5. Sub-order, Prismatic, of the Hypersthene-Augite-Andesites

(Felspar-lathes in flow-arrangement)

Formula.—Hypersth-aug, matr, flu, prism.

17genus(Vitr.) See Synopsis.
18"(Opac.)
19"(Magn.)
20"(Parv.)

The rocks of this sub-order that are represented in my collection admit easily of a general description, and since the diagnoses of the genera are given in the Synopsis, there will be no need to separately describe each genus.

Almost without exception these rocks form a constituent of agglomerates in various parts of the island; and they occur in this condition in some of the highest mountains, as Mariko, Thambeyu, and Koro-mbasanga. The exception refers to a low mound-like hill, apparently a “volcanic neck,” that rises from the basaltic plains west of Mbua (see page [58]).

In about half of the specimens the rocks are referred to section 10, where the plagioclase phenocrysts are either small and very scanty or are absent altogether. In a fair number these phenocrysts are opaque (genus 18); whilst in a few they are glassy (genus 17). The rocks are typically blackish or dark grey, and often have a pitchstone-like appearance, the groundmass being frequently semi-vitreous in character. Vesicular and scoriaceous rocks occur at times.

In all cases the felspar-lathes and pyroxene prisms are more or less in flow-arrangement; whilst pyroxene granules, if present, are subordinate. The felspar-lathes, which are either simple or once-twinned and give extinctions of acid and medium andesine, are usually small, and average in different sections ·05-·08 mm. in length. The pyroxene prisms are pale brown and are ·03 or ·04 mm. long. Most of them give oblique extinctions of over 25; but in the same slide some give straight extinctions; the proportion varies in different rocks. The pyroxene phenocrysts in all the specimens are small (not over 2 mm.), and are rhombic and monoclinic. In most sections the two forms are represented by separate crystals and are also associated in the same phenocryst. Those of rhombic pyroxene have often dark borders. There is a considerable amount of a pale brown glass in the groundmass, more or less devitrified. The specific gravity varies considerably, but is as a rule between 2·55 and 2·75, the more basic rocks containing augite in preponderance and basic andesine, whilst the less basic possess a large proportion of rhombic pyroxene and display oligoclase-andesine. Sometimes, as in the case of a rock composing an agglomerate east of Nanduri, where the porphyritic plagioclase is opaque and there is some degree of alteration, the rock looks very like a porphyrite.