368 A NATURALIST IN THE PACIFIC chap. 



repellent power of 28°. Specimens of rock obtained below the top 

 show no polarity, the mountain being well wooded except at the 

 summit. In the adjacent mountain of Koro-mbasanga,^ the 

 polaric rocks are limited to those exposed in the peak which is 

 bared of vegetation. The rocks in question are tuff-agglomerates, 

 the small blocks of pyroxene-andesite standing out from the tuff 

 having a polaric force of 14° or 15". This effect has been produced 

 in greatest intensity in the isolated peak of Navuningumu (1,931 

 feet) in the Ndrandramea region. Here the bare summit is formed 

 of a semi-vitreous, slightly vesicular, basaltic andesite with a 

 specific gravity of 2-82 in its present condition.^ This rock is 

 powerfully polaric, and rendered the compass useless, the deviation 

 generally to the westward varying from 20° to 50°. I place its 

 repellent force at about 90°, hand specimens affecting the magnetic 

 needle at a distance of 13 or 14 inches. None of the various rocks 

 obtained from the wooded slopes below displayed polarity. 



The neighbouring mountain of Ngaingai is composed entirely 

 of dacite having a specific weight of 2-57. The highest point of 

 the summit, 2,448 feet above the sea, is bare and rocky, and the 

 stone here is markedly polaric, the repellent force being about 25°. 

 Specimens from the lower wooded slopes show no polarity. Near 

 by rises the hill of Ndrandramea, which is composed in mass of 

 acid andesites or dacitic rocks. The summit (1,800 feet) is scantily 

 vegetated, and here the somewhat weathered rock which has a 

 specific weight of 2-44 (probably near 2*5 in the fresh condition) 

 has a polaric force of 14°. Specimens of a more compact rock 

 taken from the wooded slopes 300 feet below the summit 

 (sp. gr. 2-58) and from 700 feet below the top (sp. gr. 2'68) showed 

 no such effect ; but a specimen taken from a mass of agglomerate 

 in the last locality repels the needle 12°. Its specific gravity is 

 2 '6 1, and no doubt the mass had been originally a portion of an 

 exposed cliff-face.^ 



The summit of Mariko (2,890 feet), the Drayton Peak of the 

 chart, is formed of a rubbly agglomerate of a compact basic andesite. 



' This name has been wrongly applied in the Admiralty chart to the 

 mountain of Mbatini. Koro-mbasanga, 2,500 feet, lies three miles to the 

 north. 



^ This rock is described on p. 109. There is no exceptional development 

 of magnetite for a basic rock in the groundmass. 



' Unfortunately, I have no data for the peaks of Na Raro and Vatu Kaisia, 

 except that specimens obtained below the summits are non-polaric. In the 

 case of Na Raro I did not retain the specimen obtained at the top ; whilst in 

 my ascent of Vatu Kaisia I did not quite reach the summit. 



