GEOLOGY: W. M. DAVIS 
473 
of the planes, together with the theoretical spacing for the lattice de- 
scribed above. The agreement is within the limit of error of the measure- 
ments, except that some of the predicted lines are too faint to show. 
This is to be accounted for by the distribution of electrons in the atoms, 
and will be discussed in a future paper. 
THE STRUCTURE OF HIGH-STANDING ATOLLS 
By W. M. Davis 
DEPARTMENT OF GEOLOGY. HARVARD UNIVERSITY 
Communicated June 5, 1917 
The structure of high-standing atolls has seldom been studied in 
detail, and is perhaps seldom sufficiently revealed for close study. At- 
tention is therefore drawn here to only one structural feature, namely 
the relation of atoll limestones to their supposed foundation of volcanic 
rocks. According to Darwin's theory of intermittent subsidence, the 
limestones of atolls should He unconformably on an unevenly eroded, 
submountainous volcanic mass, the top of which may be buried to any 
depth, as in section M of sector L, figure 1 : the section of the volcanic 
Fie. 1. 
foundation here shown resulting from the dissection and progressive 
subsidence of a volcanic cone, as shown in sectors H, J, K. Accord- 
ing to the Glacial-control theory, which is today the only fully formu- 
lated competitor of Darwin's theory that deserves consideration here, 
the limestones of atolls should as a rule unconformably overlie a fla^. 
platform of volcanic and calcareous rocks, produced by the following 
processes: A preglacial volcanic island, sector A, figure 2, is supposed 
to have stood still so long as to have been worn down to low relief, as 
in sector B, while a reef plain was built by outgrowth around it: during 
the Glacial period, when the ocean was lowered about 40 fathoms and 
