DAMAGE DONE. 590 



especially peat-moss, predispose to swampiness, whilst per- 

 manent pasture dries up the surface- soil, but will not thrive 

 on very wet land. 



e. Density of Standing -Crop. 



Clearance of forests, especially in mountainous regions, 

 favours the formation of swamps ; it may be observed as a 

 general rule, that swampy ground, when once stocked with 

 young wood, becomes gradually drier as the wood grows older 

 and covers the ground. This is especially true for coniferous 

 forest, and the efficacy of the Scots pine for draining swamps 

 is well known in Ireland, where many bogs have been formed 

 on the former sites of forests, as is proved by the presence in 

 them of bog-oak and other woods. The damp, cold London 

 clay at Prince's Coverts near Esher, hardly ever dries up on 

 the roads through the forest, but the ash, hazel, and alder- 

 cqppice will be found to have dried up the soil on both sides of 

 an extremely wet road. 



In the forest of S. Amand, near Valenciennes, in 1843, about 

 2,000 acres was wet heather land, the soil being sand with 

 a slight mixture of clay. This was gradually planted with 

 Scots pine, and these trees have drained the soil to a depth 

 of 5 feet, so that coppice with oak standards is now being 

 introduced in place of the pines. Snipe have almost 

 disappeared. 



The valley of the Upper Ehone affords a warning example 

 of the effects of clearing forests on the formation of swamps ; 

 the high land above the river having been cleared of forest 

 at the end of the eighteenth century, in order to afford land 

 for agriculture. Field-crops, however, have made way for 

 pasture, and the quantity of hay produced annually has 

 decreased, till at last the formerly forest-clad areas have 

 become covered with moss and converted into swamps. 

 Eeafforesting the area, as in the Dammersfeld, has been 

 attempted, but is a difficult task. 



In explanation it should be noted that in mountain regions 

 with a cool, humid atmosphere, which is unfavourable to evapo- 

 ration, the amount of water taken from the soil is proportional 

 to the extent of leafy surface exposed to the air, and to the 



