Thayer (1969) has drawn attention to characteristics of several ultramafic occurrences with the theme that studies of such complexes exposed in orogenic belts may lead to understanding of processes currently operating at the mid-oceanic ridges. The author supports this approach, but considers that Thayer is incorrect in attempting to reduce these particular occurrences to a common denominator of olivine-rich ultramafic and comagmatic gabbroic rocks, and in neglecting important differences in the chemical composition, mineralogy and emplacement mechanism of various ultramafics. There is disagreement on geological interpretations which have led the author to postulate, for some ultramafic complexes, a history with high pressure, high temperature crystallization, and high temperature emplacement at shallow levels as hot, crystalline ultramafic diapirs.

This discussion will focus on aspects of such an intrusion (Lizard, Cornwall) disputed by Thayer (1967, 1969), and develop from this a more detailed model of inter-relationships between ultramafic and basaltic rocks at the mid-oceanic ridges.

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