592 



PHYSICAL GEOGRAPHY. 



remarkably clean. The clay is of a variety used for the manufacture of 

 porcelain ware ; and the sandy residuum in some cases is pure enough 

 to be mistaken for loaf-sugar, as in Acworth. Furthermore, everything 

 about the mountains of this character must be clean. The waters will 

 be clear and sparkling ; the earth will hardly soil the hands by handling 

 it; the ledges, when uncovered, will appear blanched; the sand and 

 gravelly bottoms of rivers and ponds will not be slimy and treacherous. 



When the attempt is made to measure the amount of this species of 

 denudation, the results are startling. Referring elsewhere for the details, 

 it is sufficient to state that the pre-glacial erosions of our territory, due 

 mainly to this cause, have removed from the present surface as much 

 rock as now exists above the level of the sea. The average height of 

 the land above the ocean in New Hampshire has been estimated at 1400 

 feet (p. 296). Our proposition maintains that the amount which has 

 been removed from above the existing surface is equal to a blanket 1400 

 feet thick and 9392 square miles in extent. 



The markings left behind by this kind of decomposition are less con- 

 spicuous than the others, and not so easily identified. Its tendency is 

 to crumble down bluffs, blunt sharp angles, and to act simultaneously 

 upon all sides. Should a stream of water be diverted upon certain 

 ledges undergoing atmospheric disintegration, excavation will go on 

 there more rapidly than elsewhere, since the recently separated grains 

 of rock will be washed away, and expose fresh surfaces to chemical ac- 

 tion. Such action would tend in time to produce pinnacles, such as 

 those made known to the public in the Garden of the Gods, Colorado, 

 and elsewhere among the Rocky Mountains, by means of photographs. 

 When these pinnacles stand upon high table-lands, they will bear rela- 

 tionship to the Needles or Aiguilles of the Alps, referred by Mr. Ruskiij 

 to glacial action for their formation. In this case the ice has continued 

 the wearing action commenced by the rivulets. 



The action of streams of water may next command attention. Here 

 the fact of geographical position must be taken into account. Two types 

 of valleys result from this cause. In the rainless districts of the south- 

 west part of our country, long ravines called canons are abundant. 

 Plateaus, hundreds of thousands of square miles in extent, are traversed 

 by narrow chasms cut down perpendicularly by rivers and their tribu- 



