374 On Denudation and Deposition. 



The extent of the area is an essential condition, L e., the 

 lateral dimensions of the inverted pyramid which has the area 

 for its base and the centre of the earth for its vertex. If the 

 area is small or narrow, oblique forces exerted by the parts 

 surrounding this pyramid come more into play. They enable 

 the part within the pyramid to act like a bridge ; and the 

 support thus given enables denudation, if limited to a small 

 area, to scoop out valleys, and deposition to produce ridges, 

 as may be seen in the glaciers and moraines of mountainous 

 countries. On the other hand, if the erosion due to glacial 

 action takes effect over a great stretch of country, as it does 

 in Greenland, and as it formerly did in Ireland, it causes the 

 surface to rise. 



A nearly even balance between the two opposite tendencies 

 may be seen in Egypt, where borings exhibit fluviatile de- 

 posits at great depths below the present surface, although the 

 surface is only about as much raised above the sea now as it 

 was when those ancient deposits were laid down by the Nile. 

 Each year's deposit makes the surface go down, but only 

 about as much as its own thickness, so that the new surface 

 each year is not far from being at the same level as that of 

 the preceding year. If the deposit had taken place over a 

 much greater breadth of country, the whole would have gone 

 down. It would have become a ridge if it had been confined 

 to a much narrower strip and if the river could have been 

 kept from diverging. 



A similarly instructive case is that of Brazil, where an 

 immense plateau is continuously being denuded by the vast 

 rivers that drain it. But here there is also an equally un- 

 interrupted addition to the solid materials of the earth by the 

 luxuriant tropical vegetation which everywhere prevails ; and 

 it is probably because the accessions and withdrawals are 

 nearly equal to one another, that the level of the surface has 

 been but little changed. 



Denudation may cause the surface to rise within a space 

 which is in a considerable degree more circumscribed than 

 the areas of elevation hitherto considered, if the conditions 

 are such that the stresses that come into existence round the 

 boundary of this limited space can produce faults, and pre- 

 vent the material which is outside the pyramid from being in 

 a position to help to keep down the material which is within. 

 This seems to have happened in the case of that vast mass of 

 mountains — the Himalayas, the Hindu Kush, and their asso- 

 ciated ranges — where excessive denudation accompanied by 

 the isolation secured by faults has occasioned a proportionately 

 great elevation above what was probably a humble beginning ; 



