124 



BASALTIC DIKES. 



low or deep red colour, and the chalk is highly 

 phosphorescent. 



Fig. 35. 



Ill, Chalk changed into marble. 2 2, Trap dikes. 



In Anglina, a dike 134 feet wide, consisting of a 

 rock composed of feldspar and augite, penetrates 

 strata of shale and argillaceous limestone, which it 

 cuts perpendicularly ; these are found changed to 

 a distance of thirty-five feet from the edge of the 

 dike. The shale, as it approaches the trap, be- 

 comes gradually more compact, and is most indu- 

 rated where nearest the junction. Here it becomes 

 a hard porcellaneous jasper. The limestone also 

 loses its earthy texture as it approaches the dike, 

 and becomes granular and crystalline. The altered 

 shale also contains crystals of garnet and analcime, 

 while they are found in no other 'portion of the rock. 



In the county of Antrim, also, chalk with flints 

 is traversed by basaltic dikes, and changed, for a 

 distance of ten feet from the dike, into granular 

 marble. 



In the same manner, red sandstone has been con- 

 verted into hornstone,* and soft slate clay changed 

 into flinty slate. f In Ireland, one of the greenstone 

 dikes of Antrim, on passing through a bed of coal, 

 reduces it to a cinder for the space of nine feet on 

 each side. The same fact has been noticed at 

 Cockfield Fell, in the north of England. Here spe- 

 cimens taken at a distance of thirty yards from the 

 dike resemble common pit-coal ; those nearer the 



* Geol. Transactions, first series, vol. iii., p. 201. 

 t Ibid,, 205. 



