Sect. II. 
EATE OF GKOWTH. 
107 
This fact in some degree corroborates the result of 
Dr. Allan’s experiments. The case of the schooner- 
channel, choked up with coral in an interval of less 
than ten years, in the lagoon of Keeling atoll, should 
be here borne in mind. We may also infer, from 
the trouble which the inhabitants of the Maldiva 
atolls take to root out, as they express it, the coral- 
knolls from their harbours, that their growth can 
hardly be very slow. I may add, that M. Duchassaing 
broke off all the Madrepores growing on a marked 
place in a hay at Guadaloupe ; and in the course of 
two months he found there a greater number of 
corals than before. 1 
From the facts given in this section, it may he 
concluded, first, that considerable thicknesses of rock 
have certainly been formed within the present geo- 
logical era by the growth of corals and the accumu- 
lation of their detritus ; and, secondly, that the 
increase of individual corals and of reefs, both out- 
wards or horizontally, and upwards or vertically, under 
conditions favourable to such increase, is not slow, 
when referred either to the standard of the average 
ijigas being embedded in coral rock. But it should be remembered, 
that some species of this genus invariably live, both whilst young 
and old, in cavities, which the animal has the power of enlarging 
with its growth. I saw many of these shells thus embedded in the 
outer ‘ flat ’ of Keeling atoll, which is composed of dead rock ; and 
therefore the cavities in this case had no relation whatever to the 
growth of coral. M. Lesson, also, speaking of this shell (Partie 
Zoolog., Voyage de la Coquille), has remarked, ‘ que constamment 
ses valves 6taient engages complement dans la masse des Madrd- 
pores.’ 
1 L’Institut, 1846, p. 111. 
