300 W. M. DAVIS 
still-standing platforms.t The conclusion does not seem tenable 
because the smooth floors of a good number of medium-sized atoll 
lagoons in the trade-wind belts have moderate depths, such as 
20 or 25 fathoms, and must therefore, even under the glacial- 3 
control theory, have been aggraded by 15 or 20 fathoms if their 
abraded platform lies at a depth of 40 fathoms. Moreover, the 
fact, more fully stated in the next section, that small atolls have on 
the average shallower lagoons than large atolls proves that a good 
share of their sediments is washed in from their inclosing reefs. 
If aggradation by material thus partly supplied from the margin, 
partly from locally formed organic detritus, and distributed by 
trade-wind currents in the lagoons ought to produce a slanting 
surface of deposition, these lagoon floors should not be level; their 
levelness therefore contradicts the supposed necessity of slanting 
aggradation and confirms the theory of equable distribution and 
even aggradation by the lagoon waters. Hence it may be concluded 
that lagoon floors tend to become and to remain nearly level, what- 
ever form the foundation of their inclosing reefs may have had, and 
whatever the thickness of their sediments may be; and, conversely, 
that the form of a lagoon floor gives no indication of the form of 
its buried foundation. I am therefore constrained to think that 
the general levelness of atoll-lagoon floors is no sufficient reason 
for the existence of a level rock platform at a moderate depth 
beneath. 
The depth of lagoon floors —li, then, the smoothness of lagoon 
floors is no sufficient proof of the existence of a smoothly abraded 
rock floor beneath them, we may next inquire as to evidence for 
the existence of such a rock floor that is found in the similar depth 
of atoll lagoons and of submarine banks. The depths are not all 
alike. As to atolls, Daly has shown that on the average the 
smaller ones are the shallower, and from this he draws the accept- 
able conclusion that “the smaller the platform the higher was the 
proportion of reef débris in the veneer, and the more rapidly has 
the lagoon area been shallowed”’ (183), and again that “‘the filling 
of the lagoon [by inwashed sediments] is in indirect proportion to 
tR. A. Daly, ““A New Test of the Subsidence Theory of Coral Reefs,” Proc. Nat. 
Acad. Sci., II (1916), 664-70. 
