244 DYNAMICAL GEOLOGY. 



common; twelve inches corresponds to 365 feet a year, or one mile in about 

 14|- years. 



Forbes found for the maximum in July, at his upper station on the Bois 

 Glacier, 521 inches a day, and in December 11-5 inches. In the large Muir 

 Glacier, according to G. F. Wright, the average in August, 1886, was 20 feet 

 per day; and according to H. F. Reid, in August, 1890, 10 feet. 



The Greenland glaciers are rapid in movement because the outlets from 

 the great interior mass are so narrow. At Disco Bay, the Jakobshavn Glacier 

 moves in summer at middle 65 feet per day, and a fourth of a mile from the 

 side, nearly 50 feet. Helland estimated the daily discharge of ice into the 

 sea, as icebergs, at 432,000,000 cubic feet. Eates of 35-70 and 100 feet per 

 day have been reported. The rate of 99 feet per day was observed in 

 August, 1887, in the fiord east of Upernavik by the Danish Lieutenants, 

 Ryder and Bloch. In glaciers of so great magnitude friction is reduced to a 

 minimum. 



In the summer the snow over the ice melts, sending streams and drippings 

 down the crevasses and into all accessible cracks in the ice ; as far "within as 

 the outside heat penetrates, the many air-cells inside warm up and melt the 

 ice around them, and the dirt grains and all foreign substances absorb and use 

 the heat in like manner. Moreover, the glaciers lose much at surface by 

 evaporation. 



4. Intermittent advance. — In glaciers the cycle of advance and retreat is 

 many years in length. The meteorological conditions favoring maximum 

 mass of neve, and thereby maximum rate of flow and elongation, are, as 

 already explained : Jirst, long and wet winters in the iieve region, causing an 

 extension of the neve area, which is that of the only permanent annual con- 

 tributions, and which has great breadth compared with that of the trunk 

 glacier below ; second, short and dry summers, especially below the level of 

 the neve. Thus come the largest gain and the smallest loss. 



Observation has proved that the cycle of gain and loss is a long one, 20 

 to 50 years. Forel has reported that in the Alps there have been in this 

 century five half cycles ; 1800 probably to 1815, enlargement ; 1815-30, dimi- 

 nution ; 1830-45, enlargement ; 1845-75, diminution ; and that from 1875 

 to 1890 enlargement was beginning in different glaciers. He observes that 

 the alternating periods correspond to that of a cold and rainy period, and 

 that of a warm and dry, as meteorologically deduced by C. Lang (1886). 



5. Capability offloiv in an ice-mass. — Yielding to gravity in material so 

 solid as the ice of a glacier, over uneven slopes and along valleys ever-vary- 

 ing in obstacles, is explained on the ground of the following qualities of ice 

 and glaciers, (a) The fragility of ice, in consequence of which it breaks 

 readily and so accommodates itself to obstacles; (6) the dissemination of 

 much water through the mass of the glacier, which increases the fragility and 

 approximates the condition to that of a viscid body ; (c) the plasticity of ice, 

 or the quality of molecular adaptation to conditions of pressure ; (d) slip- 

 ping along planes of bedding or straticulation in the ice ; (e) sliding of the 



