Geological Features of the North-east of Ireland, 129 



this chain sends forth ; the two latter are feeders of the Bann, tlie 

 first empties itself into Lough Foyle. 



The geological constitution of this third system is highly important 

 and interesting, it is as has been already observed, wholly secondary 

 and uniformly covered by enormous stratified masses of basalt ; this 

 covering appears to acquire its greatest thickness on the north. The 

 basaltic cap of Benyavenagh, the most northern summit of the 

 western chain measuring more than 900 feet ; and that of Knock-lead, 

 similarly situated in the eastern chain 980 feet : the average depth 

 of this superstratum may therefore be safely estimated at 545 

 feet, and its superficial extent at 800 square miles, a solid mass of 

 extraordinary and imposing dimensions. 



In the strata underlying the basalt, the English geologist is 

 agreeably surprised to recognise many of the most important of those 

 formations, which reposing upon the coal measures, occupy such an 

 extensive tract in the south and eastern counties of his own island ; 

 but which' in Ireland are entirely confined to the comparatively 

 small district now under consideration, never extending far be- 

 yond the circumference of the great basaltic area ; a circumstance 

 which almost naturally leads to the conjecture that they may have 

 been originally much more extensive, but have been elsewhere 

 removed by the agency of some destroying and denuding force, 

 to which in this quarter alone an effectual resistance was opposed 

 by the firm and massive superstratum of basalt which covered 

 and protected them. 



The beds alluded to occur in the following order, proceeding 

 from beneath the basalt downwards. 



1. Chalk. — This formation, which in England cannot be estimated 

 at less than 800 feet in thickness, does not in Ireland average more 

 than 200. It agrees exactly with the lower beds of the English 



Vol. Ill R 



