Geological Features of the North-east of Ireland. 125 



We have therefore strong grounds for believing, without incurring 

 the charge of generalizing too hastily, that the two mountain systems 

 described, should be regarded as prolongations of the great northern 

 and southern chains of Scotland, the former distinguished by the 

 prevalence of mica slate, the latter by that of grey wacke. 



In the eastern part of Tyrone which intervenes between these 

 two systems of mountainous ground, a coal formation occurs 

 associated with that variety of limestone which is usually found 

 underlying or alternating with the coal measures in Great Britain. 



The position of this coal field offers a new analogy with Scotland, 

 where the interval between the southern and northern mountains is 

 principally occupied by a broad zone of rocks connected with the 

 coal formation, upon which are placed those vast overlying masses 

 of trap formation which constitute the Campsie hill, the Ochills, &c. 

 and which, again, correspond both in their constitution and position, 

 with the basaltic group which forms our third system.* Before 



* The assertion that the basaltic group of Ireland corresponds in constitution and 

 position -with that of the Campsie hills, Ochills, &c. requires perhaps to be stated moie 

 distinctly, and more in detail ; and in the first place with regard to the correspondence 

 of these districts in the internal constitution of the component rocks. From the description 

 of the Ochills given by Mr. Mackenzie in the 2d volume of the Wernerian Transactions 

 it appears that the prevailing rock is a variety of floetz trap, possessing characters inter- 

 mediate between basalt and clinkstone, occasionally exhibiting columns, passing into 

 greenstone, and in places associated with amygdaloid. Beds of claystone porphyry, of 

 felspar porphyry and of compact felspar cap some of these eminences. The Campsie hills, 

 have been ably but more generally described by Col. Imrie in the same publication • 

 he represents them as universally covered by a thick mass of floetz trap, divided into beds 

 partly amorphous and partly columnar. The trap formation of the north of Ireland consists 

 principally of basalt divided into beds, partly amorphous and partly columnar ; the latter 

 variety resembles clinkstone in the property whence that rock derives its name"; the basalt 

 passes into greenstone, as may be seen in the mountain of Slievemish and other places 

 and it alternates with amygdaloid. A formation of clay porphyry also occurs in the center 

 of the basaltic area. 



In the second place, with regard to the position of the Scotch and the Irish trap districts,, 



