An account of the meeting for informal discussion held on Friday 14 November 1969

Abstract
The meeting for informal discussion began with the short papers, by Green, O’Hara and Walker, which are printed in this volume and were offered and discussed under the heading ‘Petrogenesis’. These were followed by a discussion under the general heading ‘Ocean crust structure and ophiolites’ on which I have notes on 39 contributions. No written contributions were received and the account which follows is a personal one based on these notes; it has not been checked with individual contributors and if any are misrepresented I offer my apologies. Introducing the discussion on the oceanic crust Matthews stressed the need to reconcile the relatively uniformly layered picture of the crust given by seismic refraction measurements, which is well established at least on the ocean basins, with the much less strongly layered assemblage of rock types revealed by petrologists. In particular we have to take note of the surprising uniformity of velocity in layer 3 which has a worldwide average of 6.69 km s -1 with a standard deviation of only 0.26 km s -1 (Raitt 1963). It would be of great interest to have many more determinations of seismic velocity on specimens of deep-sea amphibolites and greenschists. Matthews presented a cartoon showing a possible view of the formation and composition of the oceanic crust. This cartoon, modified in the light of some of the subsequent comments, is shown in figure 1. It was successful in provoking discussion.

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