290 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
drainage area of its own. The outer bank of a river's main curves 
is the one which will naturally develop tributaries. 
Foo. Rus. In the model illustrated by Plate 2 an accidental 
development of arborescent rills on the surface was produced by 
flooding. The model consisted of stratified marble dust, coal dust 
and sand, the strata overlapping. Two faint constructional terraces 
had been left in the topography. It was planned to erode with 
seepage, but the trough at the upper edge of the model accidentally 
overflowed. Almost instantly, as the rush of water flowed off the 
surface, the drainage pattern developed as shown in the plate. The 
three steeper slopes show arborescent drainage, while flood plains 
occupy the terraces. An interesting feature of this model is the 
development of arborescence not from baselevel headward, but all 
over the surface flooded, with its maximum on the steeper slopes 
and in the medial region. 
“GranD Canyon” Moper. A considerable advance beyond earlier 
experiments was made in 1901 by R. W. Stone, now of the U. S. Geo- 
logical Survey, and the author. A tank five feet (1.52 m) square and 
ten inches (.254 m) deep was used. The water was maintained ata 
constant level by a flood-gate. A rectangular island was built by 
sedimentation, 25 X 36 inches (.63 X .91 m) in area and 3 inches 
(.076 m) thick. Separate layers were sifted into a frame, the whole 
being kept moist. The model consisted of 61 very thin layers of 
marble dust, coal dust, clay and red lead, with three massive layers 
of sand each 4 inch (.013 m) thick, (Plate 3). A fine spray was 
produced by means of two special nozzles. ‘These were constructed 
so that a fine direct jet was broken up by impact against the ragged 
surface of a finely punctured tin plate, fastened at 45° inclination to 
the line of the jet. A mist-like spray was thus thrown at an oblique 
angle, while the excess of water was allowed to run off. These two 
nozzles were arranged on opposite sides of the model so as to pro- 
duce a uniform rainfall over its surface. The model was tilted, 
sprayed daily for some weeks, and photographed frequently. Arbo- 
rescent drainage developed, deep canyons with esplanades and water- 
falls were cut, and a flood plain was formed along the lower reaches 
of the confluent main streams which terminated in a lobate delta 
built out into the lagoon. A cross-section of this model, after 7183 
hours of erosion, is shown in Plate 3. On the right is shown the 
delta in section. In the middle of the section may be seen profiles 
en A 4m. m — 
