122 REGINALD A. DALY 



where mounds or fillings of snow tunnels are protected by trees over- 

 head. It is very different above tree-line, where even the weak 

 veneer of turf is broken in the burrowing, and where the millions of 

 mounds or tunnel-casts are exposed to every agent of transportation. 



(3) The transporting efficiency of wind in the treeless zone of 

 lofty mountains has, on the whole, been more emphasized by Euro- 

 pean observers than by those of America. So far as this is the case, 

 Europeans have come nearer to the highland view than we have in 

 this country. The summer quiet of alpine summits of itself gives a 

 most deceptive idea of the power of wind in the heights. During 

 the other seasons winds of almost hurricane violence are far from 

 uncommon, if we can generalize from the limited instrumental data 

 so far issued from high-lying observatories. We may believe that 

 dust, sand, and fine gravels are so rare above tree-line largely because 

 of such winds. For obvious reasons, sand-blasting there plays no 

 such role as it does in the sculpturing of rock-forms in lowland des- 

 erts; but transportation by the wind is another influence placing in 

 strong contrast the conditions of erosion in the regions above and 

 below tree-line. 



(4) Erosion and transport through avalanches are enacted in both 

 the treeless and the forested zone. In the lower zone the destruction 

 wrought by a great avalanche may be great, but it is largely a ruin of 

 tree-trunks. In the lower zone the avalanche paths are tolerably 

 well fixed from year to year, sparing much the greatest part of the 

 forested area. In the treeless zone, avalanches have generally less 

 momentum, but they are more numerous, less localized, and there- 

 fore more likely to find and sweep down loose rock debris. Above 

 tree-line their ruin is wholly rock-ruin. It seems safe to conclude 

 that snow-slides are more powerful agents of degradation above 

 tree-line than below. 



(5) The general streaming and cascading of rock-waste under 

 the direct pull of gravity are evidently immensely more rapid in the 

 treeless zone than where the strong vegetation mat binds humus, 

 soil, and bowlder to the bed-rock, though it be without perfect, 

 ultimate success. 



(6) The debris from the upper zone itself helps to protect the 

 bed-rock of the lower zone. The very rapidity of general waste 

 streaming above involves the slowing down of erosion below. 



