Or 
JAGGAR: EROSION AND SEDIMENTATION. 29, 
eroding waters were all supplied from the surface. ‘The homogeneous 
model was soaked and the excess of superficial water flowed off. 
Under these conditions the controlling drainage areas for individual 
rills are true catchment basins. The upper margin of the model (D, 
Fig. 2) is free from corrasion; here the waters accumulated, flowing 
down the slope until individual streams gained velocity and load 
sufficient to trench. Each trench then became, by reason of its 
depression below the general level, the medial line of an elongate 
drainage area, delimited by superficial divides. Lateral tributaries 
have tendency to parallelism and rhythmic spacing according as the 
general slope, initial sheet-flood, and valley-slopes were uniform. The 
angle of junction, in plan, of tributary and main stream in this model 
varies from 25 to 28 degrees. Parallel sets of spurs and streams occur 
in many places marking local rhythms; these are best seen by holding 
the plate obliquely sloping down from the eyes, and sighting down- 
stream along the main rills. The spurs are thus seen to be equally 
spaced in many parallel sets related to the general slope rather than 
to individual valleys. There are frequently parallel features of this 
sort tributary to different mains. 
The most marked parallelism and rhythmic arrangement is shown 
at the right border of the plate (A, B, C, Fig. 2). This may be viewed 
as a series of tributaries to the straight fall-line at the border. ‘Three 
sets of streams shaped in plan like half candelabra are seen, one above 
the other, the highest (A) the largest. For each group, the lowest 
Stream is compelled to eat laterally into the model farther than the 
next higher, and so on, in order to acquire its own drainage area. 
When such a stream is beyond the area which is in the shadow of the 
drainage basin next above, it wins the water of an oblique upslope 
district, and the drainage from that district unites in a channel which 
makes a distinct bend with the initial lateral channel. The bends up 
the slope, for the same group, are progressively nearer the fall line. 
The whole of group B is clearly in the shadow of group A next 
above. The highest stream of a lower group is prevented from devel- 
opment by the proximity upslope of the group next above, which catches 
all the upslope water. The stunted growth of the highest stream gives 
an advantage to the next below which acquires a longer drainage area, 
and the third still longer. In this respect each group is like the upper- 
most group, the form of which is clearly determined by the margin 
of accumulating rainfall Z at the upper edge of the plate. The larger 
rhythm of the three groups does not follow the same law, for there the 
upper group A is the largest. The reason for this is that the upper 
