SUMMARY OF ARGUMENTS. 
325 
life. 3. That lagoons and lagoon channels are materially 
enlarged by the destruction of dead coral through the 
solvent effects of sea-water. 4. That in the past history 
of the earth we find no evidence in favour of the formation 
of coral reefs in areas of subsidence, or in other words 
that fossil coral reefs are less than some 25 fathoms thick. 
1. Much stress is evidently laid upon the fact that 
many coral islands afford evidences of a certain amount of 
upheaval. This amount, in most cases, is but slight, and 
its significance appears to me to have been exaggerated. 
Undoubtedly, it proves that the record which is the most 
obvious indicates an upward and not a downward motion, 
but in so doing it introduces a difficulty which will 
presently be noticed. These indications, however, do not 
of themselves prove a general upheaval, but only oscilla- 
tion. Every geologist is aware that movements in any 
given direction are frequently neither uniform nor contin- 
uous. For instance, no one doubts that the western coast 
of Scandinavia, and, in a less degree, that of Great Britain, 
have very considerably subsided since the sculpture of their 
leading physical features, and yet from the Land’s End to 
the North Cape we constantly find proofs that the latest 
movements have been in an upward direction. Even in 
the case of the more important, but much rarer, upheaval 
of reefs, as at the island of Cuba, the coral masses are so 
thick that we must assume the practical arrest of all up- 
ward movement during the growth of the reef. In this 
case also, if the coral reef be only a sort of cap concealing 
a hill of pre-existent rock, we may reasonably be surprised 
that the ‘ ashlar- work ’ of coral limestone lias in no case 
so far yielded to the action of the atmospheric agencies as 
to lay bare its inner support. 
Doubtless there are many reefs to which either explan- 
ation might be applied, but there are some which, unless 
coral polyps can build at depths much greater than 25 
