XX HYPERSTHENE-AUGITE-ANDESITES 289 



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



Andesites 

 {Felspar-lathes in flow-arrangemenf) 



Formula. — Hypersth-aug^ matr, fiu, prism. 



17 genus . . . {Vitr.) 



18 „ ... {Opac.) 



19 „ . • . {Magn.) 



20 „ ... {Parv.) 



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- 

 orysts are opaque (genus 18); whilst in a few they are glassy 

 I genus 17). The rocks are typically blackish or dark grey, and 

 often have a pitchstone-like appearance, the groundmass being 

 Irequently semi-vitreous in character. Vesicular and scoriaceous 



1 ocks occur at times. 



In all cases the felspar-lathes and pyroxene prisms are more 

 ( r less in flow-arrangement ; whilst pyroxene granules, if present, 

 J re subordinate. The felspar-lathes, which are either simple or 

 c nce-twinned and give extinctions of acid and medium andesine, 

 e re usually small, and average in different sections -05 — -08 mm. 

 i I length. The pyroxene prisms are pale brown and are -03 or 

 •('4 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 

 ii all the specimens are small (not over 2 mm.), and are rhombic 

 a id monoclinic. In most sections the two forms are represented 

 b T separate crystals and are also associated in the same phenocryst. 

 1 hose of rhombic pyroxene have often dark borders. There is a 

 o nsiderable amount of a pale brown glass in the groundmass, 



U 



