The guiding principle of geologic investigation, brought out most clearly by Lyell, requires us to seek the explanation of past changes of the earth by observation and study of agencies which are now in operation, producing similar changes during the present epoch. From such studies of the Swiss glaciers, Agassiz, Forbes, Tyndall and others have given to us the theory of the formation of the drift by land ice, so that the comparatively small district of the Alps supplied the clue for deciphering the records of the latest completed chapter of the geologic history of northwestern Europe and the northern half of North America. Glaciers of other regions in the eastern hemisphere, notably of the Himalayas and of Norway, have also contributed much to our knowledge of the ice-sheets of the Pleistocene or glacial period. The vast ice-sheets of that time, however, are adequately exemplified at . . .