iu subprismatic basalt of the 

 " whiu sill." 



Basalt of Teesdale. The basaltic formation of Upper Teesdale 

 in Yorkshire was described by Professor Sedgwick, and its continua- 

 tion through Northumberland was traced by Mr. Button. The great 

 mass of basalt called the " whin sill " forms a layer of irregular thick- 

 ness, enclosed among strata of the Carboniferous limestone series, 

 generally on the same horizon, so that in the valley of the Tync 



