GEOLOGY: W. M. DAVIS 
649 
best determinations, lies between 4.1 and 4.3. That of the specimen 
used in this investigation is 4.19. 
iVegard, Phil. Mag., London, Nov., 1916 and Jan., 1917. 
'Williams, London, Proc. R. Soc, 93, 1917, (418). 
3 Braggs' X-Rays and Crystal Structure, 1915, (22), 
* Dana's Mineralogy, Fig. 10, 1892, (81). 
« Tutton's Crystallography, 1911, (501); or Braggs' X-Rays and Crystal Structure, 1915, (91). 
« Bragg, op. cit., pp. 120-127. 
' Kaye, X-Rays, 1917, (226). 
THE ISOSTATIC SUBSIDENCE OF VOLCANIC ISLANDS 
By W. M. Davis 
DEPARTMENT OF GEOLOGY AND GEOGRAPHY. HARVARD UNIVERSITY 
Communicated October 9. 1917 
Objection has sometimes been made to Darwin's theory of upgrowing 
coral reefs on subsiding foundations on the ground that the subsidence 
of the reef foundations should lower the ocean surface and lay bare a belt 
of recent marine deposits of smooth surface and simple shore line around 
the continental masses, while as a matter of fact the shore lines of conti- 
nents are usually more or less embayed, as if the sea had recently ad- 
vanced upon the unevenly eroded surface of the continental margins. 
The objection will, however, be found to have small weight when it is 
seen to rest upon the implied postulate that the embayments of conti- 
nental coasts have as a rule been produced by a universal rise of the 
ocean surface, everywhere of the same amount and date, whereas their 
embayments testify to no such simple origin; and to proceed upon the 
unwarranted assumption that the subsidence of reef foundations re- 
quires the subsidence of broad areas of the ocean floors, whereas the 
local subsidence of the foundations themselves is all that is necessary. 
The postulate will first be examined and refuted; the assumption will 
then be considered and its alternative will be preferred. 
The embayment of continental coasts is certainly of widespread oc- 
currence, but when the embayments are closely examined they are found 
to be of dates and dimensions so diverse that they cannot be explained 
by a universal rise of the ocean. In the first place, the most pronounced 
embayments are the fiord coasts of quaternary glaciation; there is good 
reason to believe that the great troughs of such coasts were scoured but 
deep below sea level by huge glaciers, and that, far from the ocean having 
recently risen to submerge the troughs, the coasts have in several fiord 
regions risen from the ocean, as their elevated shore lines testify; these 
