596 PROTECTION AGAINST SWAMPS. 



Fellings should take place when the water is lowest, or 

 when it is frozen. Timber should be removed up to the 

 advent of spring, and the commencement of the floods. 

 Grass-cutting and removal of litter may be carried on freely 

 in such forests, without danger of impoverishing the soil, 

 which is enriched by the annual floods ; pasture, however, 

 should not be allowed. 



SECTION III. SWAMPS. 

 1. Formation of Swamps. 



The soil of a locality becomes wet when the drainage-water 

 has not a sufficient outlet. If there be no outlet for the water, 

 swamps or peat-bogs may be formed. Either swamps or bogs 

 may be caused by rain, snow- or spring-water, or water from 

 rivers and ponds. 



Drainage may be prevented either horizontally or vertically, 

 the former if the water cannot escape superficially owing to an 

 insufficient fall of the ground ; the latter, if it cannot escape 

 by penetration into the subsoil owing to an impermeable 

 substratum of clay, turf, clayey or marly loam, soil encrusted 

 with iron, or massive rock, especially in horizontal layers. 

 Sometimes both these causes are at work, when the harm 

 done is intensified. The local causes of swampiness may vary 

 considerably. 



(a) In low-lying plains, swampiness is generally caused by 

 flowing water, owing to a slight depression in the ground and a 

 stiff soil. 



(I)} In basin-shaped valleys along watercourses (Taliiry), 

 swamps may be caused by surface-water, or by underground 

 infiltration from the stream ; the former happens after floods, 

 when the overflow cannot find its way back into the stream, 

 owing to the presence of high land along its bank. Part of the 

 overflow must then remain on the low land, especially when 

 the subsoil prevents the descent of the water. Lagoons along 

 the sea-coast are formed in this way. 



Water may spring through permeable soil from neighbouring 

 watercourses, and when it thus appears in depressions, it 



