When Louis Agassiz first suggested (1840) the idea of a time, comparatively recent in the geological sense, when northern and central Europe was buried under immense sheets of slowly moving ice, like the “ice-cap” of modern Greenland, the conception was received with incredulity. Nearly thirty years passed before this startling theory gained the general acceptance of geologists, but now it is one of the commonplaces of the science, for no other hypothesis so well explains the complicated phenomena of Pleistocene geology. One great obstacle to the acceptance of the glacial theory was the supposed fact that the Pleistocene glaciation was something quite unique in the history of the earth, a violent aberration in the development of climates. Now, however, we have every reason to believe that at least three other and very ancient periods had witnessed similar climatic changes and that “ice-ages” were recurrent phenomena. This is not the place to discuss or even to summarize the evidence which has convinced nearly all geologists of the reality of Pleistocene glacial conditions on a vast scale in Asia, Europe and, above all, in North America. The reader who may wish to examine this evidence will find an admirable presentation of it in Vol. III of the “Geology” of Professors Chamberlin and Salisbury. _North America._—There has long been a difference of opinion among students of the Pleistocene as to whether the glaciation was single, or several times renewed. That there were many advances and retreats of the ice, is not denied; the question is, whether there were truly interglacial stages, when the ice altogether disappeared from the continent and the climate was greatly ameliorated. The present tendency among American and European geologists is decidedly in favour of accepting several distinct glacial stages (Chamberlin and Salisbury admit six of these) separated by interglacial stages, and for this there are very strong reasons. While it is out of the question to present the evidence for this conclusion here, one or two significant facts may be noted. On the north shore of Lake Ontario, near Toronto, are certain water-made deposits, which rest upon one sheet of glacial drift and are overlaid by another. The fossils of the aqueous sediments are in two series, upper and lower, of which the older and lower contains plants and insects indicative of a climate considerably warmer than that of the same region to-day and corresponding to the temperature of modern Virginia. In the upper and newer beds the fossils show the return of cold conditions, much like those of southern Labrador, and this was followed by the reëstablishment of the ice, as recorded in the upper sheet of drift. Even far to the north, on the Hudson’s Bay slope, an interglacial forest is embedded between two glacial drift-sheets. In Iowa and South Dakota numerous mammals of temperate character occur in interglacial beds.