346 PROCEEDINGS OF THE AMERICAN ACADEMY. 



form of all vigorous mountains is much more due to the carviug of val- 

 leys in their uplifted mass than to irregularities in their initial form. The 

 hahit of regarding rivers as destructive agents, except near their mouths 

 where they may build deltas in bodies of standing water, seems to have 

 retarded the recognition of the imiwrtance of rivers as constructive agents. 

 It is perhaps for this reason that the first suggestion made as to the 

 origio of piedmont plains generally ascribes them to deposition in the 

 sea or in lakes. It only after closer study has been given to their com- 

 position, structure, and fossils, as well as to the activities of aggrading 

 rivers, that the prevailing fluviatile origin of most piedmont plains has 

 been recognized. Instead of having been built up from the floor of the 

 sea or of a lake basin, fluviatile plains have frequently accumulated 

 where a slow depression of the land — perhaps an isostatic movement, or 

 a compensation for a slow upheaval of the adjacent mountains — has 

 given them appropriate location. The thickness of piedmont plains on 

 continental margins, as in northern India, may then be much greater than 

 the height of their surface above sea level. The laterally confluent fans 

 of which these plains are built, each fan heading at the mouth of a moun- 

 tain valley, is good evidence of their origin in the subaerial deposits of 

 aggrading rivers. 



Many piedmont plains are to-day slightly dissected, especially near the 

 mountain base, by the rivers that formerly built them up, as in northern 

 India. Others are more strongly dissected, as in the group of great fans 

 at the base of the Pyrenees in southwestern France. Some are tilted 

 and deformed, being thus added as marginal strata to the older mountains 

 from which their materials were derived ; the marginal ridges to which 

 the Righi belongs at the northern base of the Swiss Alps, and the Siwalik 

 range at the base of the Himalayas in northern India are composed of 

 strata of this kind. Yet common as are existing piedmont fluviatile plains, 

 either of interior or exterior drainage, they have not been frequently 

 recognized among geological formations. Their prevailing absence from 

 the geological column may be objective or subjective ; due on the one hand 

 to their real absence in past geological ages ; due on the other hand to the 

 interpretation of fluviatile deposits as formations of non-fluviatile origin. 



One reason why the fluviatile deposits of piedmont plains are not more 

 generally recognized in their true relations is that they have no common 

 English name. If their flatter parts are described simply as plains, they 

 are not sufficiently distinguished from plains of other kinds. If their 

 steeper parts are described as slopes, they may be confused with the body 

 of the mountains from which they are derived. The want of an appro- 



