5 2 A NATURALIST IN THE PACIFIC chap. 



the result is a surface of white crumbling rock scored deeply by 

 the rains and carved out by the denuding forces into miniature 

 hills and dales. It is not improbable that a small crater in its last 

 solfatara-stage once existed here ; but the whitened disintegrated 

 rocks alone remain, and we can now only hazard a conjecture as to 

 the cause. 



I found a variety of basic rocks exposed on the hill slopes up 

 to 900 feet. The most frequent of the deeper-seated rocks which 

 occurred in mass at this elevation, and as large blocks on the 

 lower levels, is a dark grey rather altered hypersthene-augite- 

 andesite, referred to genus 1 of that sub-class as described on page 

 286. The specific gravity is 273, whilst the groundmass displays 

 a little greenish altered glass. Another of the deeper rocks, 

 exposed 500 feet up the slopes, is placed in the same sub-class, 

 augite and rhombic pyroxene being porphyritically developed, 

 separately and as intergrowths. The groundmass displays short 

 stout felspars, augite, and a little altered glass. The rock is 

 therefore referred to the orthophyric order described on page 290. 

 Spec. grav. 278. 



Evidence of more recent surface lava-flows here exists. In 

 one place I came upon such a bed 12 feet thick, compact in its 

 upper half and slaggy or scoriaceous in its lower half. The rock 

 is an aphanitic augite-andesite (spec. grav. 277) and belongs to 

 species B, genus 16, of the augite-andesites, as described on page 

 281. Its groundmass displays felspar-lathes in flow-arrangement 

 with a little interstitial glass. Slaggy lava is not uncommon on 

 these slopes. One specimen beside me is a semi-vitreous form of 

 the deeper hypersthene-augite-andesites of this range. 



There appears to be better evidence of sub-aerial lava-flows on 

 the lower slopes of Mount Koroma than I found in any other part 

 of the island. It should have been before remarked that one of 

 these flows lies upon a bed of a hard reddish compact tuff, which 

 appears in the thin section as an altered palagonite-tuff, containing 

 fragments of minerals including both rhombic and monoclinic 

 pyroxene, but showing neither lime nor organic remains. The 

 larger fragments are 2 mm. in size. It seems likely that this flow 

 ran into the sea during the emergence of this part of the island. 



The prevalence of rocks of the hypersthene-augite-andesite 

 type in Mount Koroma distinguishes this range from the surround- 

 ing regions of olivine-basalts and basaltic andesites. This district 

 is well worth a detailed examination, and perhaps the remains of 

 a crateral cavity may yet be found. 



