Dolerite is a volcanic rock. While similar to basalt, it contains crystals which can be seen with a hand lens. This indicates that it cooled a little more slowly than basalt. Typically it is found in volcanic plugs which channeled the basalt to the surface. Erosion since Tertiary times, 60 million years ago, has exposed these plugs. Most famous perhaps is Slemish in County Antrim, but there are many of these extinct volcanoes. These include Tievebulliagh, Corkey Rocks, Scawt Hill, Ballygalley Head and Sandy Braes, all in County Antrim.
Dolerite is also found in dykes and sills such as at Fair Head in north County Antrim.
Much of the dolerite found in these places has a greenish tinge because of the presence of the mineral olivine.
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