358 A NATURALIST IN THE PACIFIC chap. 



solidation of this mud in the recently reclaimed area that the frag- 

 ments of coral imbedded in it became silicified. In these cases 

 where the imbedded corals were already much decayed, it is proba- 

 ble that the empty cavities thus produced were filled with silica, 

 and that in this manner the nodules of chalcedony were produced. 

 Here and there a pebble or a larger block of a volcanic rock would 

 have been inclosed in the mud ; and in this case also silica largely 

 replaced the original material of the stone. I imagine that with 

 the evaporation of the water in the mud during the drying and 

 consolidating processes the proportion of silica in solution would 

 attain a degree of super-saturation and that the silicification would 

 hence be brought about. 



With the consolidation of the mud the deposition of silica 

 ceased ; and in the case of any coral fragments, where the trans- 

 formation was not completed, decay would often commence. In 

 the instance of some bits of coral found imbedded in foraminiferous 

 mud- rock in the Lambasa plains the process of the change had 

 been suspended, and the fragments were in a state of decay, and 

 coloured red by iron oxide. If silicification occurred in a sub- 

 marine deposit only after it became a portion of an old land-surface 

 we ought not to find incompletely silicified corals inclosed in it. 

 For these reasons I do not consider that silicification would occur 

 in the case of submarine deposits long after they have been raised 

 above the sea. 



On the other hand it would seem that the deposition of silica 

 in the hard parts of dead organisms does not proceed in the 

 shallow-water calcareous mud of coral reef coasts previous to 

 emergence. Silicified corals have never as far as I know been 

 found under such conditions. Nor could the coral fragments now 

 lying on the Kalikoso plains, often only elevated some 20 or 30 

 feet above the sea, have undergone this change whilst exposed on 

 the land-surface as they now lie. They must have been inclosed 

 in some material containing abundant free silica ; and it is reason- 

 able to suppose that this material was the chalky mud of the reef- 

 flats on which they once lived. If this is admitted, then it follows 

 that since, as above assumed, silicification does not occur in such a 

 mud either before upheaval or long after it has been raised above 

 the sea, it must take place in the intermediate period, or in other 

 words whilst the recently exposed submarine deposits are con- 

 solidating and drying. 



Several objections at once occur with reference to this explan- 

 ation of the silicification of corals in this island ; but much more 



