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 2·73, 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. 2·78.
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. 2·77) 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 surrounding 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.