116 MAN AND THE GLACIAL PERIOD. 



ward movements. We may presume, however, that in 

 general the marks left by the retreating ice correspond 

 closely with those actually made and obliterated by the 

 advancing movement. 



From observations upon the glaciers of Switzerland 

 and of Alaska, it is found that neither the advance nor 

 the retreat of these glaciers is constant, but that, in obe- 

 dience to meteorologic agencies not fully understood, 

 they advance and retreat in alternate periods, at one time 

 receding for a considerable distance, and at other times 

 regaining the lost ground and advancing over the area 

 which has been uncovered by their retreat. 



" M. Forel reports, from the data which he has col- 

 lected with much care, that there have been in this cent- 

 ury five periods in the Alpine glaciers : of enlargement, 

 from 1800 (?) to 1815 ; of diminution, from 1815 to 

 1830 ; of enlargement, from 1830 to 1845 ; of diminution, 

 from 1845 to 1875 ; and of enlargement again, from 1875 

 onward. He remarks further that these periods corre- 

 spond with those deduced by Mr. C. Lang for the varia- 

 tions for the precipitations and temperature of the air ; 

 and, consequently, that the enlargement of the glaciers 

 has gone forward in the cold and rainy period, and the 

 diminution in the warm and the dry."* 



When, now, we attentively consider the combination 

 of causes necessary to produce the climatic conditions of 

 the great Ice age of North America, we shall be prepared 

 to find far more extensive variations in the progress of 

 the continental glacier, both during its advance and dur- 

 ing its retreat, than are to be observed in any existing 

 local glaciers. 



With respect to the arguments adduced in favor of a 

 succession of glacial epochs in America the following 

 criticisms are pertinent : 



* American Journal of Science, vol. cxxxii, 1886, p. 77. 



