[V MOUNT KOROMA 51 



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



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



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



)luish scoriaceous basaltic andesite containing much glass in the 



;jroundmass 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 



i nd formed of a vesicular rather than a scoriaceous basaltic andesite 



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



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



f henocrysts 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 

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

 liaes representing the trend of the two dykes above noticed would 

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

 r( )cks of the dykes differ conspicuously from the prevailing doleritic 

 n )cks that form, as before remarked, the ancient flows, the average 

 k ngth of the felspar-lathes in the former being •i-'2 mm., in the latter 

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

 n( )w the exact situation would not be easy to discover, on account 

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



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

 be tween Wailea Bay and Lekutu is named Koroma and attains a 

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

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

 pi; lins, covered with the usual " talasinga " vegetation, reach inland 

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

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

 dn.ined by the Mbua river and its tributaries, the rock usually 

 ex Dosed at its surface being a decomposing porphyritic basaltic 

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

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

 fac : forms a part. 



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

 in 1 stream at the foot of the south slope of Mount Koroma. 

 W] ilst crossing some low wooded outlying hills in this locality, 

 [ ( ame suddenly upon what seemed like a desert in miniature, 

 :jui e bare of vegetation and occupying an area of some acres, 

 fie e a porphyritic basic rock, from some cause unknown to me, 

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



E 2 



