mm m te 
JAGGAR: EROSION AND SEDIMENTATION. 289 
flowing sheet from under the lateral cliffs. The river bed itself is a 
locus of seepage wherever the stream corrades. 
UNDERGROUND DRAINAGE AREAS AND STREAM SHADOW. The 
foregoing analysis brings out two principles which may be defined . 
before going farther. Each tributary in this model of inclined strata, 
has an underground drainage area. So has every natural orifice of 
exit of seeping waters in nature. Every spring controls such drain- 
age area. Every brook is apt to be a line of loci of springs, and col- 
lectively they control a certain underground drainage area. Every 
forking system of tributaries, on larger scales, controls a similar under- 
ground area. The supply of every river system is dependent on this 
underground area (King, F. H., 1899), on the one hand, and a surface 
catchment basin, on the other. If, as in this model (Plate 1, fig. 2), 
strata variously aquiferous slope with the drainage, then each im- 
pervious stratum is an underground surjace on which are separated 
the underground drainage areas of different orifices. Hence there 
may exist underground divides dependent on initial attitude of strata, 
and quite independent of initial surface topography. Apparently the 
presence of drainage surfaces as distinct from pores favors digitation. 
This is shown by the contrast between figs. 1 and 2, Plate 1. This 
is borne out by the fact that ‚the spray models hereinafter described 
also develop arborescent drainage, the drainage surface in such case 
being the superficial topography. In contrast to these, in a model 
supplied wholly by seepage (Plate 1, fig. 1), there is no initial drain- 
age surface. 
The control by a tributary of a certain drainage or accumulation 
area of its own prevents that area from supplying water to any orifice 
farther down the general slope. Whether this be a superficial or an 
underground area, the effect is the same. Accordingly any such lower 
orifice may be said to be in the shadow of the higher stream and its 
drainage area. In Plate 1, fig. 2, the lowest of the four rhythmic tribu- 
taries on the right bank is thus shadowed by the next higher, and so 
on. Headward development by undermining will go on by a multiple 
ramification of those streams which can free themselves from shadow. 
The development of tributaries is more difficult mouthward than head- 
ward, because the mouthward region of a straight river is always in the 
Shadow of the headward tributaries. If a river can for any reason 
bend laterally, new tributaries may develop wherever a curve under- 
Mines a portion of the general slope not in shadow. This principle is 
illustrated in Plate 1, fig. 2. If a lower tributary can eat laterally far 
enough to go beyond the shadow of higher branches, it may acquire a 
. 
