172 Dr. Berger on the Geological Features 



The chalk Is frequently traversed by basaltic dykes, and often 

 undergoes a remarkable alteration near the point of contact ; where 

 this is the case the change sometimes extends eight or ten feet from 

 the wall of the dyke, being at that point greatest, and thence gra- 

 dually decreasing till it becomes evanescent. The extreme effect 

 presents a dark-brown crystalline limestone, the crystals running in 

 flakes as large as those of coarse primitive limestone ; the next state 

 is saccharine, then fine grained and arenaceous ; a compact variety 

 having a porcellaneous aspect and a bluish-grey colour succeeds : 

 this towards the outer edge becomes yellowish white, and insen- 

 sibly graduates into the unaltered chalk. The flints in the altered 

 chalk usually assume a grey yellowish colour ; the altered chalk 

 Is highly phosphorescent when subjected to heat. 



Examples of this conversion of the chalk into granular marble 

 may be seen on the east slope of Divis mountain, near Belfast, in a 

 ravine to which Dr. Macdonald has given the name of Allan's 

 Ravine, in honour of a mineralogical friend. 



They are again exhibited in the neighbourhood of Glenarm, 

 where a singular compound dyke, consisting of three branches, tra- 

 verses the chalk : the included masses of which are altered in the 

 manner above described. 



A dyke, very similar to the preceding, occurs in the isle of 

 Rathlin, near Church bay, and produces the same effect on the 

 chalk. This dyke appears to be resumed on the opposite point of 

 the Antrim coast at Kenbaan head, where the granular marble is 

 likewise found. (See section, plate 10. and the explanatory notes.) 

 A dyke near Ballintoy affects the chalk in the same manner. 



In the south-west extremity of Antrim the same marble is said to 

 occur near a whin dyke ; the spot assigned is Bamersglen, near 

 Trummery, about one mile north-east from Moira. 



