XXV LIMONITE AND IRON-SAND 357 



hydrochloric acid ; it gives off water and is evidently impure 

 limonite. The larger fragments, i to 2 inches in size, represent 

 the partial conversion into limonite of a basic volcanic rock with 

 much glass in the groundmass which formed probably the surface 

 of the basaltic flows of the plateaux. There must be an enormous 

 amount of this iron-stone in the island. The finer gravel has a 

 concretionary character, some of the pieces appearing like bits of 

 stick that have been converted into limonite. It seems to have 

 been formed during the disintegration of the rock on the moist 

 surface of these densely wooded basaltic plateaux ; the process was 

 not accomplished in ponds or swamps, but was carried out on 

 ordinary damp ground. 



It must be observed in the above connection that the soil in 

 the areas of basalt and basaltic andesites, which occupy a large 

 portion of the surface of the island, contains a large amount of fine 

 magnetic iron-sand. After heavy rains the foot paths glisten with 

 this fine material which has been washed out on the ground. 

 This is especially the case in the extensive scantily vegetated 

 " talasinga " regions where the basaltic rocks are disintegrating for 

 a considerable depth. The river-sand of these areas, after a little 

 washing, yields about 75 per cent, of magnetic iron grains which give 

 in some cases a slight titanium reaction. The amount of magnetic 

 iron-sand in these rivers, as for instance in the Yanawai and the 

 Wainunu, must be very great. In the beds of the small sluggish 

 streams on the surface of the Wainunu table-land the amount is also 

 very large. 



Any explanation of the origin of the extensive silicification 

 evidenced by the occurrence of silicified corals and siliceous con- 

 cretions on the surface in various parts of the lower regions of the 

 island will have to include that of the formation of the limonite 

 fragments so often accompanying them. The necessary conditions 

 would, I think, be afforded by an emerging land-surface during 

 the consolidation of the exposed calcareous muds and the subse- 

 quent draining of the new surface. On parts of the newly formed 

 land, there would follow the successive stages of sea-water, brack- 

 ish, and fresh-water swamps, such as are clearly indicated by the 

 abundance of silicified coral fragments that strew the surface of the 

 low-lying and often swampy districts around the fresh water lake 

 of Kalikoso. 



In such a locality as that of Kalikoso, there were no doubt at 

 the time of the emergence large tracts covered with chalky cal- 

 careous mud derived from reef-debris ; and it was during the con- 



