FLORA OF THE SUNDRIBUNS. 
239 
among these sandy patches. This subsoil extends, as borings in the 
Gangetic delta show, to a depth of 120 feet, where it rests on a fairly 
uniform layer of semi-fluid mud 40 feet in thickness, which is sue" 
ceeded by a formation of the same character as that which overlies 
it. 
The heavy flow of water in the larger channels that marks the 
rainy season frequently causes the erosion of the bank against which 
a current sets. Banks are thus at times washed bodily 'away ; more 
frequently, however, the root-system of the riparian vegetation holds 
the actual bank in position and the current only undermines it. When 
the waters fall to a lower level in the cold weather, such banks, de- 
prived of the support supplied by the pressure of the water, often 
subside bodily into the stream, with the vegetation growing upon 
them still intact. The submersion to which the trees are subjected 
during the higher tides of the subsequent monsoon suffices to kill 
them, but does not necessarily effect their removal, and the obstruction 
they now offer to the flow of the stream is apt again to alter the set of 
the current and to lead to a similar attack by the river on another 
part of its bank. Where erosion of this kind takes place there is not 
infrequently a coincident and compensating accretion of shelving 
muddy bank on the opposite and convex side of the river -reach. 
Such newly formed banks become covered with grass which serves 
to bind the mud already deposited and helps to arrest silt and floating 
seeds. The latter germinate freely and lead to an extension of the 
forest over the newly formed land. The peculiar root-systems of 
many of the resulting species help still further to bind the soil and, by 
arresting more and more silt, to raise the general level of the bank. 
The strong storms from the nortti-west, so prevalent in the Bengal 
plain from March till May, and the cyclones that occasionally sweep 
up from the sea of Bengal at the commencement and the close of 
the south-west monsoon, do considerable damage to the forests by 
overturning the taller trees, which break those that check their fall. 
The trees along the coast-line are, moreover, markedly affected by the 
steady monsoon winds that blow for half the year; they have in 
consequence a gnarled and bent and stunted habit of growth. 
Throughout much of the western Sundribuns, except in the most 
northerly islands, the vegetation is largely of the mangrove type 
though even here the mangroves [Rhizophorace^) are accompanied 
by Gengwa {Excoecaria Agallocha), by Hital [Phoenix paludos a) and 
by Satari [^gialitis annulata). In the southern islands of the 
central Sundribuns, where the influence of the tides is strong, the 
predominance of the mangroves is equally marked. Throughout the 
