GEOLOGICAL CONCLUSIONS. 147 
abound in rivers, this African coast has only some small streamlets. 
Thus the influence of Continental waters and detritus on the distri- 
bution of reefs, has been shown to be very marked. But about the 
Pacific islands, where streams are small, the same cause has had little 
effect, seldom doing more than modifying somewhat the shores and 
bottom of a harbour. We have ascertained that in different groups, 
as the Ladrones, the Sandwich Islands, Samoa, New Hebrides, there 
is an inverse relation between the extent of reefs and the evidences of 
recent volcanic action in the islands; and that the largest reefs exist 
where there is no proof of former igneous action, or where it has long 
ceased. The influence of volcanic agency on the planting and increase 
of coral reefs is thus satisfactorily exhibited. ‘The existence of large 
reef-islands in open seas, where the neighbouring lands are mostly 
destitute of coral reefs, has farther supported our conclusions, as such 
islands are in general removed from the deleterious influences just 
mentioned. 
The modifications of form and interruptions of reefs arising from 
abrupt or sloping shores, and tidal or local currents, have also been 
exemplified. ‘The origin of the distant barrier has been traced to a 
sinking of the land which it once simply fringed; and the lagoon 
island to a continuation of this subsidence till the original land had 
disappeared. 
This account of coral reefs and islands may be closed by a state- 
ment or recapitulation of some deductions which have a special bearing 
upon geology. 
IV. GEOLOGICAL CONCLUSIONS FROM THE STRUCTURE 
AND COMPOSITION OF CORAL REEFS AND ISLANDS. 
I. The coral reef-rock has been described as solid limestone of 
coral origin. In some parts it is a coral conglomerate, or breccia, 
made up of fragments firmly cemented. Over much larger areas it 
is a fine white limestone, as compact as any secondary marble, and 
as homogeneous in texture. It is often free from any traces of 
organic life, or proofs of an organic origin. Only now and then an 
imbedded shell or some other relic evinces that animals of any kind 
were living in the seas. ‘This white limestone breaks with a con- 
choidal fracture, a splintery surface, and rings under the hammer. 
These facts are of great importance in deciding upon the origin of the 
