Ch. VI. 
RECAPITULATION. 
193 
with respect to the whole amount of subsidence neces- 
sary to have produced the many atolls widely scattered 
over immense spaces, the movement, as already shown, 
must either have been uniform and exceedingly slow, 
or effected by small steps separated from each other by 
long intervals of time, so as to have allowed the reef-con- 
structing polypifers to bring up their solid frameworks 
to the surface ; and this is one of the most interesting 
conclusions to which we are led by the study of coral- 
formations. We have little means of judging whether 
many considerable oscillations of level have usually 
occurred during the elevation of large areas ; but we 
know from clear geological evidence, such as trees still 
standing upright at successive levels and covered by 
marine strata, that this has frequently been the case ; 
and we have seen on our map, that some of the same 
islands after having subsided, have been upraised ; and 
that others after having been uplifted, have subsided. 
We may therefore conclude that the subterranean 
changes which cause some areas to rise and others to 
sink, have generally acted in a closely similar manner. 
Recajntulation . — In the three first chapters, the 
principal kinds of coral-reefs were described in detail, 
and they were found to differ little, as far as relates 
to the actual surface of the reef. An atoll differs from 
an encircling barrier-reef only in the absence of land 
within its central expanse ; and a barrier-reef differs 
from a fringing-reef only in being placed, relatively 
to the probable inclination of its submarine foundation, 
