2 54 POPULAR SCIENCE MONTHLY. 



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 pre- 

 cision. 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 midsuromer 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 meteor- 

 ological 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 



