300 JOUENAL OF THE! EOYAL HORTICULTURAL SOCIETY. 



slight elevation would drain the water off the submerged lands which 

 form the Irish Sea and the German Ocean, and restore a continuous 

 continental edge extending from the Pyrenees to the West Coast 

 of Ireland and thence northward towards Scandinavia. We have to 

 deal, then, with a continental shelf, in connection with which, the 

 geologists tell us, there is abundant evidence of fluctuations of level in 

 past ages. 



To go back to a very distant epoch, we find that at the close of the 

 Silurian period the earth's crust along the continental edge was sub- 

 jected to severe pressure from the north-west and south-east, which 

 threw the surface into a series of great folds running at right angles 

 to the direction of thrust — namely, north-east and south-west. This 

 folding formed the great mountain ranges and valleys of Scotland which 

 still give to that country its character ; and in the West of Ireland the 

 county of Donegal was similarly crumpled, as were portions of Mayo 

 and Galway; and far to the northward Scandinavia owes its present 

 contour to the same mighty folding. On the land thus uplifted exten- 

 sive lakes formed later on, and during the succeeding Devonian period 

 a great thickness of sandstones and slates w^as laid down on the lake 

 bottoms. The land sank at length, the sea flowed again over much of the 

 British Isles ; and during the Carboniferous period which succeeded, 

 the greater part of Ireland was thickly covered with a vast series of 

 limestones and shales laid down on a surface now deeply submerged 

 beneath the ocean. Again the land rose — we are now passing by millions 

 of years in as many seconds — and once more a period of severe earth- 

 pressure set in, folding not only the old rocks already distorted by the 

 former period of pressure, but also, in a notable degree, the Devonian 

 and Carboniferous rocks that now overlay them. The result is seen on 

 a glorious scale in the south-west of Ireland, in the vast ribs of slate 

 and sandstone rocks that form the mountain ranges and promontories 

 of Kerry and Cork, and the deep corresponding valleys where alone 

 are left traces of the Carboniferous limestone which formerly covered 

 the country. Thus, then, are the three great mountain districts of 

 western Ireland accounted for, those of Donegal and of Mayo-Galway 

 owing their origin to the earlier, and that of Kerry-Cork to the later 

 period of crumpling. In the spaces between these three great buttresses 

 of ancient uplifted rock, the Carboniferous beds, more destructible, 

 reposed in a more or less undisturbed condition; and the Atlantic, beat- 

 ing incessantly against the western coast, and the rains dissolving the 

 limestone have together worn down and eaten out these softer rocks, 

 so that the country covered by them now presents green plains and 

 deep sea-inlets, contrasting with the heathery heights and bold promon- 

 tories of the folded areas of harder rocks. 



We are in a position now to turn to the botany of this region. We 

 see that ancient geological changes have provided for the plants an area 

 in which tracts largely covered with limestone, and mostly low in 

 elevation, alternate with mountainous tracts of rocks other than lime- 

 stone, two main types of plant-habitat being thus provided; and we 



