614 



BOCKS ALTEEED BY TEAP DIKES. 



[Ch. XXIX. 



till it becomes evanescent. " The extreme effect," says Dr. Berger, 

 "..presents a dark brown crystalline limestone, the crystals running in 

 flakes as large as those of coarse primitive (metamorphic) limestone 

 the next state is saccharine, then fine grained and arenaceous ; a com- 

 pact variety, having a porcellanous aspect and a bluish-gray color, 

 succeeds : this, towards the outer edge, becomes yellowish-white, and 

 insensibly graduates into the unaltered chalk. The flints in the 

 altered chalk usually assume a gray yellowish color." * All traces of 

 organic remains are effaced in that part of the limestone which is 

 most crystalline. 



The annexed drawing (fig. 684) represents three basaltic dikes 





Kg. 684. 









^ _____!_. 





-■— 



''" :;; " :; iiii iii' hi 



1 '/ " ill 



11 



|-.v. 



Chalk in-y'.x || 



l^i m i 



p 



\ - il 



Chalk 



v.:--:-V'i!|:;:l!i;|!:i l Mrjr i : }\ ■■/=/::::::•.:: :: \ \ \ \\ 



I.! 11 



k 





Dike 35 feet. Di 



1 foot. 



Dike 20 feet. 



Basaltic dikes in chalk in island of Eatklin, Antrim. 

 Ground plan, as seen on the beach. (Conybeare and Buckland.t) 



traversing the chalk, all within the distance of 90 feet. 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 contrast in the composition and 

 color of the intrusive and invaded rocks, in these cases, renders the 

 phenomena peculiarly clear and interesting. 



Another of the dikes of the northeast of Ireland has converted a 

 mass of red sandstone into hornstone. By another, the shale of the 

 coal-measures has been indurated, assuming the character of flinty 

 slate ; and in another place the slate-clay of the lias has been 

 changed into flinty slate, w T hich 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 greenstone dikes of 

 Antrim, on passing through a bed of coal, reduces it to a cinder for 

 the space of 9 feet on each side. 



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

 observed. Specimens taken at the distance of about 30 yards from 

 the trap are not distinguishable from ordinary pit-coal ; those nearer 

 the dike are like cinders, and have all the character of coke ; 



* Dr. Berger, Geol. Trans., First Series, vol. iii. p. 1^2. 



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



% Ibid., p. 213 ; and Playfair, Illust. of Hutt. Theory, s. 258. 



