302 W. M. DAVIS 
before their testimony can be used. In any case, even in atolls 
of similar diameter, the depths of the lagoons vary so much that 
they alone cannot be taken as proving that the atol! reefs have 
been built up with the rise of the ocean from stationary platforms 
of uniform depth. 
The fact that the maximum depth of atoll lagoons seldom 
exceeds 40 fathoms does, however, suggest the existence of some 
control that has prevented the occurrence of greater depths; but 
this control may be found elsewhere than in the abrasion of plat- 
forms at a uniform depth below sea-level. For example, if the 
subsidence of an atoll or a barrier-reef island is relatively rapid, 
the reef will be somewhat submerged, the inwash of detritus from 
the reef will be active, and the increase of lagoon depth will be 
retarded; if subsidence is, on the other hand, relatively slow, the 
reef will be maintained at sea-level and will broaden its surface, 
sand islands will be formed along its edge, and inwash of detritus 
into the lagoon will practically cease; hence, in spite of slow 
subsidence the lagoon will not be rapidly shoaled. Thus, unless sub- 
sidence be unusually rapid, there appears to be a series of spontane- 
ous reactions which tend to prevent lagoon depths from varying 
by large measures. Wherever unusually rapid subsidence occurs, 
the atoll would be drowned and converted into a submarine bank. 
The scarcity of such banks in the Pacific, as far as it is now explored, 
suggests very strongly that subsidence has rarely been unusually 
rapid; and slow, equable, or intermittent subsidence, being a 
near approach to long-continued stability, does not appear to be 
particularly incredible. But, however that may be, the known 
depths of atoll lagoons can be explained as well by the theory 
of intermittent subsidence as by the glacial-control theory. 
The depth of submarine banks will be discussed in a later 
section. 
The volume of existing reefs—The glacial-control theory sug- 
gests that a very uniform upgrowth of reefs should take place in 
the uniformly rising postglacial ocean, and that all existing reefs 
should therefore be of similar surface breadth and of similar volume. 
Daly finds this to be the case; he says that “‘the widths as well as 
the heights of the existing barrier and atoll reefs are of the proper 
