104 
RATE OF GROWTH. 
Ch. IV. 
instead of this, it had quietly subsided, surely every 
part of the island which offered a solid foundation, 
would in a like manner have become coated with living 
coral. 
Through steps such as these, any thickness of rock 
composed of a singular intermixture of various kinds 
of corals, shells, and calcareous sediment, might be 
formed ; hut without subsidence, the thickness would 
necessarily be determined by the depth at which the 
reef-building polypifers can exist. If it be asked, at 
what rate in years I suppose a reef of coral favourably 
circumstanced could grow up from a given depth ; I 
must answer that we have no precise evidence on 
this head. It will, however, be hereafter shown that 
in certain large areas where subsidence has probably 
been long in progress, the growth of the corals has 
been sufficient to keep the reefs up to the surface ; and 
this is a much more important standard of comparison 
than any cycle of years. 
It may, however, be inferred from the following 
facts, that the rate under favourable circumstances 
would be far from slow. Dr. Allan of Forres has 
given, in his MS. Thesis deposited in the library of 
the Edinburgh University, the following account of 
some experiments, which he tried during his travels 
in the years 1830 to 1832 on the east coast of 
Madagascar. ‘ To ascertain the rise and progress 
of the coral family, and fix the number of species 
met with at Foul Point (lat. 17° 40'), twenty spe- 
cies of coral were taken off the reef and planted 
