515 
of Edinburgh, Session 1879 - 80 . 
that the reefs have commenced close to the shore and have extended 
seawards, first on a foundation composed of the volcanic detritus of 
the island, and afterwards on a talus composed of coral debris, and 
the shells and skeletons of surface organisms.* 
The lagoon channel was subsequently slowly formed by the solvent 
action of the sea water thrown over the reefs at each tide, and the 
islets in the lagoon channel are portions of the original reef still 
left standing. The reefs have extended outwards from the island 
and have been disintegrated and removed behind in the same way 
as the atoll has extended outwards after reaching the surface. 
Where reefs rise quite to the surface, and are nearly continuous, 
we find relatively few coral patches and heads in the lagoons and 
lagoon channels. Where the outer reefs are much broken up, the 
coral growths in the lagoon are relatively abundant. Where the 
water was deep and the talus to be formed was great, the outward 
growth has been relatively slow,f and the disintegrating forces in the 
lagoons and lagoon channels gaining in the struggle, the reefs would 
become very narrow and might indeed be broken up. This, however, 
would admit the oceanic waters and more food, and growth would 
again commence on the inner as well as the outer sides of the still 
remaining portions. In the great barrier reef of Australia, where the 
openings are numerous and wide, the reefs have a great width. Where 
the openings are few and neither wide nor deep (as in lat. 12° 30') 
the reefs are very narrow and “ steep to”— on their inner side. 
At the Admiralty Islands, on the lagoon side of the islets on the 
barrier reefs, the trees were found overhanging the water, and in some 
cases the soil washed away from their roots. It is a common obser- 
vation in atolls that the islets on the reefs are situated close to the 
lagoon shore. These facts point out the removal of matter which is 
going on in the lagoons and lagoon channels. 
Elevation and Subsidence .- — Mr Darwin has given many reasons 
for believing that those islands and coasts which have fringing reefs 
had recently been elevated, or had long remained in a state of rest. 
* A dredging in 155 fathoms, close to the harrier reef of Australia (between 
it and Raine Island), gave a coral sand, which was, I estimate, more than two- 
thirds made up of the shells of surface animals. 
t Hence in barrier reefs, where the deptji outside is very great, we find the 
reefs running closer to the shore than whes^ the depth is less, and conse- 
quently the talus to be formed is smaller. 
