10 TOPOGRAPHY AND GEOLOGY. 



aie weathered out of it, forming superb features in the landscapes north 

 of Shgo. 



The whole of the limestone region is marked, as in other countries, by 

 the disappearance and reappearance of streams, which often run for long 

 distances underground, and by the prevalence of caves produced by solution 

 along these subterranean waterways. 



At Dungannon, south-west of Lough Neagh, in a country where the lime- 

 stone surface is more irregular than in the south, a 

 . patch of Coal Measures, containing ordinary house- 

 Tyrone Coal-tield. j^^j^ ^^^j^ |^^g ^^^ ^^^^ fortune been preserved. It is 



in part covered by later deposits, and forms an un- 

 expected region of mining industry, close to the moors of Tyrone and 

 Londonderry, where the Caledonian chains, and even still older ridges, 

 come to light. 



The Hercynian folding lifted the Carboniferous beds to a fatal height 

 upon these north-west highlands, and only outliers remain to show their 

 former extent. From Dungannon to Lough Foyle, however, a fairly con- 

 tmuous band of sandstones represents the shore deposits of the old Car- 

 boniferous sea. As already mentioned, coal occurs in these strata away to 

 the north at Ballycastle. 



It must not be supposed that the Coal Measures were removed during a 

 definite part of one geological period. Denudation, starting on the Her- 

 cynian chains, has been checked here and there for a time, and has then got 

 to work again on the old surface of attack. The sum-total of the vicissi- 

 tudes of the Irish region shows a large balance on the side of denudation. 



After the great uplift at the close of Carboniferous times, the Permian 

 sea flowed in upon the north, as it did over the corresponding English area. 

 Then the Triassic period set in ; and continental land, by a new swing 

 upward, spread away for some 900 miles to the south-east. On its surface, 

 deserts and shallow lakes occurred, the latter often drying up, and depositing 

 layers of gypsum and rock-salt. Thus the conditions 

 Rock-Salt of in Triassic Ireland were much hke those of L^tah at 

 Carrickfergus. the present day. The gypsum of Kingscourt and the 

 Belfast district, and the rock-salt now mined near Car- 

 rickfergus, show that deposits were laid down, comparable to those of 

 Cheshire. The conglomerates of the same period have given a name to 

 Red Bay, in Co. Antrim, where the red soil, when ploughed, reminds one of 

 eastern Devonshire. From Portadown to Magilligan Point at the entry of 

 Lough Foyle, these soft Triassic sandstones are traceable above the Car- 

 boniferous deposits. Yet they lie more often on the Carboniferous Lime- 

 stone and the Lower Carboniferous Sandstone than on the Coal Measures, 

 thus proving how far the denudation that accompanied the Hercynian up- 

 heaval had already stripped away the coal. 



Ireland was still destined to be denuded, rather than to be compensated 

 for her previous losses. The Rhaetic and Jurassic sea, which stretched in 

 Ligain from the south-east, met with a shore in the ancient hills of Donegal. 

 The downward dip of the area only allowed of the deposition of Liassic 

 strata ; while the continued subsidence in England, on the other hand, 

 produced the well-known oolitic limestones of Bath and Portland, which are 

 famous among building stones. The thin Irish representatives of the 

 Jurassic system, the Lias clays of Co. Antrim, have a curious effect upon 

 the landscape. Though little noticeable in themselves, they produce catas- 

 trophic landslips along the coast. The mass of chalk and basalt deposited 



