The island comprises a large central lowland of limestone with a relief of hills surrounded by a discontinuous border of coastal mountains which vary greatly in geological structure. The mountain ridges of the south are composed of old red sandstone separated by limestone river valleys. Granite predominates in the mountains of Galway, Mayo and Donegal in the west and north-west and in Counties Down and Wicklow on the east coast, while a basalt plateau covers much of the north-east of the country. The central plain, which is broken in places by low hills, is extensively covered with glacial deposits of clay and sand. It has considerable areas of bog and numerous lakes.
The island has seen at least two general glaciations and everywhere ice-smoothed rock, mountain lakes, glacial valleys and deposits of glacial sand, gravel and clay mark the passage of the ice.Influenced by the Gulf Stream and with the prevailing winds predominantly from the south-west, the climate is equable and temperatures are fairly uniform over the whole country. The coldest months are January and February with mean daily air temperatures of between 4 C and 7 C, while July and August are the warmest (14 C to 16 C).
May and June are the sunniest months, averaging 5 to 7 hours of sunshine per day. In low-lying areas average annual rainfall is mostly between 800 and 1200mm (31" to 47") but ranges from less than 750mm (30") in some eastern areas to 1500mm (59") in parts of the west. In mountainous areas annual rainfall may exceed 2000mm (79").
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