Ch. XXIX.] EOCKS ALTERED BY TEAP DIKES. 48l 



Fig. G31. 



- — - 



i ' ';v/' 



' 1 '::.■}:. 



— ^- 



Chalk 



VI .^^"'.:% ->'' 



\ : . flp 



Chirik 





:-:^^ililiiiiiilliililliili:^ 



^Niiiliite 





Dike 35 feet. Dike Dike 20 ft 



1 foot. 

 Basaltic dikes in clialk in island of Eatlilin, Antrim. 

 Ground plan, as seen on the bcacli. (Conybeare and Buckland.*) 



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-meas- 

 ures 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, which still retains numerous impressions of ammonites. f 



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 cin- 

 ders, and have all the character of coke ; while those close to it are con- 

 verted into a substance resembling soot.| 



As examples might be multiplied without end, I shall merely select 

 one or two others, and then conclude. The rock of Stirling Castle is a 

 calcareous sandstone, fractured and forcibly displaced by a mass of green- 

 stone which has evidently invaded the strata in a melted state. The 

 sandstone has been indurated, and has assumed a texture approaching to 

 hornstone near the junction. In Arthur's Seat and Salisbury Crag, near 

 Edinburgh, a sandstone which comes in contact with greenstone is con- 

 verted into a jaspideous rock. 



The secondary sandstones in Skye are converted into solid quai-tz in 

 several places, where they come in contact with, veins or masses of trap ; 

 and a bed of quartz, says Dr. MacCulloch, found near a mass of trap, 

 among the coal strata of Fife, was in all probability a stratum of ordinary 

 sandstone, having been subsequently indurated and turned into quartzite 

 by the action of heat.§ 



But although strata in the neighborhood of dikes are thus altered ia 



* Geol. Trans. 1st series, vol. iii, p. 210 and plate 10. 

 f Ibid. p. 213; and Playfair, Illust. of Hutt. Theory, s. 263. 

 X Sedgwick, Camb. Trans, vol. ii. p. 37. 

 § Sjst. of Geol. vol. i. p. 206. 

 31 



