514 Hope. — The ‘ Sadd ’ of the Upper Nile . 
Then a little island is gradually formed, and mocca-mocca 
roots get stranded and begin to grow, and in a year or two 
a dense living palisade of Montrichardia protects the small 
island from being washed away. ‘ The little island lengthens 
down stream to a considerable distance. The semicircle of 
gigantic arums is sufficiently elastic to bend before the weight 
of water ; the plants may be torn off by the roots, but never 
broken off.’ 
4 The great rivers of Guiana all contain islands of different 
sizes, some as many as ten miles long, and it may be con- 
fidently stated that nearly all have been built up in this way, 
by means of the mocca-mocca, with the assistance of the host 
of thorny Papilionaceae.’ 
From Mr. Rodway s description it appears that the obstruc- 
tions caused on the Guiana rivers are caused or intensified by 
the mocca-mocca being stranded in the water, or growing up 
in it from roots which have sunk to the bottom, and by the 
interlacing of floating grass with its thickly planted stems, and 
the c host of thorny Papilionaceae.’ On the other hand, the 
‘ Sadd ’ of the Upper Nile consists mainly of plants which grow 
in the lateral lakes and lagoons, or on the banks of the main 
channel, and are forced in masses into the river by floods and 
winds and swept down by the current until they unite and find 
a point on the bank or in a narrow part of the channel which 
brings them to a stand, when they form a floating block under 
which further supplies of material coming from up-stream are 
sucked in and go to increase the vertical thickness of the mass 
both above and below water. The consequence of a block of 
‘ Sadd ’ on the Nile is increased flooding of the land on either 
side of the river, and disastrous loss of water in the lakes and 
lagoons, and by evaporation, for it does not appear that there 
is any such solid ground, within reach of the current, into 
which the river can eat and form a new channel, as there is 
in the case of the Guiana rivers ; and the floating power of 
the hollow-stemmed and buoyant mocca-mocca prevents the 
blocks formed by it and the floating grass from being any- 
thing like so permanent as those on the Nile. 
