The great glaciation of the Permian period, referred to in the next chapter, is a totally different matter. During this time the ice-sheets appear to have reached their maximum area, and to have extended to sea-level, in countries which are at present close to the equator, while lands now in high latitudes remained unglaciated. It is true that at the present day glaciers exist at high latitudes under the equator itself, and given a ridge sufficiently steep and a snowfall sufficiently heavy such glaciers would possibly extend to sea-level; but even these conditions would not give rise to the enormous deposits of true boulder-clay which have been discovered, and there seems no way of avoiding the supposition of an enormous difference in the position of the pole relatively to the continents at this time. Wegener’s theory alone, however, requires that glaciation should always have been proceeding in some part of the globe (unless both poles were surrounded by wide expanses of ocean), which is hard to reconcile with the extremely definite and limited glaciations which geological research has demonstrated. In these circumstances we may tentatively explain the pre-Tertiary glacial periods by combining Wegener’s theory of the movements of continents and oceans as a whole with the theory of changes of elevation and of land and sea distribution which is outlined below. That is to say, we may suppose that the positions of the continents and oceans have changed, relatively both to each other and to the poles, slowly but more or less continuously throughout geological time; while at certain periods the land and sea distribution became favourable for extensive glaciation of the regions which at that time were in high latitudes.