1878.] S29 [Niles. 



mountains the lowest zone maybe wanting, and the zone of glaciation 

 may extend to the base. A great variety of mountain forms may 

 result from such modifications of these three elements of the typ- 

 ical mountain slope. I believe that the mountains which have 

 these zones well characterized, should be regarded as presenting the 

 types of those orographic features which arise from the predominance 

 of certain physical agencies at certain altitudes. 



Furthermore, I believe that careful studies of these typical features 

 may lead us to a better analysis of the features of our own mount- 

 ains. Although we have no mountains in the Appalachian system 

 which reach the snow-line, yet the summits of many are of sufficient 

 elevation to acquire, in our rigorous climate, at least a mild express- 

 ion of the features produced by weathering at greater altitudes. 

 In some instances the rock has been broken till now the summits 

 are almost or entirely composed of the fragments, while upon other 

 summits the mechanically sculptured, but not completely dissevered, 

 rock still remains. Upon most of our hills in this and in higher 

 latitudes, the region of glaciation is the most extensive and best 

 characterized one, although its vegetation is rarely of alpine character. 

 There is no discrepancy in this, for the topographic features of this 

 region were mostly formed under the forces of a past climate, while 

 the alpine vegetation survives at its present altitude upon the mount- 

 ains, under the conditions of the present climate. Hence many of 

 our hills, from their summits to their bases, are characterized by the 

 topographic features of the zone of glaciation, and yet sub-alpine and 

 deciduous forests are found upon their slopes. 



I have spoken of these topographic features as distributed in zones, 

 because I have considered them only in their relations to the slopes 

 of mountains. I do not claim that all of the features of an entire 

 country, from its greatest elevations to the level of the sea, may be 

 classified in three zones. Descending such an extensive range of 

 surface, we find a repetition of these features; a region of aqueous 

 agencies is frequently succeeded by one of glaciation at a lower level, 

 and this may be followed by another of aqueous agencies. But the 

 individual slopes of any district may exhibit a part of the same law 

 which we recognize more distinctly upon the slopes of mountains. 

 The aqueous agencies act with greatest power upon the lower portions 

 of such slopes, and this is as true of the sea as of streams. But the 

 recession of the cliff-like slopes of sea-coasts and valleys is not en- 

 tirely from the aqueous denudations at their bases. The mechanical 



