OBJECTIONS TO THE THEORY OF SUBSIDENCE. 279 
The Elevation Theory. — The view has been presented 
that, in place of subsidence, elevation is at the bottom in 
the origin of barriers and atolls. Coral reefs may, like sea- 
beaches, be made at different heights on the slopes of rising 
land; but this is not the result of elevation which is implied ; 
for barrier reefs and atolls are the objects whose origin is to 
be accounted for. 
a. Mr. G. C. Bourne’ found at Diego Garcia, an atoll of 
the Indian Ocean southeast of the Chagos Bank, that in 
wells sunk over the island, the rock, for a short way down, 
consists of horizontal layers, eighteen inches to three feet 
thick, of coral sand in two or three alternations with coral 
shingle containing corals in coarse masses; and he concludes 
that the coarser layer corresponds to a layer of growing corals 
and the sand layer to a deposit of sand that was spread over 
and killed the corals; and that this process was repeated two 
or three times or more. Then he says: “ aise the forma- 
tion to the surface and you get that stratification which you 
see on so many parts of the island, a stratification which can- 
not be explained on any theory of subsidence.” He does not 
seem to be aware that the elevation did not make the strati- 
fication; and that the stratification is on his view positive 
evidence of a period of submergence; and that the thicken- 
ing of such a series by additions above, as in the process 
described, would require a continued subsidence previous to 
the final elevation. In any case, the final emergence is all 
there is in such facts to support an elevation theory of coral 
islands. 
The case of Christmas Island, 1,200 feet high (p. 274), 
is one of upheaval; but the upheaval, if the horizontal ter- 
races are parts of successive layers, did not make the layers. 
1 Bourne, Nature, April 5, 1888. 
