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PERIODS OF ERUPTION OF THE BREIDDENS. 



parts of Shropshire. The mine of Snailbatch, it has been shown, lies upon a band of igneous eruption, 

 and these varied and altered rocks of Wotherton, Middleton, &c. only differ from it in containing 

 less of the more valuable ores of lead. 



The previous details have shown, that the Breidden Hills, like those of Corndon and 

 Shelve, present two classes of trap rock, the one contemporaneous with the formation 

 of the associated shale and sandstone, the other injected after their consolidation. 

 But at what period after the consolidation of the Silurian deposits were the chief masses 

 of the Breidden thrown up ? The evidences prove that the Silurian rocks had been 

 first elevated to some extent before the coal-measures were accumulated, while the latter 

 are so much dislocated as they approach these hills, that there can be no doubt that 

 they have been also affected, though at a subsequent period, by the outburst of volcanic 

 matter. For example, the extremity of the Shrewsbury coal-field is worked at Coed 

 Wae, within half-a-mile of the north-eastern termination of these trappean ridges, and 

 there the strata are so highly inclined that the workmen were obliged to board up the 

 drifts. The exact relations, however, of this, and all other parts of the contiguous coal- 

 field, extending in a narrowband to Pontesbury, are ill-developed, the junction between 

 them and the various members of the Silurian System on which they rest, being much 

 obscured by sloping accumulations of local detritus. But there are other examples of 

 high inclination, like this of Coed Wae, and also numerous faults, some of which have 

 been previously adverted to. Hence, although we have no positive demonstration 

 of trap in contact with coal, as in the cases of the Clee Hills and Coalbrook Dale, we 

 might, from such evidence alone, speculate with the highest probability on some of the 

 eruptions of the Breidden, like those of the Wrekin, having been posterior to the 

 formation of the coal-measures. Following up this inquiry, I will now proceed to 

 offer other independent proofs, that volcanic action has been continued along the same 

 line of eruption, into periods subsequent to the consolidation of the New Red Sand- 

 stone, and even of the Lias. 



Trap Dykes on the line of the Breiddens which have cut through the New Sand- 

 stone, explanatory of the period of certain lines of elevation and dislocation in 

 North Shropshire and Staffordshire. 



I have previously stated that there are few geological points more difficult to define 

 accurately than the age of trap rocks. They have frequently been described as of the 

 age of those strata with which they are found in contact. Such distinctions, however, 

 are altogether untenable, except where the volcanic rocks were actually evolved during 

 the accumulation of the sedimentary deposits with which they are associated; and 

 numerous instances of such contemporaneous traps have been already described. I 



