VII THOKA-SINGA 103 



columnar in structure. Kala-kala, about 1,600 feet above the sea, 

 is an imposing-looking hill with perpendicular cliff-faces on some 

 of its sides. I did not ascend it, but found at its base a rock of the 

 same andesitic type, differing from that of Ndrandramea in the 

 more crystalline character of the groundmass, and having a specific 

 gravity of 2-61. West of Kala-kala is the outlying hill of Vatu 

 Mata with a flat top and rising only about 400 feet from its base. 

 It has all the appearance of being composed of the same andesitic 

 rocks. It is shown on the left-hand in the illustration. 



Lying south of Kala-kala are the two peaks of Mako-mako and 

 Thoka-singa, rising respectively 1,400 and 1,300 feet above the sea. 

 I ascended the last-named, which has a rounded summit covered 

 with trees. Approaching it from Nambuna on the east, I found at 

 its foot a large mass of pitchstone-agglomerate, formed of fragments 

 of vitreous basic rocks, such as occurs around the lower part of 

 Soloa Levu on the other side of the valley. The slopes of Thoka- 

 singa, between 200 and 450 feet below the summit, are strewn with 

 masses of another kind of agglomerate made up of blocks 3 to 8 

 inches across, occasionally rounded, and composed of the same 

 felsitic andesite, of which the mass of the hill is formed. This last- 

 named rock is exposed in bulk in the upper part, but on the summit 

 the agglomerate reappears. It has a granitoid appearance, and is 

 distinguished from the acid andesites of the other hills of the 

 Ndrandramea district by its greater specific gravity (272 to 2-74), by 

 its holocrystalline texture, and by the coarse grain of the mosaic 

 of its felsitic groundmass, which is probably quartz-bearing but is 

 relatively scanty. It is, however, referable to the same group of 

 felsitic andesites, but is to be placed at the basic end of the series. 

 (Its description is given on page 302.) In Thoka-singa we have 

 therefore a hill which is evidently formed in mass of these holo- 

 crystalline felsitic andesites but covered in places with an agglomer- 

 ate of the same materials. I have already referred to this feature 

 in the structure of Ndrandramea. Since the blocks are sometimes 

 rounded, such agglomerates may represent the result of marine 

 erosion during the emergence of this part of the island. In the 

 case of Navuningumu, where they lie abruptly on calcareous clays 

 containing tests of foraminifera and shells of pteropods, a different 

 explanation appears to be needed. 



The Hill of Soloa Levu. — This isolated hill, which 

 presents another type of these acid andesites, has a broad rounded 

 summit ; and though elevated about 1,600 feet above the sea, the 

 hill itself rises only 800 or 900 feet above the country at its base, 



