400 W. M. DAVIS 
banks should not be regarded as the product of abrasion acting on 
still-standing reef-encircled preglacial islands while the ocean was 
lowered in the glacial period. 
As to the depths of submarine banks being so accordant as to 
be beyond. explanation, except as a consequence of abrasion at 
some such depth as 35 or 40 fathoms: It has already been pointed 
out that the actual depths are discordant, for they vary from 20 or 
25 fathoms in the smaller banks north of Fiji to 40, 50, or 60 fathoms 
in the large banks of the China Sea and Indian Ocean, and to 64 
fathoms in the exceptional Saya de Malha bank. Accordance is 
found only by subtracting larger measures of postglacial aggrada- 
tion from the depth of the shallower banks and smaller measures 
from the depth of the larger banks; and even then the depth of the 
Saya de Malha bank is not brought into accord with that of its 
fellows. Daly has pointed out, as has been previously noted, 
that small banks are, on the average, shallower than large ones, and 
has explained this relation by showing that small banks should — 
be more rapidly aggraded than large ones, in so far as the detritus 
is supplied from their rim; but according to this principle small 
atolls drowned by subsidence should also be more rapidly aggraded 
and therefore shallower than large ones: hence these variations of 
depth cannot serve as ground for valid choice between the two 
theories. As the depth of actual aggradation of submarine banks 
is absolutely unknown, measures of aggradation satisfactory to 
either theory may be freely assumed, but neither theory is thereby 
strengthened. Choice between them must be made on other 
grounds. 
Possible balance of processes acting on submarine banks.— 
It is evidently conceivable that all the banks mentioned above 
may be neither drowned atolls built on subsiding foundations, nor 
ancient still-standing islands reduced to abraded platforms now 
more or less aggraded, but still-standing submarine masses of any 
origin now for the first time in process of building up toward sea- 
level, as postulated in the Rein-Murray theory of atolls. This 
conception cannot be proved or disproved by soundings; it can be 
tested only by indirect evidence, such as is afforded by the history of 
adjacent islands or continents and by the general action of organic 
