internal Structure of the Magnesian Limestone. 63 



from examples in the north of England, in the north of Ireland, in the 

 Hebrides, and in many other parts of the world, that trap dykes are not con- 

 fined to the carboniferous order. They are found in very great abundance 

 in many deposits of a much newer epoch. Hence, many of the trap dykes in 

 our coal-fields may belong to a comparatively recent age ; and the only ex- 

 amples of any direct value in proving the first conclusion (viz. that such dykes 

 do not pass up into the overlying beds), are those in which the trap cuts 

 through portions of the coal strata in immediate contact with the magnesian 

 limestone. 



In Nottinghamshire, Derbyshire, and Yorkshire, I never found a single 

 example of a trap dyke near the great overlying terrace. In the counties of 

 Durham and Northumberland (notwithstanding the common occurrence of 

 trap in the coal formation) there are but two examples of dykes which bear 

 upon the present question ; and they appear to lead to opposite conclusions. 

 1 , The well known dyke which descends from Bolam to Houghton-le-Side, 

 comes to the eastern edge of the magnesian limestone, and is lost under the 

 alluvial and diluvial covering. But beyond the eastern boundary of the for- 

 mation another dyke, agreeing both in its direction, inclination, and minera- 

 logical characters with the former, breaks out from beneath the diluvium, and 

 ranges without interruption into the moors south of Whitby. If these two 

 trap dykes form one continuous mass (which is at least very probable), they 

 must undoubtedly cut through the magnesian limestone. 2. The next ex- 

 ample is at Quarrington Hill (three or four miles south-east of Durham), 

 where a trap dyke rises almost perpendicularly through the carboniferous 

 beds, but does not penetrate the capping of limestone. It is therefore pro- 

 bable, though by no means certain, that this dyke assumed its present form 

 before the limestone was deposited*. 



The trap dyke at the south-west end of Tynemouth Castle cliff is unfortu- 

 nately of no assistance to this inquiry ; because the capping of yellow lime- 

 stone does not extend to that extremity of the cliff where the dyke is present. 



Such is the imperfect evidence, or rather such is the absence of all direct 

 evidence in favour of the conclusion, that the trap dykes in our nortiiern coal- 

 fields belong to an age which is anterior to the deposition of the magnesian 

 limestone. That in some instances there may be probable reasons in favour 

 of this conclusion I do not pretend to deny ; but this is not the proper occa- 

 sion for discussing them. 



* For a detailed account of the two trap dykes mentioned above, see Transactions of the 

 Cambridge Phil. Soc. vol. ii. pp. 21, 40. 



