CONCLUSION. 49 
vedges, and chemical action by the carbonic acid of the atmosphere 
did by the organic acids of vegetation upon all calcareous rocks and 
jven to some extent upon siliceous ones. 
Of the three topographic types described on page 23, the plateau 
;ype may be ascribed largely to the toughness of its granular, granitic, 
i>r gneissic material, which offered more uniform resistance to erosion. 
Here structure was of minor and often of no consequence. The ridge- 
and-valley type is probably due to the complex structure and the 
mutual relations of its soluble and its schistose rocks and the varying 
resistance of both structure and material to erosion. This is some- 
times reflected in the vegetation, for limestone, when near the surface, 
promotes fertility by its additions to the subsoil, and even when deep 
and covered with drift and alluvium oftener underlies the arable 
land, while schist areas, constituting the hilltops, being more rugged, 
usually bear less drift and weather less readily, and suffice for the 
support of forest growth. The Hudson- Champlain valley type, con- 
sisting of an undulating surface with several series of low hills, is 
traceable to the behavior under erosion of soft but insoluble rocks in 
small folds with occasional more resistant miniature anticlinoria or 
synclinoria. Here, however, the general rock surface has been modi- 
fied by the addition of morainal and terrace material. 
To such a history and to such causes the Taconic landscape owes 
its varied and picturesque features. 
