36 



NORTH AMERICAN FAUNA. 



[no. 27. 



immense pressure behind it breaks its way seaward, occasionally 

 becoming dammed and raising the level of the water until the in- 

 creased pressure again clears a channel. At Fort Simpson, near lati- 

 tude 62°, the ice continues to drift in quantity for some days after its 

 disruption. About ten days, on the average, after the Mackenzie 

 opens at this point, or about the time that the breaking ice has reached 

 latitude 65°, the upper Mackenzie opens and the channel is filled 

 again with floating ice. Sometimes a third consignment of floes, 

 from the 'Little Lake ' or from Great Slave Lake, fills its current. 



The following table shows the dates of the opening and closing of 

 the Mackenzie at Fort Simpson during a series of years: 



Table showing condition of Mackenzie at Fort Simpson. 



° From the Liard. The ice from the Mackenzie above the Liard began to run Oct. 18. 

 ''Had not closed on Nov. 24. 

 ''Had not begun to drift Nov. 1. 



The following table shows the dates of occurrence of certain phe- 

 nomena at Fort Norman, distant from Fort Simpson about 180 miles 

 by the river. Some of the years being represented in the Fort 

 Simpson table also, the rates of progress of the opening and closing 

 of the river mav be noted : 



Table shoxoing condition of Mackenzie and date of first snow at Fort Norman, 



Mackenzie. 



In the autumn of 1903 the ice in Peel River at Fort McPherson 

 commenced to drift on October 4, and the river set fast on October 12. 

 In the following spring it opened on May 23. The lowest tempera- 

 ture registered there during the same winter was — 60°. 



