664 
GEOLOGY: R. A. DALY 
appearance due to the unequal reflection of light where the fibers run 
in different directions. 
The structural phenomena discussed in the preceding paragraphs 
all furnish evidence tending to confirm the writer's theory of the ori- 
gin of cross-fiber veins, and they are difficult or impossible of expla- 
nation under any other hypothesis of vein formation hitherto advanced. 
Conclusions. Cross-fiber veins are formed through a process of lat- 
eral secretion, the growing veins making room for themselves by pushing 
apart the enclosing walls. The fibrous structure is to be attributed 
largely to the mechanical limitation of crystal growth through the ad- 
dition of new material in only one direction. In the case of the asbesti- 
form minerals the fibrous structure is accentuated by a normal prismatic 
habit and cleavage. 
* The experiments herein referred to are described in my more extended paper on "The 
Genesis of Asbestos and Asbestiform Minerals" which will appear in an early number of Bull. 
Amer. Inst. Min. Eng. 
2 For a review of these theories, see Fritz Cirkel, Chrysotile asbestos, its occurrence, ex- 
ploitation, milling and uses, Ed. 2, Canada Dept. of MineSy Mines Branch, Report No. 69 
(1910); O. B. Hopkins, A report on the asbestos, talc and soapstone deposits of Georgia, Geol. 
Survey Ga., Bull. No. 29 (1914); and J. A. Dresser, Preliminary report on the serpentine 
and associated rocks of Southern Quebec, Canada Dept. Mines Geol. Survey Mem. No. 22 
(1913), 
' Dresser, J. A., Ibid., p. 65. 
4 Pratt, J. H., Asbestos, U. S. Geol. Survey Mineral Resources, 1904, pp. 1137-1140 (1905). 
6 Diller, J. S., Asbestos, U. S. Geol. Survey Mineral Resources, 1907, pt. II, pp. 720-721 
(1908). 
^ Taber, Stephen, The growth of crystals under external pressure, Amer. J. Sci., ser. 4, 
41, 532-556 (1916). 
A NEW TEST OF THE SUBSIDENCE THEORY OF CORAL 
REEFS 
By Reginald A. Daly 
DEPARTMENT OF GEOLOGY AND GEOGRAPHY, HARVARD UNIVERSITY 
Received by the Academy, October 27, 1916 
According to Charles Darwin's much-discussed theory, a barrier 
reef represents the upgrowth of a coral reef which originally fringed a 
sinking (generally volcanic) island; and an atoll reef represents a fur- 
ther upgrowth, completed in typical form after the central island has 
sunk below sea-level. Between the upgrowing reef and the subsiding, 
central island is a concavity or *moat.' Through the accumulation of 
detritus and shells and skeletons of organisms inside the reef, the moat 
is slowly filled. Above the detrital covering of the moat surface is the 
water of the 'lagoon.' The subsidence is supposed to have progressed 
