Very important questions of time are involved in this idea of Post-glacial man, and much will depend, in the solution of these, on the views which we adopt as to the rate of subsidence and elevation of the land. If, with the majority of British geologists, we hold that it is to be measured by those slow movements now in progress, the time required will be long. If, with most Continental and some American geologists, we believe in paroxysmal movements of elevation and depression, it may be much reduced. We have seen in the progress of our inquiries that the movements of the continents seem to have occurred with accelerated rapidity in the more modern periods. We have also seen that these movements might depend on the slow contraction of the earth's crust due to cooling, but that the effects of this contraction might manifest themselves only at intervals. We have further seen that the gradual retardation of the rotation of the earth furnishes a cause capable of producing elevation and subsidence of the land, and that this also might be manifested at longer or shorter intervals, according to the strength and resisting power of the crust. Under the influence of this retardation, so long as the crust of the earth did not give way, the waters would be driven toward the poles, and the northern land would be submerged; but so soon as the tension became so great as to rupture the solid shell, the equatorial regions would collapse, and the northern land would again be raised. The subsidence would be gradual, the elevation paroxysmal, and perhaps intermittent. Let us suppose that this was what occurred in the Glacial period, and that the land had attained to its maximum elevation. This might not prove to be permanent; the new balance of the crust might be liable to local or general disturbance in a minor degree, leading to subsidence and partial re-elevation, following the great Post-glacial elevation. There is, therefore, nothing unreasonable in that view which makes the subsidence and re-elevation at the close of the Post-glacial period somewhat abrupt, at least when compared with some more ancient movements.