chap, ii.] MANURING THE SOIL. 31 
in the course of years, cause the bog to burst 
its bounds, and overflow the surrounding 
country; as the Solway-moss did many years 
ago, and as bogs in Ireland have done fre¬ 
quently. An excess of vegetable matter on 
a silicious sub-soil, differs from the common 
black-peat in retaining less water; and in 
being mixed with a portion of the primi¬ 
tive earth, which, from its loose texture, be¬ 
comes easily detached from the sub-soil. 
Peat in this state is called heath mould. 
The most productive soils are those in 
which several ingredients are combined in 
proper proportions; and if any one of the 
primitive earths preponderates, the soil be¬ 
comes comparatively unfertile. Thus the 
best soil for gardening purposes is generally 
allowed to be a calcareous loam on a chalky 
sub-soil; and this sort of soil is composed of 
nearly equal parts of lime, sand, and clay, 
enriched depositions of decayed animal and 
vegetable matter. The next best soil is a 
sandy loam, composed of clay and sand, en¬ 
riched by decayed animal and vegetable 
substances, and resting on a sandy or gra¬ 
velly sub-soil. The worst soils are peat and 
