TABLELANDS 141 



broad arches and troughs which run in a general east 

 and west direction. In some cases, as in the Knock- 

 mealdown Mountain, the arch is composed entirely of 

 Old Red Sandstone flanked with Carboniferous strata. 

 But in most instances an underlying core of Silurian 

 rocks has been exposed along the centre of the arch. 

 As not only the Carboniferous Limestone, but the rest 

 of the Carboniferous system covered the south of 

 Ireland and participated in this plication, the amount 

 of denudation from these ridges has been enormous. 

 On the Galty range, for example, it can hardly have 

 been less but may have been more than 12,000 feet. 

 The third and latest group of Irish mountains is that 

 of Mourne and Carlingford, which may with some 

 probability be referred to older Tertiary time when 

 the similar granitic and porphyritic masses in Mull and 

 Skye were erupted. 



The Tablelands of Britain strictly include the moun- 

 tains, which are in general only prominences carved 

 out of tablelands. There are still, indeed, large areas in 

 which the plateau character is well shown. Of these 

 the most extensive and in many respects the most in- 

 teresting is the present tableland or plain of Central 

 Ireland. As now exposed, this region lies upon an 

 undulating eroded surface of Carboniferous Limestone. 

 But it was formerly covered by at least 3,000 or 4,000 

 feet more of Carboniferous strata, as can be shown 

 by the fragments that remain. 1 The present system 

 of drainage across the centre of Ireland took its origin 

 long before the ancient tableland had been reduced 



1 A striking instance of one of these fragments is to be seen on 

 the summit of Slieve League in Donegal. See p. 61. 



