What was the period of this climatic pendulum? The answer comes to us vaguely in echoes of Niagara's voices. The cataract began its existence at Lewiston during the retreat of the latest ice sheet. Since that time the gorge has been cut back from Lewiston to the present site of the falls, and it is possible to estimate roughly what time the task has consumed. This episode is one-half or less than one-half of the time elapsed since the beginning of the retreat of the ice from its most advanced position. Thus indefinitely, we may count that something like 40,000 years sped while the climate changed from those Greenland conditions to these which we now enjoy. By similar conservative studies of the effects of deposition and erosion accomplished before the latest glaciation, the duration of the interglacial epoch is found to be several times that of the post-glacial interval; that is, in numbers, 80,000 or 120,000 years or more.
The significance of these figures does not depend upon their precision. They confessedly do but indicate the general magnitude of the times. But they serve to show that those times were more than sufficient for the operation of the causes assigned to produce the observed effects, and thus they sustain the hypothesis. Furthermore, they serve to bring glaciation near to us. In earth history, whose eras are measured by millions of years, events which occurred a hundred thousand years ago are of recent date. We live within the operation of the causes which may hinder or promote glaciation, and, though the present is an age of comparative mildness, we cannot be sure whether this be the spring of a great era or midsummer of an epoch. Are the gnomes of the under-world wearied of mountain building, and have they sunk to rest? Are the shafts of the sun's heat as they traverse the air effectively caught and stored? Does man, consuming fossil carbon in his manifold activities, unconsciously postpone the return of winter?
The cause of a glacial epoch may be found when an adequate cause of cold is linked with the occurrence of glaciation, but the spread of an ice mantle is dependent on snowfall as well as on temperature, and it is through this relation that the peculiar distribution of Pleistocene glaciers may be explained. It will suffice here to state the meteorological conditions which, according to Chamberlin, determined the most striking centers of accumulation, those which were situated in the plains of north-northeastern America.
Studies of polar currents, which free the northern coast of Siberia of ice and crowd it upon the American Arctic Archipelago, combined with the partial data available as to the barometric conditions of the Arctic zone, lead him to the conclusion that in the northern hemisphere the grand movement of the atmosphere from west to east about the globe is oblique to parallels of latitude, and is upon an axis which has
