xx HYPERSTHENE-AUGITE-ANDESITES 289 



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



Andesites 

 {Felspar-lathes in flozv-arrangement) 



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



See Synopsis. 



The rocks of this sub-order that are represented in my col- 

 lection admit easily of a general description, and since the diag- 

 noses 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 pheno- 

 crysts 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 — 'OS mm. 

 in length. The pyroxene prisms are pale brown and are "03 or 

 •04 mm. long. Most of them give oblique extinctions of over 

 2 5 ; 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, 



u 



