Abstract

DR M. J. Le Bas offered his congratulations to Dr Walker on his lucid presentation of the role of density during magmatic ascent. However, density effects alone cannot explain the majority of continental basalts which have been extruded through less dense materials. That density control can be important is shown at Carlingford and Skye, where layered gabbros intrude as sheets above sialic crust and at the base of the Tertiary lava pile: Bailey’s mechanism for ring dyke formation is almost impossible here. Dr Walker likened the inclined sheets of Iceland to cone sheets, but the former are planar and dip to the riftzone which gives a different stress field from that of the conical sheets in the Hebrides. It seems that cone sheets raise their central mass of country rock as a result of vertical forces, and that this mechanism is well exemplified by highlevel carbonatite complexes. How did he reconcile the fact that his model appears to depend on acid preceding basic magma, contrary to the relations at most Hebridean centres? Had he considered the mechanical strength of the crust through which magma penetrates: the nature and extent of joints and fractures in strata along which magma can find weaknesses? Finally, did he distinguish between the different effects, apart from viscosity, of ‘wet’ and ‘dry’ magmas, which can either soften the envelope of an intrusive boss followed by deformation and folding, or give brittle-type fracture of the country rocks permitting sheet intrusion?

DR R. R. Skelhorn: It was very

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