GROWTH OF REEFS. 
317 
which is not determined by their existence. Moreover, 
to prove formation from drifting does not suffice to dis- 
prove subsidence. The length of Sandy Hook varies in 
consequence of the action of currents, yet this does not 
disprove Professor Cook’s conclusion that the New Jersey 
Coast is subsiding. 
Further, it must be borne in mind that subsidence has 
undoubtedly taken place in the region around Florida, 
though at present we can only prove this to have lasted into 
the earlier Pleistocene, the difficulty of dealing with this 
being augmented by the occurrences of elevated coral reefs, 
in Jamaica at 2,000 feet above the sea, and in Cuba perhaps 
even at the same, and certainly at 1,000 feet. The former, 
while obviously proving elevation, are considered by Mr. 
Crosby to prove also the occurrence of epochs when move- 
ment was in the opposite direction (see p. 309). 
The next section calls attention to the vague character 
of the evidence adduced for the building up from deep 
waters of important banks, composed of organic debris, to 
serve as the foundations of a coral reef, though doubtless 
such a thing may occasionally occur. But in regard to the 
existence of submerged mountain masses, which have been 
indicated as suitable for the like purpose, it may be well 
to give Professor Dana’s quotation of the argument, and 
his comment thereon. Mr. Murray ‘ observes that the 
“ soundings of the Tuscarora and Challenger have made 
known numerous submarine elevations : mountains rising 
from the general level of the ocean’s bed, at a depth of 
2,500 or 3,000 fathoms up to within a few hundred fathoms 
of the surface.” But “ a few hundred fathoms,” if we make 
“ a few ” equal 2, means 1,200 feet or more, which leaves a 
long interval yet unfilled.’ 
Lastly, Professor Dana reviews the proposed explana- 
tion of the ring-like shape of atolls, and of the channels 
which exist in the enclosure of atolls or between atolls ; 
22 
