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DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



.surrounding volcanic rocks is obviously the same. On the west side of Belfast Lough a 

 boss of similar rock, about 1200 feet in diameter, rises at the very edge of the basalt 

 escarpment into the eminence known as Carnmony Hill (fig. 25). On its northern side 

 it presents along its wall a mass of interposed volcanic agglomerate.* Of the other 

 doleritic necks scattered over the surface of the Antrim plateau, I will refer to only one 

 which occurs on the hill slopes between Glenarm and Larne. It forms a prominence 

 known as the Scawt Hill, and. consists of a boss of basalt, which, in rising through a vent 



Fig. 25. —Section of Volcanic Vent at Carnmony Hill (E. Hull). T, Lower basalt ; C, Cretaceous beds ; 



L, Lower Lias ; M, Triassic marls. 



in the plateau-sheets, has carried, up with it and converted into marble a large mass of 

 chalk which is now exposed along its eastern wall (fig. 26). 



As examples of the similar necks which have been exposed by denudation outside the 

 present limits of the plateau, I may allude to those which rise through the Cretaceous 

 and other Secondary strata on the northern coast near Ballintoy. One of the most 

 striking of these may be seen at Bendoo, where a plug of basalt, measuring about 1400 

 feet in one diameter and 800 feet in another, rises through the Chalk, and alters it around 



Fig. 26. — Section of the east side of Scawt Hill, near Glenarm. a, bedded basalt ; b, mass of chalk ; c, basalt neck. 



the line of contact (fig. 27). Another remarkably picturesque example is to be seen near 

 Cushendall, where a prominent doleritic cone rises out of the platform of Old Red Sand- 

 stone, some distance to the north of the present edge of the volcanic escarpment. 



The greater coarseness of grain of the material filling these pipes, compared with that 

 of the sheets in the terraces, is only what the very different conditions of cooling and 

 consolidation would lead us to expect. There is no essential difference of composition 



* This neck was recognised by Du Noyer in 1868 as "one of the great pipes or feeders of the basaltic flows." 

 See Prof. Hull, Explanation of Sheets 21, 28, and 29, Geol. Survey of Ireland (1876), p. 30. 



