288 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
produced by seepage through an inclined sand beach. The model ` 
differed from that of Plate 1, fig. 1, höwever, in that it was built of 
alternate sedimentary layers of marble dust, clay and sand, whereas 
No. 1 a was homogeneous sand. ‘The oozing waters of the main stream 
bed undermined water-bearing strata on the walls of its gorge. The 
waters so released into the valley undermined superjacent beds, start- 
ing lateral channels. The loci of these channels are interdependent, 
because each tributary has a potency of position dependent on an 
initial underground drainage area. The moment a stream flows freely 
it discharges the waters of a certain upslope district and that district +; 
thereafter slowly enlarges until it is delimited by boundaries which are 
underground divides from other drainage. Such divides are not topo- 
graphic elevations in any sense; they are the boundaries of what might 
be called the ‘sphere of influence” of any stream. No lower tributary 
in the same up-and-down zone of flow can use from the same aquifer- 
ous stratum any of the underground waters of an area above a higher 
tributary; hence a lower tributary, to gain a drainage area of its own, 
must eat laterally until it is beyond the zone of higher tributaries, 
or make use of aquiferous strata stratigraphically below their level. 
Moreover a higher tributary is always apt to take off the underground 
waters of a lower, and therefore rob it in fact, though none of the 
ordinary superficial evidences of piracy may appear. Initial rhythm 
occurs where uniform undermining affects uniformly a cliff of uniform 
height for some distance; instance the rhythmic spacing of the four 
lower tributaries on the right! bank of the main stream in Plate 1, 
fig. 2. A similar notching, having a tendency to rhythm, may be 
seen on both cliffs which bound the widening flood plain farther 
down stream. The four dextral tributaries mentioned are graduated 
in length, the highest being the longest. The reason for this is that 
the highest controls the headward drainage area underground, the 
next lower stream is thereby impoverished of supply, and so on to the 
smallest. As a stream gains width it increases the number of orifices 
of exit of water, its volume and its underground drainage area. This 
process has reached its maximum in the wide flood-plain area of the 
main stream, which by both scour and lateral planation has opene 
a broad water channel beneath the flood plain. That channel satis” 
fies the discharge of a large drainage area on either side, hence there 
are no lower tributaries in the cliffs bordering the plain. Wells dug 
in the flood plain would find abundant water anywhere representing ? 
m eel 
1“ Right” and “left” in this paper always refer to an observer facing downstream. 
