Page:Popular Science Monthly Volume 37.djvu/114

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THE POPULAR SCIENCE MONTHLY.

able to that now existing in northern Germany, justify geologists in concluding that this era was one of long duration, and characterized in Germany by climatic conditions apparently not less temperate than those that now obtain.

4. To this well-marked interglacial era succeeded a second overflow of Scandinavian inland ice, confined to a region much narrower than that covered by the first. Its boundaries are shown not only by the geographical distribution of the youngest bowlder-clay, but by the direction of rock-striae, the trend of erratics, and the position of well-marked moraines.

Concerning the ground-moraines of the Alpine lands of central Europe, the only question that has recently given rise to much discussion is the origin of the materials themselves. The observations of able investigators appear to Prof. Geikie to have demonstrated that these materials have been derived, in chief measure, from the underlying rocks by the erosive action of the ice that overflowed them. German geologists are not agreed upon this much-debated question of glacier-erosion—a few still maintaining that glaciers have little or no eroding power. But where the evidences of erosion have been studied over a wide region, from which the ice has completely disappeared, rather than at the lower ends of existing glaciers, some of the strongest opponents of glacier-erosion have been compelled to go over to the other camp. As an example, Prof. Geikie quoted Dr. Blaas, who, through his observations on the glacial formations of the Inn Valley, has recanted his former views and become a formidable opponent of the very theory which he once upheld. To his books and to memoirs by Penck, Brückner, and Bohn, and especially to the chapter on glacier-erosion by the lastnamed author, Prof. Geikie refers those who may be anxious to know the last word on this question.

Observations by Drs. Brückner and Penck have led to the opinion that the loess is of interglacial age. Examining a wider range of evidence, Prof. Geikie has little doubt that the loess belongs to no particular horizon, though it must be considered strictly a Pleistocene accumulation. Concerning its mode of formation he discussed the various theories advanced, and gave it as his opinion—an opinion formed from what he has himself seen of the loess in various parts of Germany, from reading, and from conversation with those who have worked over loess-covered regions—that it is for the most part of aqueous origin, formed in the slack waters of the great rivers, and in the innumerable temporary lakes which occupied or partly occupied many of the valleys and depressions of the land. Probably some may have been derived from the denudation of bowlder-clay, some from "rainwash" while much of the so-called Bergloess with its abundant