268 A NATURALIST IN THE PACIFIC chap. 



the altered rocks. Most of the lathes show a single median twin- 

 line, and when broader they display twin-lamellae. The angles of 

 extinction indicate acid and basic andesine. 



Three out of the eight species distinguished by the length of the 

 felspar-lathes occur in my collection, that with the longest lathes 

 ("3 — "5 mm.) being not represented. 



2. Genus of the Augite-Andesites 

 Formula. — Au£-, matr, non-flu, gran, phen, opac. 



Characters. — In the groundmass the felspar-lathes and 

 prisms are not in flow-arrangement and the augite is granular. 

 Opaque plagioclase phenocrysts. 



Description. — This genus may be divided into two groups 

 according to the size of the plagioclase phenocrysts, the first 

 " porphyritic," where they average 3 mm. and over, the second 

 " non-porphyritic," where they are smaller than 3 mm., usually not 

 over 2 mm. The former would include some of the " porphyrites," 

 and to this only one of the rocks sliced is to be referred. All the 

 rest belong to the non-porphyritic type ; and several of them are 

 rocks that have undergone the propylitic change, as indicated by 

 the formation of pyrites, chlorite, calcite, and other alteration- 

 products. 



(a) Porphyritic Sub-genus. — A greyish rock, with sp. gr. 

 278, showing abundant porphyritic opaque plagioclase (4 to 7 mm.), 

 from the vicinity of Satulaki. These phenocrysts are often 

 aggregates of lesser crystals, or they may be extensively cross- 

 macled. They are traversed by numerous fine cracks and show 

 much dust-like included material. They are in part corroded by 

 the magma and give evidence of fracture in their present position, 

 the re-union being sometimes effected by the growth of new 

 substance. Their lamellar extinctions (10° to 20°) are those of 

 andesine. The groundmass displays a plexus of stout felspar- 

 lathes, averaging -i mm. long, with the meshes occupied by coarse 

 augite granules, "03 to '05 mm., with little, if any, interstitial glass. 

 The felspars are often lamellar and give extinctions like those of the 

 phenocrysts. 



{b) Non-porphyritic Sub-genus. — Reference will first be 

 made to some of the propylitic rocks of the dykes of the Ndriti 

 Basin which belong to this group (see p. 70). They are greenish 

 or greyish, with sp. gr. 276 to 2-8, and often sparkle with pyrites 



