108 LYELL'S ELEMENTS OF GEOLOGY. 



Rocks altered by Trap Dikes. 



having a porcellanous aspect and a bluish-grey colour, suc- 

 ceeds : this, towards the outer edge, becomes yellowish-white, 

 and insensibly graduates into the unaltered chalk. The flints in 

 the altered chalk usually assume a grey yellowish colour." * All 

 traces of organic remains are effaced in that part of the lime- 

 stone which is most crystalline. 



The annexed drawing (Fig. 96.) represents three basaltic 

 dikes traversing the chalk, all within the distance of ninety feet. 



Fig. 96. 



Clialk 



Dike 35 ft. Dike Dike 20 ft. 



1 foot. 



Basaltic dikes in chalk in island of Rathlin, Antrim. Ground plan as seen on the 

 bead). (Conybeare and Buckland.)f 



The chalk contiguous to the two outer dikes is converted into a 

 finely granular marble, m m, as are the whole of the masses 

 between the outer dikes and the central one. The entire con- 

 trast in the composition and colour of the intrusive and invaded 

 rocks, in these cases, renders the phenomena peculiarly clear 

 and interesting. 



Another of the dikes of the north-east of Ireland has con- 

 verted a mass of red sandstone into hornstone4 By another, 

 the slate elay of the coal measures has been indurated, and has 

 assumed the character of flinty slate ; and in another place the 

 slate clay of the lias has been changed into flinty slate, which 

 still retains numerous impressions of ammonites.|| 



It might have been anticipated that beds of coal would, from 

 their combustible nature, be affected in an extraordinary degree 

 by the contact of melted rock. Accordingly, one of the green- 

 stone dikes of Antrim, on passing through a bed of coal, re- 

 duces it to a cinder for the space of nine feet on each side.lT 



At Cockfield Fell, in the north of England, a similar change 

 is observed. Specimens taken at the distance of about thirty 

 yards from the trap are not distinguishable from ordinary pit 

 coal ; those nearer the dike are like cinders, and have all the 

 * . . . 



* Dr. Berger, Geol. Trans., First Series, vol. iii. p. 172. 



t Geol. Trans., First Series, vol. iii. p. 210. and plate 10. 



t Ibid. p. 201. $ Ibid. p. 205. 



II Ibid. p. 213. ; and Playfair, Illust of Hutt Theory, s. 253. T Ibid. p. 206. 



