Ireland may be described as basin-shaped, since it consists of a central plain, more or less surrounded by a rim of mountains. In the north and south these mountains cover wide areas, but in the east and west they are more restricted and less continuous. In the north-west, where they follow the fold lines of the Scottish Highlands, they consist of crystalline and granitic rocks, while in the south-east of Ulster, where they were once continuous with the Southern Uplands, and in Leinster, where they formed part of the same mountain area as Wales, they are of Cambrian and Silurian rocks which have been pierced, however, by the large granitic masses that now give them their most characteristic features. The Antrim Plateau, in the north-east, is built up of layers of basalt. In the south and south-west, where the mountains have been folded along the same lines as those of south-western England, the ridges consist of Old Red Sandstone, while in the intervening valleys Carboniferous rocks appear. The low-lying Central Plain, formed by the denudation of the upper layers of the Carboniferous rocks which once covered the greater part of Ireland, is underlain by a floor of Carboniferous Limestone which, in fact, only comes to the surface in a few places, as it is generally concealed beneath a covering of glacial drift. Over the whole of Ireland, indeed, the drift is widespread, and, by obstructing the watercourses, has done much to aid in the formation of the bogs which are so characteristic a feature of Irish scenery.
CLIMATE.—The British Isles fall within the climatic area of North Western Europe, and lie in the belt of westerly and south-westerly winds, which modify alike the heat of summer and the cold of winter. In summer, the land is heated by the direct insolation of the sun, which is then north of the equator, and temperature decreases in a northerly direction. Owing to the cooling influence of the westerly winds blowing from the ocean, however, Ireland has at this season of the year a temperature about 2°F. lower than that part of Great Britain which lies within the same parallels of latitude ; while places on the west coast of Great Britain have a somewhat lower temperature than corresponding places on the east coast. In July, the warmest month of the year, the isotherms range from 63° F. in south-eastern England to 55° F.
in the north of Scotland. In winter, on the other hand, when the sun is south of the equator, the British Isles receive the greater part of their warmth not from it directly, but from the westerly and south-westerly winds, which reach them from the Atlantic. Accordingly, the western coasts, which are most exposed to these winds, receive the greatest benefit from them, and temperature decreases, not from south to north, but from west to east. At this season of the year, therefore, Ireland has a mean temperature several degrees higher than that of corresponding parts of Great Britain, and the west coasts of both islands are warmer than the east coasts. In January, the coldest month of the year, the mean sea-level temperature ranges from 44° F. in the south-west of Ireland to 38° F. in the south-east of England, while the isotherm of 40° F. runs from Cape Wrath to the Isle of Wight. The upland regions are, of course, colder at all seasons of the year than the sea-level temperatures indicate. On the whole, however, it may be said that in the British Isles the summers are warm and the winters cool.
The western parts of the British Isles have generally a heavier precipitation than the eastern parts. This is due in the first place to their more exposed position in relation to the winds blow ing from the Atlantic, which is the great source of moisture ; and in the second place to the presence of mountain masses, which force the winds upwards, so that they are cooled, and the moisture, which they carry, condensed. The eastern parts of the country, on the other hand, lie in the rain shadow cast by the mountains, and have, therefore, a much lower rainfall. Generally speaking, it may be said that on the mountainous districts of Ireland and of the west of Great Britain there is a mean annual rainfall of at least 40 inches, while on the lowlands, in both countries, there is less than that amount. But parts of the Western Highlands, of the Lake District, and of Wales, have as much as 60, 80, and even 100 inches. The greater part of the Irish plain, and the Scottish Lowlands, have between 30 and 40 inches ; the Central Plain of England, and much of the east coast of Great Britain, have between 25 and 30 inches, while the eastern counties of England have less than 25 inches.