Igneous Action and Metamorphism.—I have already referred to the fact that igneous eruptions were frequent during Palæozoic time. As far as there is any evidence at present, they do not appear to have taken place during the Cambrian Epoch either in Europe or in North America. In Europe contemporaneous tuffs or ashes are found for the first time in the succession in the Arenig rocks; and in America about the same period, or perhaps a little earlier. By referring to the Table, p. 156, it will be seen that the contemporancous traps also occur only in the areas which had undergone most depression, or where the greatest amount of sediment had accumulated. Sir J. Herschel has urged as a reason for this that the crust after a continued period of depression would descend to a point where the heat would be sufficiently great to melt it through, or to weaken it; and that as a consequence fractures would take place, and there would then be a corresponding rising of heat and fiery matter from below, to that of the accumulation of the sediments.