The two Palaeozoic ranges, the Caledonian and the Hercynian, were comparable in size with the Alps. In his chapter on the European Altaids, F. E. Suess1 says “Recent investigations have brought evidence that the original Variscan mountain chain of which now only scanty remains are left, was once not inferior in size and extension to the Alps to-day”. The remains of both ranges are now at a low level, and it was for some time tacitly accepted that that low level was due to denudation through the long period since they were formed. That low level is not, however, consistent with isostatic theory. Isostatic theory requires that a mountain must have strong roots immersed in a heavier plastic medium so that in effect the chain and its roots float “hydrostatically” in that medium. Accordingly if a layer h km. thick and density p is removed from the top of the range, the level of the range will fall by h (1 − /σ) where σ is the density of the plastic medium. If p = 2·6 for granite, and σ = 2·85 for the intermediate layer, the level will fall by − km.