The heart of geology has always been represented by a synthesis of diverse disciplines. In the early days of our science, it was from this synthesis that the great historical pageant of the last four thousand million years was built up. A hundred years ago, the generalisations of geology culminated in the contribution they made to the history of the Earth. It seemed then that the goal of every geologist, whatever his discipline, was to add a page, a chapter or an appendix to a record of events that embraced everything from the origin of the Earth to the dominance of man.

But this is no longer the case. Now it is not a record of events that lies at the heart of our science. Our curiosity is busy with another question. It is a question set by the history itself. We can see that we are not dealing with an arbitrary repetition of cycles. There is an overall, inexorable, dynamic change relentlessly driving forward. The Earth is changing in nature, and the synthesis that we are looking for now is one that explains that change. What processes are at work? How far is geological evolution a part of a more general process, affecting all planets? Can geological processes be grouped together as a special case of planetary evolution?

The concept of planetary evolution is opposed to that of a 'steady state'; it is not opposed to the more sacred principle of uniformitarianism. If the Earth, in not opposed to

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