76 



BEDDED TRAP IN CUMBERLAND AND WALES. 



But if analogous proofs of this nature be sought from other parts of the island, the 

 great basaltic dyke which ranges from a focus of volcanic action near the sources of the 

 Tees, to Robin Hood's Bay on the coast of Yorkshire 1 , intersecting all the formations 

 from the lowest coal-measures to the inferior oolite inclusive, affords the best example of 

 the impossibility of inferring the age of trap, from that of the sedimentary rock with which 

 it is found in intrusive contact. The observer whose researches did not extend beyond the 

 coal-measures, might assign the date of irruption to that formation ; while another exa- 

 mining the Yorkshire coast would prove that it was of an age posterior to the oolites. 

 Lastly, as if tending to exclude all limitation, the North of Ireland tells us that basaltic 

 and other trap rocks, even including syenite 9 , have been erupted through chalk, the very 

 youngest of our secondary formations ! Where then, we may ask, can the geologist 

 arrest his steps in deciding upon the age of many of these masses of eruptive trap ? 

 How can he venture to assert, that the very basalt which has flowed over the coal of 

 the Clee Hills, or pierced the coal of Staffordshire, was not emitted after the accumu- 

 lation of the most recent of our secondary rocks ? or that some of these eruptions may 

 not have accompanied those movements of elevation, by which even the youngest 

 tertiary deposits of our island were raised from the bed of the ocean ? 



Whilst, therefore, it is always difficult, and often impracticable, to define the age of 

 some of the intrusive trap rocks, the more gratifying is it to retrace in other masses of 

 the same class formed anterior to the accumulation of the Old Red and Carboniferous 

 Systems, the clearest proofs of intermittent and repeated volcanic emissions ; for, 

 although they have been poured out in a period of high antiquity, we are still able to 

 read off in them distinct analogies to Nature's present operations. The types of the 

 Silurian System and the associated volcanic rocks have remained so clear, that the 

 geologist has in them a record never to be mistaken, — one which will enable him to 

 descend from their horizon into those deeper-seated rocks, amid which Professor Sedg- 

 wick has detected analogous relations ; showing that in Cumberland and Wales there 

 are numberless bands of porphyry, interstratified with, and participating in all the 

 flexures of the slates, the whole of which have been subsequently pierced through by 

 other and intrusive masses of igneous origin. But although in these regions, the vol- 

 canic operations have been upon a grander scale than in the Silurian country, there is 

 not (I speak on the authority of my friend) , that clear demarcation between the pro- 

 ducts of fire and of water, to which I shall in subsequent chapters direct attention, the 

 whole mass of rocks being generally crystalline and altered (metamorphic) , and very 

 rarely presenting any traces of organic remains. And hence these regions are ill fitted 



1 See an excellent account of this basaltic dyke and the associated phenomena by Professor Sedgwick, Trans. 

 Phil. Soc. Cambridge, vol. ii. p. 139. 



2 Mr. Griffith discovered this phenomenon and announced it to the British Association at Dublin, 1835. 

 Subsequently Professor Sedgwick and myself visited the locality in company with Mr. G. See Geol. Proc, Phil. 

 Mag., vol. viii. N.S., p. 559. 



