CARTER : THE EVOLUTION OF THE DON RIVER-SYSTEM. 391 
river's feeders, for the fall of each of them would be increased 
by every deepening of the main channel, and so they would 
have increased ability to cut back into the secondary water- 
sheds, and thus in all directions the power of the predominant 
river would be increased and its permanent success be assured 
in the struggle for pre-eminence. A simple diagram will make 
this plain (Fig. 1). Consider RS as the main watershed, down 
the dip-slope of which flow three rivers, dividing it into valleys 
separated by the secondary watersheds WX and YZ. Suppose 
for simplicity that the central 
stream B has all three advan- 
tages, and so holds a command- 
ing position with regard to its 
lateral rivals. It will thus 
intrench itself more deeply 
than its neighbours, and be 
able to cut back more rapidly 
into the main watershed. The 
deepening of the principal 
channel B will give the feeders 
F and G, which begin to work 
along the outcrops of the two 
prominent grit beds 00' and 
PP', a steeper slope, which 
will increase their velocity, and they in their turn will be more 
effective in cutting back into the secondary watersheds WX 
and YZ. In time these advantages will enable them to invade 
the water-catchment areas of their competitors, and to filch 
from them some of their tributaries (E, H). Every gain of this 
sort will increase tlie volume of the principal stream, and thus 
its cutting power, and enable it to make fresh depredations on 
its neighbour's territory. Ultimately this may go on until the 
river B is able to cut back so far into A's drainage area that it 
can capture its head stream D. The original streams. A, B, C, 
flowing down the slope of the main watershed, are called dip 
or consequent streams ; the lateral feeders developed along the 
strike of the beds are termed subsequent streams. After capture 
of the head waters, the residuary streams below the angle of 
Fig. 1. 
DIAGRAM OF RIVER CAPTURJ:. 
