Dr. JanetWatson, gave the opening address and concluded the discussion of the three papers (in this number of the journal) by Coward, Francis and Simony, dealing with Lewisian geology and crustal regeneration in northwest Scotland:
Now that the antiquity of much of the continental crust has been established it is clear that the metamorphic and tectonic processes of reworking at depth have played a major role in the geological evolution of the crust. These processes have provided the means by which old rock complexes have been reshaped and readjusted, without loss of coherence, in response to new crustal regimes; they have enabled the crust to adapt itself repeatedly to global changes in the distribution of stable and mobile regions and in the siting of zones of high temperatures. The papers published in this volume are all concerned with the effects of deep- seated regeneration as displayed by rocks of the Lewisian gneiss complex which underlies most of northwest Scotland. Taken together, they have a bearing on the operation of a major geological process.
The oldest components of the Lewisian are at least 3000 million years in age. The time-span of the events discussed by each author was well over a thousand million years and because there is evidence that more than one major change in crustal regime took place during the period of regeneration it seems legitimate to think of the evolution of the complex in terms of several stages and to regard the products as polycyclic.