Geologically, most of the islands are the remnants of igneous intrusions. The Isle of May's rock is fine grained basalt of a dark-grey colour with tinges of green and greenstone. Fidra is also largely basalt and The Bass is a phonolite volcanic plug. Craigleith is a laccolith made up of essexite which is popular for making curling stones and Cramond island is made up of dolerite. Inchmickery and Inchgarvie are of igneous origin and the latter is partly made up of picrite. Studies of the landscape beneath the waters of the firth have revealed that the visible surface of Inchgarvie is only the top of a larger crag and tail structure similar in structure to Edinburgh Castle and the Royal Mile created by glacial action. The tidal islet of Eyebroughy is about to the west of Fidra. Its component rock is trachyte and Lower Carboniferous in origin.