Abstract
Summary In an earlier paper it was suggested that it was failure in the region between 250 and 600 km below the surface which determined the periodicity of orogenic phenomena. In this paper estimates of the cross-sectional areas of the oceanic troughs and of the mass deficiencies reflected in the strips of large negative gravity anomalies are given and it is inferred that the interval between successive periods of failure is 50 million years. It is pointed out that if the troughs are formed in the manner described in the earlier paper then a regression of sea-level of about 70 metres will occur as a result of their formation. Finally some aspects of the development of the trough regions into mountain ranges are discussed qualitatively.
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