The basement is often oceanic basalt. Continental islands, such as Madagascar, are composed of continental rocks, such as granite, and originate through the movement of continental basement attached to tectonic plates. Distance to the mainland is not a clear characteristic, because oceanic islands can arise at the borders of plates, get attched to one plate and get carried to the passive margin of a continent where it may overthrust the margin and be raised as an island near the coast. These coastal islands of oceanic origin are often characterized by occurring in arcs and having thick carbonate deposits. The fauna of far oceanic islands reflects their de novo origin - they ar eoccupied by chance migrants.
Continental islands carry the descendents of mainland populations.
Nearshore oceanic islands have a more complex mixture of historical isolates and more recent flora and fauna derived from a large continental pool. Oxford University Press, Oxford. Scientific American Library, W.H. Freeman and Company, New York. A Natural History of the Islands of the World. Natural History Press, New York.