REPORT OF THE CHIEF ASTRONOMER 



639 



SESSIONAL PAPER No. 25a 



(6) The debris from the treeless zone naturally helps to protect the bed-rock 

 of the forested zone ; the faster the wasting . above, the slower is the bed-rock 

 destruction below. This exceedingly important argument needs no expansion. 



(7) The chemical solution of rock is, to be sure, probably more rapid beneath 

 the forest-cap than it is above tree-line where the amount of vegetable acid 

 is at a minimum. This cause may, however, be believed to do little toward 

 •counterbalancing the effect of the combined causes just enumerated. .Erosion 

 in alpine mountains takes place primarily by the removal of masses; in com- 

 parison, molecular transfer of rock material to the low grounds has but a very 

 minor control. 



Conclusion. — A review of the conditions of general degradation shows 

 clearly its differential character above and below tree-line. Summits already 

 reduced to the tree-line are bound henceforth to be stubborn against further 

 erosion. Summits bearing a treeless zone are as clearly bound to continue 

 wasting rapidly so as to tend to approach accordance of summit levels with 

 their tree-covered neighbours. Since the glaciated zone of alpine mountains 

 is, in general, well within the treeless zone, the special degradation due to local 

 glaciers harmonizes with general erosion in the development of accordance. 



The different mountain ranges of the Forty-ninth Parallel section all show 

 summit-level accordance and, in each case, at an elevation closely similar to that 

 of the effective tree-line. The following table shows the observed position of 

 this line in the higher ranges. 



Clarke range.. 

 Gal ton range. . . . 

 Purcell range . . . 

 Selkirk range. . . 

 Columbia system 

 Okanagan range . 

 Hozomeen range. 

 Skagit range .... 



Average range in 



position of tree-line. 



Elevations in feet abov 



sea level. 



7,500- 

 7,500 

 7,100- 

 7,100- 

 7,000 

 7,200- 

 7,500- 

 6,800- 



7,200 

 7,300 

 6,800 

 6,800 

 6,000 

 6,500 

 6,500 

 5,800 



Average elevation 



of tree-line, below which 



forest effective in 



retarding erosion : 



approximate measure 



in feet. 



7,200 

 7,20u 

 7,000 

 7,000 

 6,500 

 6,600 

 7,000 

 6,000 



In all cases the ' turf -line ' is higher, occasionally rising to 1,000 feet or 

 more above the effective tree-line. An inspection of the maps shows the 

 sympathy between effective tree-line and summit-level accordance, and power- 

 fully suggests that Dawson was right in explaining it as largely due to the 

 tree-line's influence. 



On the other hand, Willis opposes Dawson's explanation with the following 

 argument : — 



