ch. xxv SILICIFIED CORALS AND FLINTS 351 



of contact or general metamorphism that we have here to deal ; 

 but with the remarkable surface-collections of silicified corals, 

 nodules and flints of chalcedony, fragments of white quartz-rock, 

 bits of jasper, and certain curious siliceous concretions, that occur 

 often in association with fragments of limonite in these low-lying 

 regions. All the siliceous materials above named have, as the 

 microscope indicates, a common character, chalcedonic silica in a 

 greater or less degree being the basis of all of them, whether 

 coral, flint, white quartz-rock, or jasper. It soon became apparent 

 whilst examining these districts that one general condition pre- 

 vailed whilst this extensive deposition of silica and the formation 

 of the beds of limonite were in progress. It cannot, however, be 

 pretended that these processes are actually in operation on the 

 plains now. Except in the case of the limonite in a few localities 

 the processes have been suspended ; but they were in active 

 operation not long ago : and an examination of the general char- 

 acters of the districts will probably disclose some of the conditions 

 under which these products have been formed. 



On the surface of the Kalikoso plains, where these materials 

 are most abundant, we find silicified corals associated with frag- 

 ments and nodules of chalcedony, flints, white quartz-rock, limonite, 

 concretions of carbonate of iron, &c, in the low-lying and often 

 swampy district around the fresh-water lake, the whole region 

 being only elevated between 20 and 60 feet above the sea. This 

 is an area of decomposing acid rocks (quartz-porphyries, trachytes). 1 

 On the other hand in most of the regions where these materials 

 occur on the surface we have areas of basic rocks (basalts and 

 basaltic andesites) incrusted in places with submarine tuffs and 

 foraminiferous clays, the volcanic rocks undergoing extensive 

 disintegration. Such for instance are the Lekutu, Sarawanga, 

 Ndreketi, and Lambasa plains. In the Lambasa plains, which 

 are described in this connection on page 139, we find besides the 

 corals and flints and nodules of chalcedony, fragments of jasper. In 

 the Sarawanga and Lekutu lowlands, we find silicified corals and 

 limonite ; but here the crystallised silica of the corals contains a 

 large quantity of water, whilst in its lesser degree of hardness and 

 in its low specific gravity it comes near to semi-opal. In these 

 and other localities, as in the level country around Ndranimako on 

 the right side of the Yanawai estuary, we find curious concretions 

 of the same kind of hydrous silica more or less crystalline. These 

 concretions are described below. 



1 The region is described on pp. 224.-22S. 



