xvi INTEODUCTION 



bold coast-line, has a very different origin, being, comparatively, a 

 creation of yesterday, and built up by the outpourings of Eocene 

 volcanoes. The recent inroads of the sea upon this lava-plateau 

 have produced the bold coast scenery of Antrim and Derry. The 

 great Central Plain, of Carboniferous limestone, touches the sea 

 only at intervals, and the contact of these low-lying and more 

 soluble rocks with the ocean often corresponds with deep inden- 

 tations of the coast, as is noticeable in Donegal Bay, Clew Bay, 

 and Galway Bay. 



Mesozoic rocks, which play so important a part in the building- 

 up of England, are practically absent from Ireland, and the Tertiary 

 sedimentary rocks, with their crumbling clays, light soils, and 

 gravelly wastes, are likewise conspicuous by their absence. The 

 only post-Carboniferous deposits which can be held to in any way 

 affect the flora are those of the Pleistocene period, and these are of 

 great importance. A vast sheet of Boulder-clay, often of consider- 

 able thickness, is spread over the greater part of the country, except 

 on the high grounds. In the Central Plain, this deposit is composed 

 almost entirely of the debris of the limestone, and forms a tough 

 blue clay full of pebbles and blocks of that rock. The soil which 

 results from its decay is highly calcareous. The eskers, too, which 

 raise then' sinuous green ridges all over the Central Plain, consist of 

 limestone gravel. Around the flanks of the mountain -groups the 

 drift is mostly coarse, often sandy, and is composed of the waste of 

 the rocks which occupy the adjoining high grounds. In this way 

 the surface geology may enthely upset any deductions as to soil 

 and flora which might be drawn from the solid geology of an area. 

 For instance, the limestone valley of the Blackwater is so choked 

 with non-calcareous drift from the Old Eed Sandstone hills on either 

 side, that hardly one of the characteristic plants of the limestone is 

 to be found there. In North Down, which is formed of grey 

 Silurian slates and grits, the Boulder-clay is calcareous, bright 

 red in colour, and full of blocks of basalt, the result' of glacial 

 denudation of the Chalk, New Red Sandstone, and Eocene lavas of 

 the adjoining hills of Antrim. Or the surface deposits of higher 

 grounds may actually represent the denudation of the rocks which 

 now occupy the plain below, as on the northern slopes of the Dublin 

 mountains, where the granite glens are choked with limestone 



