Causes of Fertility/ or Ban-eniiess of Soils. 
171 
generally uniform, extending in tliis country from the north and 
west: consequently we find the post-tertiary erratics ranging 
from the west and north, towards the east and south, fringing the 
western coast up the valleys, extending into Scotland, on either side 
of the Pennine chain or central ridge of England, which appears 
in most cases to have formed a barrier to further progress, though 
this is occasionally surmounted in the lowest spots. The cha- 
racter of these deposits, and consequently the soils found upon 
them, present every variety of form and nature, from the blowing 
sands to the huge boulder stones, from the fine arenaceous to the 
coarse gravelly soil. Thirdly and lastly, soils are derived from 
alluvial deposits, found occupying the line of most of the great 
rivers in this country. Accumulated by water, in its passage 
through various strata, they are of a very mixed and fertile 
character, especially near the mouths, where the currents of salt 
and fresh water mingling caused the death of multitudes of 
Infusoria, which sinking down were deposited along with fine 
mud, and affect the value of these soils to a considerable extent: 
similar processes may be seen going on at the mouths of many 
rivers at the present day. The sedimentary deposit has, in more 
than one instance, been turned to advantage by damming up the 
stream, and retarding its onward progress, until all the materials 
gathered together during its long journey through various strata 
were deposited as mud ; when it was allowed slowly to pass off 
into the sea. Much land of the first quality has thus been gained 
both at the mouth of the Humber and in the Netherlands ; in 
the latter country great expense and untiring energy are exhibited 
in preserving the valuable acquisitions from the ravages of the 
sea: works of great cost having been erected in the form of 
walls and barriers, the occasional destruction of which, and con- 
sequent inroad of the sea, is regarded as a national calamity. 
Considerable tracts of alluvial soil occupy the vallej^s of our 
principal rivers, such as the Thames, the Severn, the Avon, the 
Trent, the Dee, and the Mersey ; varying in physical and 
chemical characters, according to the nature of the beds from 
whence derived. Thus a tract which occurs on the sea-coast, ex- 
tending from Lynn to Barton in Lincolnshire, is a rich sandy 
loam, containing considerable quantities of animal matter ; while 
the alluvium on either side of the Thames, in the neighbourhood 
of London, is of a clayey nature ; but, however they may differ, 
the general character is great richness, due to their depth, fine 
state of division, and complicated nature. 
From whichever of the above sources soils are immediately 
derived, they are all primarily produced by the decomposition of 
the older rocks. Thus the granite, upreared into mountain 
