large body of material composing the loess except by assuming the existence of great deserts to windward. But there seems to be little or no evidence of such deserts where they could be effective. The mineralogical character of the loess of Iowan age proves that the material came from granitic rocks, such as formed a large part of the drift. The nearest extensive outcrops of granite are in the southwestern part of the United States, nearly a thousand miles from Iowa and Illinois. But the loess is thickest near the ice margin and thins toward the southwest and in other directions, whereas if its source were the southwestern desert, its maximum thickness would probably be near the margin of the desert. The evidence cited above seems inconsistent not only with the hypothesis that the loess was formed at the end of an inter-glacial epoch, but also with the idea that it originated at times of maximum glaciation either from river-borne sediments or from any other source. A further and more convincing reason for this last conclusion is the probability and almost the certainty that when the ice advanced, its front lay close to areas where the vegetation was not much thinner than that which today prevails under similar climatic conditions. If the average temperature of glacial maxima was only 6 deg.C. lower than that of today, the conditions just beyond the ice front when it was in the loess region from southern Illinois to Minnesota would have been like those now prevailing in Canada from New Brunswick to Winnipeg. The vegetation there is quite different from the grassy, semi-arid vegetation of which evidence is found in the loess. The roots and stalks of such grassy vegetation are generally agreed to have helped produce the columnar structure which enables the loess to stand with almost vertical surfaces.