596 MU. J. J. LISTER ON THE GEOLOGY 



Nomuka itself is a limestone island, and will be described later; 

 but Mango, Tonua, Nomuka-iki, Tonuraeia, and probably Kelefasia 

 are formed of volcanic tuffs. 



The island of Maugo lies about six miles E.S.E. of Nomuka. 

 The main part of the island is some two miles in length, and of an 

 oval shape, with the long axis running east and west. Projecting 

 to the south of the west end of the island there is a small peninsula 

 connected with the main mass by a narrow neck. At the eastern 

 and western ends of the island there are rounded hills attaining 

 a height of about 150 feet above sea-level. 



This island is composed for the most part of layers of volcanic tuffs 

 which var}' widely in character. The summit of the eastern hill 

 is formed of thick and approximately horizontal layers of fine white 

 material which is largely calcareous, but contains a fine scoriaceous 

 residue. The western hill is composed of layers of a coarse sandstone 

 dipping at an angle of about 2° or 3° towards the W.S.W., and con- 

 sisting of rounded fragments of lava embedded in a calcareous 

 matrix. Fragments of coral, some of them 6 inches in diameter, 

 are mixed with the volcanic fragments. Between tlie two hills 

 layers of the two kinds of rock alternate with one another. 



The southern peninsula is formed of a mass of breccia traversed 

 in various directions by cracks, but presenting no regular stratifi- 

 cation. The fragments are embedded in a calcareous matrix and 

 vary greatly in size. Lying on the surface are some large boulders, 

 some of coral, some of volcanic rock, which appear to have been 

 isolated by the gradual removal of the finer material in which they 

 were embedded. The largest of these was a rounded mass of coral 

 which measured 10 feet in length, 7 feet in breadth, and 3J feet in 

 height. In the cliff which surrounds this part of the island, frag- 

 ments of coral were thickly scattered among the volcanic con- 

 stituents. I counted 43 pieces exposed in one square yard. Though 

 Mango is surrounded by a broad fringing reef, I failed to find any 

 raised coral-rock upon it. 



The island appears to have been originally formed as a sub- 

 marine bank, probably of volcanic origin, on which corals grew. On 

 a return of volcanic activity the coral-reef was broken up, and the 

 fragments of it mixed with the volcanic materials have formed the 

 breccias of which the island is composed. 



It appears that the southern promontory was the nearest point of 

 the present island to the centre of the eruption, for there the con- 

 stituents of the breccia are largest and mixed indiscriminately 

 without stratification. The nearly horizontally stratified rocks on 

 the eastern and western hills were laid out now in finer, now in coarser 

 layers by the action of the water. The island has subsequently 

 been elevated to its present height, the rounded outline of the hills 

 being due to subaerial denudation. 



It is remarkable that while the island is now surrounded by 

 flourishing coral-reefs, there should be no old raised reef upon it. The 

 most probable explanation appears to be that elevation took place 

 too rapidly to allow of the growth of reefs of any considerable extent. 



