IV MOUNT KOROMA 51 



referable to genus 12 of the augite-andesites, described on page 275.) 

 A few paces west of this spur a vertical dyke, 20 feet wide and 

 trending N.W. and S.E., appears on the beach. It is formed of a 

 bluish scoriaceous basaltic andesite containing much glass in the 

 groundmass and showing imperfectly developed felspar lathes. It 

 is included in genus 4 of the augite-andesites described on page 270. 



A little east of the spur there is another dyke apparently vertical 

 and formed of a vesicular rather than a scoriaceous basaltic andesite 

 referred to genus i of the augite-andesites (page 267). It differs from 

 the rock of the previous dyke in the presence of small plagioclase 

 phenocrysts which contain abundant magma-inclusions ; but it 

 resembles it in the characters of the groundmass. This dyke is 

 about 40 feet in thickness and trends N.E. and S.W. 



It may be inferred from the foregoing remarks that there was at 

 one time a volcanic vent in the district west of Nukunase. The 

 lines representing the trend of the two dykes above noticed would 

 if extended meet at a common focus a little way inland. The 

 rocks of the dykes differ conspicuously from the prevailing doleritic 

 rocks that form, as before remarked, the ancient flows, the average 

 length of the felspar-lathes in the former being •1-2 mm., in the latter 

 •3-'4 mm. Both, however, belong probably to the same vent of which 

 now the exact situation would not be easy to discover, on account 

 of the re-shaping of the surface through the denuding agencies. 



Mount Koroma. — The highest peak of the hills lying inland 

 between Wailea Bay and Lekutu is named Koroma and attains a 

 height of 1,384 feet. I did not ascend its slopes higher than 900 

 feet, and approached it from the Mbua or south side. Extensive 

 plains, covered with the usual " talasinga " vegetation, reach inland 

 from the shores of Mbua Bay to the foot of this range without 

 attaining a greater elevation than 100 feet. This low district is 

 drained by the Mbua river and its tributaries, the rock usually 

 exposed at its surface being a decomposing porphyritic basaltic 

 andesite. It is again referred to on page 56 in connection with the 

 low-lying level region of this portion of the island of which it in 

 fact forms a part. 



A basic non-calcareous fine-grained tuff-sandstone is exposed 

 in a stream at the foot of the south slope of Mount Koroma. 

 Whilst crossing some low wooded . outlying hills in this locality, 

 I came suddenly upon what seemed like a desert in miniature, 

 quite bare of vegetation and occupying an area of some acres. 

 Here a porphyritic basic rock, from some cause unknown to me, 

 has decomposed in the mass to a depth of 20 feet and more ; and 



E 2 



