Geology and Mineralogy. 169 



1800-1811 ? 



1811-1816(1822) Advance. 



1818-1820 Grand maximum. 



1820-1830 Slight recession, uncertain. 



1830-1850 Contradictory movements. 



1855 Maximum. 



1856-1900 Recession. 



1875-1892 Advance for some Swiss glaciers. 



1890-1900 Same for some Austrian glaciers. 



In regard to the duration of the periodic movement in general, 

 there are to be recognized two distinct periods. First, the 

 annual, caused by the fact that during the winter months no 

 melting takes place, and the state of the glacier is stationary ; the 

 advance, consequently, continues, so that there is a temporary 

 increase between October and April or May. During the sum- 

 mer months, however, the melting predominates and a decrease 

 shows itself. This annual period causes in one case an accelera- 

 tion, in another, a retardation, of the general variation going on, 

 whatever that may be. 



There is also a cyclic period, the duration of which is not easily 

 established, but which the author connects with the thirty-five- 

 year climatic period called the Cycle of Bruckner. Thus, in 1889, 

 after considering all the facts available, he arrived at the conclu- 

 sion that the average value of the phase of increase was 10-5 

 years, that of decrease 27*4 years, or together 37'9 years for the 

 entire period. This conclusion was also reached by E. Richter 

 (1891) after discussing the observed maxima from 1600 down to 

 1840-50. Forel, however, is inclined to regard the actual periodic 

 relations as much more complex. He would divide the Swiss 

 glaciers for the nineteenth century into three classes. First, 

 those with a single period, like that of the Aar, with a maximum 

 about 1870; second, those of two periods, as the Rhone, maxi- 

 mums 1820, 1855 ; and finally, those of three periods, as the Trient 

 and des Bossons, with maximums 1820, 1855, and 1892. While 

 the glacial increase in general is due to an excess of precipitation, 

 the period coinciding probably with the Briickner cycle, the 

 actual advance must depend upon the length of time which the 

 snow-fall requires in its journey from the upper neve regions to 

 the glacial front, varying for glaciers of different length. The 

 actual effect observed will be modified by the temperature condi- 

 tions existing. The author adds that the minimum state is to be 

 regarded as the normal magnitude for the glacier, the forward 

 impulses being, as it were, accidents. — Bibl. Univ., Nov. 15, 1900. 



In this connection, attention may be called to the 5th Report 

 (for 1899) on the periodic variations of glaciers by E. Richter, 

 President of the International Commission of Glaciers (see this 

 Journal, ix, 71). In this report (Bibl. Univ., July 1900,) it is 

 noted that for the Swiss glaciers the phase of advance which 

 began about 1875, has spent its force, so that now, for 1899, only 

 one glacier (Boveyre, Rhone) is certainly and only nine others 

 probably advancing: which nineteen are certainly, and forty-four 

 probably, retrograding. 



