1907.] 



NATURAL SCIENCES OF rillLADELrillA. 



561 



'Test Picture. — The annual test picture was taken from Rock W, on 

 August 19, 1907, and shows a continued shrinkage on both sides and 

 in dei)th of the ton<j;ue. This last feature is particularly marked when 

 walking over the slopes of ice on a line with the test plates. When the 

 location for these was selected it was largely on account of the even 

 slope and inicrevassed siu'faces. Now great crevasses are found, but 

 particularly on the left side, and the slope in places approximates 35 

 degrees, whereas ten years ago it was but 22 degrees. A careful study 

 of the rock at the sides of the ice stream and the probable form of the 

 bed on which the glacier flows would indicate that the tongue has now 

 almost reached the upper edge of the nearly flat bed moraine which has 

 been gradually uncovered for at least twenty years, and should reces- 

 sion continue the ice edge will begin to mount over ledges of bed rock 

 which have been worn and polished for ages beneath the glacier mass. 



Flow of the Glacier above the Tongue. — The six steel plates laid out 

 on July 12, 1906, were located again on August 12, 1907, after a period 

 of exactly thirteen months or 396 days. After a good deal of difficulty 

 they were all found, but it was not possible to triangulate the position 

 of No. 6 on the extreme left of the glacier, which had taken a position 

 on a blade of ice between two very deep crevasses, and so far below the 

 general level of the glacier that a tower forty feet high would have been 

 required to bring the stadia within the field of view of the transit. 



The following table gives the total motion of the five plates during the 

 interval of thirteen months, and the average daily motion computed 

 from this total. In the last cohmm has been repeated the observed 

 daily motion of these plates laid out on July 12, 1906, and located 

 again twelve days later.^ 



Motion of 1906 Plates on Surface of Illecillewaet Glacier, July 12, 1906, 



to August 12, 1907. 



Number of 

 Plate. 



Total motion 396- 

 day interval 

 (inches) . 



Average daily mo- 

 tion 396-day in- 

 terval (inches). 



Average daily mo- 

 tion twelve-day 

 interval summer 

 of 1906 (inches). 



960 

 1,056 

 2,136 

 2,66-4 

 2,436 



Plate lost. 



7.00 

 11.33 



9.75 

 10.25 



8.85 



» Compare "Observations made in 1906," Proc. Acad. Nat. Set. Philn., Decem- 

 ber, 1900, pp. 573, 574. 

 37 



