The oldest rocks of Scotland are the Lewisian gneisses, which were formed in the Precambrian period, up to 3,000 Ma (million years ago). They are among the oldest rocks in the world. They form the basement to the west of the Moine Thrust on the mainland, in the Outer Hebrides 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. One of these intrusions forms the summit plateau of the mountain Roineabhal in Harris. The rock here is anorthosite, and is similar in composition to rocks found in the mountains of the Moon.