356 A NATURALIST IN THE PACIFIC chap. 



beautiful globular aggregate, each globule having a nucleus of the 

 iron oxide and giving a black cross in polarised light. In another case 

 the globular structure is less perfect, and the chalcedonic groundmass 

 is penetrated by a multitude of fine cracks filled with iron oxide. 



The deposits of limonite vary in character in different localities, 

 and evidently they have not all the same history. The soil of the 

 low-lying plains around Wai-ni-koro and Kalikoso, and especially 

 in the vicinity of the fresh-water lake, is often coloured a deep 

 ochreous red. Small fragments of an earthy yellowish-brown 

 limonite occur on the surface in quantity and are particularly 

 abundant near the lake. They yield much water when heated. 

 In some places in this district, as in the country traversed between 

 Wai-ni-koro and Kalikoso, the surface is strewn with a number of 

 small round concretions of the size of small marbles (6 to 12 mm.) 

 which are composed of a mixture of carbonate of iron and limonite, 

 but show no recognisable structures. They are somewhat friable 

 and give off much water when heated, whilst they effervesce freely 

 in hot hydrochloric acid. It is probable that some of the earthy 

 limonite of the Kalikoso district contained originally iron carbonate 

 and has been produced from concretions such as I have just 

 described. 



The variety of limonite found in fragments on the surface of 

 the plains of Mbua, Lekutu, and Sarawanga, at elevations usually 

 of 100 or 200 feet above the sea, is a heavy compact kind with a 

 specific gravity of 3 to 3*5, and closely resembling red hematite. 

 Since, however, it is lighter in weight and still contains a little water, 

 it may be regarded as in the transition stage. It occurs as portions 

 of cake-like masses varying usually from a third of an inch to 

 rather over an inch in thickness. As a rule it is found in localities 

 where no lakes or swamps now exist and may be associated, as in 

 the Sarawanga and Lekutu plains, with silicified corals and silice- 

 ous concretions ; but in some cases, as in that of the Mbua plains, 

 ponds and swamps are still scantily represented in the vicinity, 

 and the water of the stagnant streams is deeply coloured with iron 

 (see page 56). 



Ironstone gravel occurs in great quantity strewn over the 

 surface of the basaltic table-lands, especially in the case of that 

 between the Wainunu and the Yanawai rivers. The smaller 

 gravel varies usually between one eighth and one third of an 

 inch in size, the larger fragments being about an inch. The 

 specific gravity is 3*1 to 3'2. The material forming the finer gravel 

 dissolves with but little effervescence and scanty residue in hot 



