By ALFRED K. WALLACE, F. R. S.
ERRATIC BLOCKS AND ICE-SHEETS.
IT is little more than fifty years ago that one of the most potent agents in modifying the surface features of our country was first recognized. Before 1840, when Agassiz accompanied Buckland to Scotland, the Lake District, and Wales, discovering everywhere the same indications of the former presence of glaciers as are to be found so abundantly in Switzerland, no geologist had conceived the possibility of a recent glacial epoch in the temperate portion of the northern hemisphere. From that year, however, a new science came into existence, and it was recognized that only by a careful study of existing glaciers, of the nature of the work they now do, and of the indications of the work they have done in past ages, could we explain many curious phenomena that had hitherto been vaguely regarded as indications of diluvial agency. One of the first fruits of the new science was the conversion of the author of Reliquiæ Diluvianæ — Dr. Buckland, who, having studied the work of glaciers in Switzerland in company with Agassiz, became convinced that numerous phenomena he had observed in this country could only be due to the very same causes. In November, 1840, he read a paper before the Geological Society on the Evidences of Glaciers in Scotland and the North of England, and from that time to the present the study of glaciers and of their work has been systematically pursued with a large amount of success. One after another crude theories have been abandoned, facts have steadily accumulated, and their logical though cautious interpretation has led to a considerable body of well-supported inductions on which the new science is becoming firmly established. Some of the most important and far-reaching of these inductions are, however, still denied by writers who have a wide acquaintance with modern glaciers; and as several works have recently appeared on both sides of the controversy, the time seems appropriate for a popular sketch of the progress of the glacial theory, together with a more detailed discussion of some of the most disputed points as to which it seems to the present writer that sound reasoning is even more required than the further accumulation of facts.
The works referred to are: Do Glaciers Excavate? by Prof. T. G. Bonney, F. R. S. (The Geographical Journal, vol. i, No. 6); The Glacial Nightmare and the Flood, by Sir H. H. Howorth, M. P., F. R. S.; Fragments of Earth Lore, by Prof. James Geikie, F. R. S.;