§ 26. STRATA ALTERED BY CONTACT WITH BASALT. &55 



presents the appearance of a pavement composed of 

 enormous angular blocks of stone ; whence has originated 

 the popular name of the Giants' Causeway. In the cliffs, 

 a natural cavern has been excavated by the inroads of the 

 waves, about sixty feet high, and of great picturesque 

 effect ; the entrance is nearly thirty feet in width, and the 

 walls are formed of dark basalt. 



But the great interest of this spot, in a geological point 

 of view, is the altered structure observable in the sedimen- 

 tary rocks wherever they are in contact with the basalt. 

 The Chalk in this part of Ireland, constitutes a line of cliffs 

 traversed by trap, which occurs in vertical dikes, and in 

 extensive beds, and has a columnar structure. 



12 4 3 2 5 2 1 



Lign. 193. — Trap Dikes traversing Chalk: in the Isle of Rathlin. 



1.1. Chalk. 



2. 2. 2. Chalk changed into granular marble from contact 



with the Trap-Dikes. 



3. A narrow Trap Dike or vein traversing altered Chalk. 



4. 5. Trap Dikes. 



The chalk strata have a total thickness of about 270 

 feet, and rest on a green sandstone, called mullattoe, 

 which is the equivalent of the firestone of the south-east of 

 England (ante, p. 296) ; it contains flint nodules, ammo- 

 nites, belemnites, echinites, terebratulse, and other usual 

 fossils of the cretaceous formation. 



In the Isle of Kathlin, nearly vertical dikes of basalt are 

 seen intersecting the chalk (as in this sketch, Lign. 193)^ 

 which at the line of contact, and to an extent of several feet 



