Niles.] 328 [March 6, 



and cliffs to a certain degree, though not so powerfully as in the 

 summit region. But more important is the abundance of streams 

 which traverse this zone and furrow it with ravines, gorges, and val- 

 leys. These are often so near each other that the mere ridges between 

 them have received all their characteristic features through their 

 erosive action, and the whole zone is sometimes a more or less regular 

 succession of such ravines and ridges. 



Admitting the great difference of existing opinions as to the relative 

 or predominating power which glaciers or streams of water have 

 exercised in the excavation of these valleys, I believe careful study 

 and comparison will satisfy the observer that the steepness of the 

 slopes and the most characteristic contours and features of this zone 

 have been produced by, or are inseparably associated with, some 

 form of aqueous action. It is, therefore, on account of the predom- 

 inance of this form of geological action in the presence of others, 

 that I call this region the Zone of Aqueous Agencies. If we accept 

 the above conclusion, it will follow that the topography of this zone is 

 not essentially dependent upon climate. We accordingly find this 

 zone represented, even among the Alps, considerably below the 

 climatic zone of sub-alpine vegetation. 



Considering now these three zones in their climatic relations, we 

 recognize that in the features of the zone of weathering we have a 

 topographical expression of the geological agency of the existing 

 climate. In the features of the zone of glaciation we have a pre- 

 served record of the action of a power which originated in the pecu- 

 liarities of a past climate, while the features of the zone of aqueous 

 agencies are not of climatic origin. We may, therefore, speak of 

 these zones as climatic, past-climatic, and non-climatic in their origin. 



It will be understood that the above descriptions apply only to 

 those mountain slopes upon which each of these zones are well charac- 

 terized. But there are Alps which have the sub-alpine, the alpine, 

 and the snow-regions distinctly marked, and yet one or two of the 

 physiographic zones here described are wanting. This usually arises 

 from peculiarities of geological constitution. When, for example, 

 the rocks at the base are softer, or yield more readily than those of 

 the upper portions, the denuding agents may drive back the face of 

 the lowest slope till it joins the uppermost zone directly by obliterating 

 the intermediate one, and one steep, precipitous slope from the base 

 to the summit of the mountain may be the result. Among other 



