XXV CONDITIONS OF SILICIFICATION 359 



investigation is needed to establish any view on the subject. In 

 the Kalikoso plains, however, we have a critical locality for the 

 pursuit of this inquiry. Concretions of carbonate of iron and 

 deposits of earthy limonite are here associated with silicified corals 

 on the surface of a level and often swampy district around a fresh- 

 water lake in a region which is only elevated 20 to 60 feet above 

 the sea. We are dealing here with an area of land that has 

 emerged in comparatively recent times as far as the history of the 

 island is concerned. The element of time is limited, and the 

 problem is not complicated, as it would be in the case of an old 

 land-surface, raised some hundreds of feet above the sea, by the 

 intrusion of many other disturbing agencies. Nature has simplified 

 matters here for the inquirer. 



The evidence of recent emergence with regard to the whole 

 island is discussed in Chapter II., and need not be again referred to 

 here ; whilst the general description of the Kalikoso district is 

 given in Chapter XVI. In this connection it may be remarked that 

 before their emergence the Kalikoso plains were covered by the 

 waters of a large irregular sea-water lagoon or lake, which though 

 more or less surrounded by hills had free communication with the 

 sea on the north along the line of the passages now occupied by 

 the Wai-ni-koro and Langa-langa rivers. Both massive and 

 branching corals then thrived in the waters of the lagoon. There 

 is no ground for supposing that during the emergence there was an 

 intermediate stage characterised by brine-ponds and salt-swamps. 

 The drainage from the slopes of the mountains to the southward 

 would have prevented it. Whilst this change of level was in 

 operation, brackish water collected in the deeper part of the 

 original lagoon, forming a lake which as evidenced by the present 

 distribution of limonite on the surface of the plains was then far 

 larger than it is now. As the plains became exposed large flats 

 covered with chalky mud in which dead corals were more or less 

 imbedded were bared ; and there and then as the drying and con- 

 solidation proceeded silicification took place in the manner before 

 surmised. This deposit was of no great thickness, and has been 

 since removed by the denuding agencies, whilst the silicified corals 

 remain behind. 



When in the Solomon Islands I was unable to find the source 

 of the chalcedonic worked flints of such frequent occurrence in that 

 region. In my general work on those islands (pp. "jj to 80) refer- 

 ence is made to this subject. It will probably be shown that there 

 as in Fiji most of the flints are silicified corals. 



