1906.] NATURAL SCIENCES OF PHILADELPHIA. 571 



Illecillewaet Glacier, Recession of Tongue of Ice from Rock C. 



Date of Observation. 



Recession of Ice since 

 previous Year. 



Aug. 17,1898. 



July 29,1899. 



Aug. 6, 1900. 



Aug. 5, 1901. 



Aug. 26,1902. 



Aug. 25,1903. 



Aug. 14,1904, 



July 25,1905. 



July 24,1906 



It is interesting to note that while the recession between 1890 and 

 1898 showed an average of 56 feet per year, for the eight years from 

 1898 to 1906 this average has been but 33.3 feet per year, or about 

 three-fifths. It will also be observed, by reference to the map, that the 

 measurement on July 24, 1906, was not made to the point of greatest 

 extension in the ice. Should this point have been measured the course 

 would not have been in the same line as previous years, the tongue 

 having moved to the left, but the recession for the year ending in 1906 

 would have been 64 feet, instead of 84 feet as noted in the table. 



The change noted between any two years is not a good indication of 

 the amount of recession or advance which may have taken place, as 

 the local weather conditions, rainfall, and even the condition of the 

 crevasses above, all have a marked influence in determining changes, 

 and it is only when these are eliminated by including a longer interval 

 that the true amount of change may be determined. 



Flow of Glacier above Tongue. — In 1899, to determine the rate of 

 flow of the ice at a point about 1,300 feet above the tongue, eight plates 

 were laid out across the glacier. These were accurately placed by 

 means of a transit in a true line almost at right angles to the direction of 

 flow, and their change in position accurately determined after stated 

 intervals. The positions of these plates have been plotted on the 

 map, and the table on page 572 gives a summary of the changes that 

 have taken place at the times noted. 



With the exception of the comparisons made between July 31 and 

 September 5, 1899 (see first part of table, page 574), the motion indi- 

 cates the movement of the glacier over the period of approximately a 

 year, and thus includes both the summer flow which should be greater 

 and the winter flow which should be less than the averages given. 



Several of the 1899 plates have been lost from one cause or another. 



