302 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
tude and rate of cutting in neighboring streams of the same class are 
approximately the same, then the adjacent divides should approximate 
equal altitudes” (Tangier Smith, loc. cit., p. 164). “Ifat the begin- 
ning of the cycle of erosion the upland sky-line is markedly irregular, 
x kit will depend on circumstances whether or not the uplands will 
tend to approach uniformity of altitude after graded slopes have 
been attained” (p. 170). “Adjusted slopes are graded slopes” (p. 
165). That slopes tend to become graded and equalized, and that 
summits or crestlines follow the lead of adjacent slopes is a fact of 
observation clearly stated by Tangier Smith and further illustrated 
by the models under discussion here. But Tangier Smith through- 
out his paper has failed to mention the controlling feature of the 
process, namely, underground water. The underground water- 
surface varies with the topography, rising higher under the higher 
divides and approaching the valleys to form springs. (King, 1899, p- 
97-99.) On the slopes adjacent to higher water-table, there will be 
higher hydrostatic spring pressure and consequently more under- 
mining. Other things being equal, therefore, a high water-table 
tends to pull down adjacent slopes and consequently, by Tangier 
Smith’s law, lowers superjacent summits. When the water-table 
level, and with it the summit level, equal those of neighboring divides, 
the spring-pressure is equalized and the opposing slopes of a valley 
become adjusted. Accordance of summit levels is probably largely 
controlled by this process (see also R. A. Daly, 1905, p. 105), with 
the tree-line and snow-line as correlated levels. A corollary of this 
statement of the levelling controlled by the water-table is that in a 
zone of overloaded streams depositing on flood-plains, the water- 
table may be dammed back into a relatively high position in the low 
interstream divides. If this be true the profile of the water-table 
from head-zone to mouth-zone along an interstream divide line 
should be concave upward with the two ends relatively higher than 
the middle portion above the catenary curve of the adjacent stream- 
beds. The water-table will thus have a headward bevel like that of 
the general surface mentioned above (p. 301) and shown in Plate 5. It 
should be noted, however, that a headward bevel is different from a 
headward slope. While still sloping mouthward, the underground 
water-surface is bevelled headward to the zone of maximum number 
of tributaries, because the divides in that direction are lower relatively 
to the original upland surface. 
No attempt is made in this paper to discuss “peneplanation,” 
meandering, or the mechanism of flood-plain aggradation. It is 
