The word 'terrane' is therefore a deliberate corruption of the common word 'terrain', which refers only to a land surface. Its scientific meaning refers to a block of continental material from its surface right down to its base just above the mantle. An 'exotic terrane' extends the concept to a block of material that has broken away from a continent, and moved until it collides and fuses somewhere else, a process sometimes referred to as 'docking'. where its presence can give rise to an apparently confused geology over a fairly small area.

Not all terranes are small, of course. India is a large example, and whilst on passage from Gondwanaland near the South Pole to its present location docked on to southern Asia, it must have been isolated like present day Australia, which is probably an undocked terrane destined to join onto another continent in the far distant future. Greenland and Cyprus may also be examples, with the Seychelles as further candidates as they are made of continental materials which raise special problems in their present location.

The idea of terranes was first developed in the United States, where the concept made an immediate impact on the understanding of the geology of western coastal regions. Indeed even the inexplicable geology of Alaska has lately succumbed to investigation. Alaska has been found to be "a collage of terranes dismembered and repositioned over the past 160 million years by the wanderings and collisions of crustal plates, the flotsam of the ancient, vanished ocean that preceded the Pacific". (This is a direct quotation from the paper also titled "Terranes" by David G Howell)

Locally, too. the idea seems to be fruitful. There are several areas of this country where geology appears to change too suddenly for ordinary explanations, or where other anomalies exist such as traces of volcanic rock 'with no apparent volcanoes, or ophiolites where rocks not far away show no sign of abnormality. Suspicions are arising that this country, in common with other areas fringing the continental basement. contains many exotic terranes. some of which docked a long time ago and therefore share much of their geological history with the region on which they docked, but some more recently so that they share at most only a thin veneer of local sediments. Students of Geology may well have seen and puzzled over such features in Scotland, North Wales, Anglesey and Cornwall, during field trips. But the thought may now be entertained that adjacent areas may possibly have had a different geological history as part of a different territory or even a different continent.

Elegant, simple ideas like this deserve to be true, but there is still work to be done to explain how exotic terranes can cross oceans, if oceans always grow from mid-ocean ridges towards the continents under which they are subducted. Suggestions are being made that perhaps they move only along the edges of continents, in the way that, for example. Baja California is moving because of the San Andreas Fault, but this certainly does not fit for India, Australia, or Alaska.

It occurs to me, also, that there is still room for amateur speculation in such recent ideas. For example. the stability of small pieces of continent once they have developed an independent existence does not seem to have been investigated. Pieces of continent floating on the mantle will not be exempt from the general rules governing flotation and stability, of which some insight can be gained by floating odd-shaped pieces of wood on water. A thin flat slab will not float with its surface vertical, but will roll over until the surface is parallel with the water. A squarish block is unlikely to have any surface parallel to the water when it becomes stable, and might even finish with its diagonal vertical. Thus, a wide slab broken from a continent will obviously remain level as it floats on the mantle below, but a piece narrower than the continent is thick, typically about 35 kms. would be expected to roll over on to its side, no doubt revealing some very interesting rocks in the process. The island of Naxos, in the Aegean Sea could well be an example of this effect, although orthodox opinion has a much more complex explanation. An intermediate slab up to perhaps three or four times as wide as it is thick would not remain level, raising one edge and lowering the other, causing a regional dip to stratified rocks that would probably be preserved after docking. The high side might also raise fragments of ocean floor - this would be the place to look for ophiolites the presence of which would also suggest that neighbouring granites may well have been uplifted rather than intruded.

No doubt time will resolve these problems. New work like this shows once again that Geology is not a dry, static subject, as many suppose, but is dynamic and entertaining.