Note 173 — Chapter VIII

These rocks present in microscopical structure a decided resemblance to the Carboniferous lavas of the Bathgate Hills (Falconer, Trans. Roy. Soc. Edin., Vol. XLV., 1906, p. 133) in Scotland. With the exception that the doleritic types are somewhat less weathered, the description and classification of the Bathgate rocks may be applied almost word for word to these Tertiary lavas of Nigeria. Doleritic rocks (Falconer, op. cit., Plate I., Fig. 2) with phenocrysts of augite and olivine, and more rarely of felspar are common on the Morrua plateau in eastern Nassarawa and along the base of the escarpment between Darroro and Assab (928, 1573, 1630). Felspar is abundant in the ground-mass and the augite occurs in granules, granulitic aggregates, small ophitic plates, or minute idiomorphic crystals between the felspars. The basalts of Zammagan (1578) and Buratai (1640) show a typical fluxional arrangement of the felspars (Falconer, op. cit., Pl. I., Fig. 3). The basalt of Kwoia (1654) exhibits an intimate mixture of felspar and augite in the ground-mass (Falconer, op. cit., Pl. I., Fig. 4). The basalt of Assab (934) contains scattered felspars with abundant minute granular aggregates of augite, while a similar rock from Buratai and Dukshi (1641) contains some residual brown glassy material and passes into limburgitic varieties with phenocrysts of augite and olivine in an ill-defined microlitic base.