276 A NATURALIST IN THE PACIFIC chap. 



semi-ophitic. No plagioclase phenocrysts. Augite phenocrysts 

 when present less than 2 mm. in size. 



DESCRIPTION. — These rocks come near to the non-porphyritic 

 group of genus 9 ; but differ in the absence or rarity of plagioclase 

 phenocrysts, in their more frequently vesicular and scoriaceous char- 

 acter, and in the fresher condition of the rock. Their sp. gr. is about 

 277. They present themselves usually as blackish-brown doleritic 

 rocks and form ancient flows in the coast-plains, sometimes 

 exhibiting a columnar structure as in the crossing of the Ndreketi 

 above Mbatiri. They are, however, of limited occurrence and are 

 mostly represented in the Ndreketi plains and in the district 

 between the Lekutu River and Wailea Bay. 



Typically they display in thin sections no phenocrysts either of 

 plagioclase or of pyroxene, and exhibit a plexus of usually long 

 stout felspar-lathes partly invested by the lesser augites in a 

 copious smoky glass. . . . The felspar-lathes, '25 to '4 mm. in 

 length, give lamellar extinctions of io° to 20° (andesine) and contain 

 a few magma-inclusions. . . . The semi-ophitic augites, *i to *2 mm. 

 in size, are sometimes twinned. . . . The smoky glass polarises 

 feebly and displays dark feathery aggregates of microliths. Within 

 it are brownish-yellow semi-opaque patches of residual glass, which 

 polarise faintly and behave like palagonite. 



The two species with felspar-lathes - 2 to '3 mm. and "3 to - 5 mm. 

 are represented in my collection. 



4. Granular Sub-order of the Augite-Andesites 

 Formula. — Aug, matr>flu> gran. 



13. Genus 



FORMULA. — Aug, matr, flu, gran, phen, vitr. 



Characters. — In the groundmass the felspar-lathes or prisms 

 are in flow-arrangement and the augite is granular. The plagioclase 

 phenocrysts are glassy. 



Description. — This genus readily splits up into two sub-genera, 

 the non-porphyritic, where the plagioclase phenocrysts are less than 

 3 mm. in size, and the porphyritic where they are larger. 



1. NON-PORPHYRITIC SUB-GENUS. — Dark-brown or blackish 

 rocks displaying small plagioclase phenocrysts, usually only 1 or 2 

 mm. in size. Three of the four species defined by the length of 

 the felspar-lathes are represented in my collection. 



