352 A NATURALIST IN THE PACIFIC chap. 



It may be remarked that nearly all the districts in which the 

 silicified corals and concretions, siliceous minerals, and Hmonite 

 occur, are scantily vegetated " talasinga " lands ^ with reddish soil. 

 Except in the instance of the Kalikoso plains, the swamps and 

 lakes have as a rule long since disappeared, their sites being alone 

 indicated by the limonite on the surface. In the Mbua plains, 

 however, there are occasional small ponds and swamps, and there 

 is no doubt that the limonite so bountifully represented on the dry 

 districts is still in process of formation. 



Before drawing some general inferences as to the conditions 

 under which this deposition of silica and iron took place, I will refer 

 to the characters of the materials thus produced. 



The silicified corals include massive corals of the Astraean and 

 " Pontes " kinds and branching specimens of the Madrepore type 

 or habit. The former are rarely larger than 7 or 8 inches across 

 and are merely fragments. The latter are always portions of 

 branches, never exceeding 3 or 4 inches in length. In the last case 

 it is sometimes possible to show, as in the case of a specimen found 

 on the Kalikoso plains, that before silicification occurred the dead 

 fragment of branching coral had been extensively eroded by solvent 

 agencies and had been penetrated by burrowing molluscs. The 

 larger blocks of massive corals have usually been extensively 

 chipped by the natives in obtaining flints. In past times they 

 were carried from one place to another, the result being that 

 occasionally they were brought to me in the mountain-villages, all 

 showing evidence of their having supplied flints to a past genera- 

 tion. 



These corals are as a rule completely silicified. When a 

 massive specimen is broken across it is not infrequently found 

 that whilst the coral structure is preserved in its outer part, the 

 inner portion is composed of a compact seemingly structureless 

 mass of bluish-white or pale-grey flint, which has the characteristic 

 microscopical appearance of chalcedony and a specific gravity of 

 2"59.2 It is from the more compact parts of the silicified massive 

 corals that the " worked " flints found on the surface were obtained, 

 though in some of them, as in the case of a " scraper " in my 

 collection, the traces of coral structure are still apparent to the eye. 

 Wichmann observed in the case of the silicified corals from Fiji 

 that the whole petrifying process appears to consist in the satura- 

 tion of the coral with silica, the coral structure being usually 



^ For the meaning of "talasinga" see p. 55. 



'^ The portion exhibiting the coral structure has a specific gravity of 2'54. 



