316 ROCKS METAMORPHOSED BY BASALT. 



Contact Metamorphism by Basalt. The stratified rocks in 

 contact with the trap have undergone remarkable changes in several 

 localities. 



At Portrush, the lava, a rudely prismatic dolerite, overlies and 

 perhaps alternates with a flinty slate, which contains numerous 

 impressions of Ammonites, belonging to the Lias shales. This trans- 

 formation of Lias shale reminds us of the more extensive phenomena 

 of the same kind in Savoy. Most of the alterations of stratified rocks 

 on this coast are produced by basaltic dykes, which divide both the 

 overlying masses of trap and the subjacent strata. At the foot of the 

 hill called Lurgethan, basaltic dykes traverse the red sandstone con- 

 glomerate, which is altered near the contact so as to resemble compact 

 hornstone. 



The coal-measures, underlying the basalt of Fairhead, are crossed 

 by dykes which have changed the ordinary shale into flinty slate, 



Fig. 65. The Giant's Causeway. 



hardened and pyritised the sandstone for 1 5 yards, and converted the 

 coal to cinder. The chalk is traversed by many dykes, and is con- 

 verted into a real marble for 10 feet or more from the contact with 

 the basalt. 



The effects in approaching the contact are first a yellowish tinge of 

 colour, then a bluish-grey colour and compact texture, then a fine- 

 grained arenaceous aspect, next a saccharoid granulation, and finally, 

 close to the dyke, the chalk is altered to a dark -brown crystalline 

 limestone, with flaky crystals as large as those in limestone. The 

 flints in the altered chalk assume a grey-yellowish colour ; the altered 

 chalk is highly phosphorescent when headed. Examples are seen 

 near Belfast, at Glenarm, in Eathlin, and other places. Near the top 

 of the chalk which crowns the cliffs of Murloch Bay is an interposed 

 bed of wacke 5 or 6 feet thick. 1 



Volcanic Mud Streams of the Hebrides. Before the volcanic 



1 Conybeare, Trans. Geol. Soc., vol. iii. 



