For a decade following the general acceptance of the glacial origin of “diluvium,” the deposits were embraced as “drift” and treated as the products of one long period of glacial activity, and throughout the controversy of iceberg and glacier the unity of the glacial period was unquestioned. Beds of peat and fossiliferous lacustrine deposits in Switzerland, England, and in America and the recognition of an “upper” and a “lower” diluvium by Scandinavian geologists suggested two epochs, and as the examples of such deposits increased in number and it became evident that the plant fossils represented forms demanding a genial climate and that the phenomena were seen in many countries, the belief grew that minor fluctuations or gradual recession of an ice sheet were inadequate to account for the phenomena observed. It is natural that this problem should have found its solution in America, where the Pleistocene is admirably displayed, and where the State and Federal surveys were actively engaged in areal mapping. In 1883 Chamberlin[64] presented his views under the bold title, “Preliminary Paper on the Terminal Moraine of the Second Glacial Epoch,” and the existence of deposits of two or more ice sheets and the features of interglacial periods were substantially established by the interesting debate in the Journal led by Chamberlin, Wright, Upham and Dana.[65] Contributions since 1895 have been concerned with the degree rather than the fact of complexity, and continued study has resulted in the general recognition of five glacial stages in North America and four in Europe. _The Loess as a Glacial Deposit._ A curious side-product of the study of glaciation in North America is the controversy over the origin of loess. The interest aroused is indicated by scores of papers in American periodicals and State reports of the last quarter of the 19th century—papers which bear the names of prominent geologists. The “loess” in the valley of the Rhine had long been known, but the subject assumed prominence by the publication in 1866 of Pumpelly’s Travels in China.[66] Wide-spread deposits 200 to 1,000 feet thick were described as very fine-grained yellowish earth of distinctive structure without stratification but penetrated by innumerable tubes and containing land or fresh-water shells. Pumpelly considered these deposits lacustrine, a view which found general acceptance though combated by Kingsmill (1871),[67] who argued for marine deposition. Baron Von Richthofen’s classic on China, which appeared in 1877, amplifies the observations of Pumpelly and marshals the evidence to support the hypothesis that the loess is wind-laid both on dry land and within ancient salt lakes. The conclusions of Von Richthofen were adopted by Pumpelly whose knowledge of the Chinese deposits, supplemented by studies in Missouri, of which State he was director of the Geological Survey in 1872–73, placed him in position to form a correct judgment. He says:[68]