small and remote islands. While Rona used to support a small population who grew grain and raised cattle, is an inhospitable rock. Thousands of northern gannets nest here, and by special arrangement some of their young, known as , are harvested annually by the men of Ness. The status of Rockall, which is to the west of North Uist and which the Island of Rockall Act 1972 decreed to be a part of the Western Isles, remains a matter of international dispute. Geology Most of the islands have a bedrock formed from Lewisian gneiss. These are amongst the oldest rocks in Europe, having been formed in the Precambrian period up to three billion years ago. In addition to the Outer Hebrides, they form basement deposits on the Scottish mainland west of the Moine Thrust and on the islands of Coll and Tiree. These rocks are largely igneous in origin, mixed with metamorphosed marble, quartzite and mica schist and intruded by later basaltic dykes and granite magma. The gneiss's delicate pink colours are exposed throughout the islands and it is sometimes referred to by geologists as "The Old Boy". Granite intrusions are found in the parish of Barvas in west Lewis, and another forms the summit plateau of the mountain Roineabhal in Harris. The granite here is anorthosite, and is similar in composition to rocks found in the mountains of the Moon. There are relatively small outcrops of Triassic sandstone at Broad Bay near Stornoway. The Shiant Islands and St Kilda are formed from much later tertiary basalt and basalt and gabbros respectively. The sandstone at Broad Bay was once thought to be Torridonian or Old Red Sandstone. Climate The Outer Hebrides have a cool temperate climate that is remarkably mild and steady for such a northerly latitude, due to the influence of the North Atlantic Current. The average temperature for the year is 6 °C (44 °F) in January and 14 °C (57 °F) in summer. The average annual rainfall in Lewis is and sunshine hours range from 1,100 to 1,200 per year. The summer days are relatively long and May to August is the driest period. Winds are a key feature of the climate and even in summer there are almost constant breezes. According to the writer W. H. Murray if a visitor asks an islander for a weather forecast "he will not, like a mainlander answer dry, wet