330 
APPENDIX II. 
example, the blocks of recent limestone which were dredged 
by Professor A. Agassiz off the Florida reef. 1 Under what 
circumstances, then, will the sea- water act as a solvent on 
the dead coral ? I think we must reply, When the fluid 
is rather rapidly altering its position in regard to the sub- 
stance attacked. Thus rain and streams are important 
solvents, and so might be breaking waves or tidal ebb and 
flow, but when the water is at rest or is only spreading 
with a slow, diffusive movement, its solvent action is ex- 
tremely slight. For instance, chalk often is, and must 
often have been, saturated with water, yet numbers of the 
minute organisms which enter into its composition are still 
perfectly distinguishable. The same is true of many 
other limestones ; indeed the effect of water often seems 
conservative rather than destructive. It sinks down into 
the body of the rock, carrying with it the carbonate of lime 
which has been obtained from the exposed superficial part 
of the mass, but on reaching the level of saturation, when 
it only percolates by diffusion, it commonly deposits its 
burden, filling up with mineral calcite the interstices of the 
organic materials. Hence the comparatively quiet waters 
of a lagoon would be favourable to the consolidation rather 
than to the destruction of the dead coral, save only within 
a very limited distance from the surface. Moreover, the 
remains of organisms, when once the interstitial animal 
tissues have been replaced, appear to be less soluble than 
the other parts of a rock, as is indicated by the familiar 
1 weathering out ’ of fossils. Beef rock also appears very 
apt to assume a solid and semi-crystalline condition 
(p. 17), and in regard to this we must not overlook a 
peculiarity of coral which, as' it seems to me, has an 
important bearing on the subject. Dead coral is very 
readily converted into dolomite, which is a much less 
soluble salt than calcite. Further, the conditions which 
1 See p. 288 of this work. 
