272 A NATURALIST IN THE PACIFIC chap. 



filled with alteration products, that traverse them. Although much 

 of their original material has often disappeared, they still display 

 the lamellar twinning of medium andesine (15° to 20°) .... The 

 phenocrysts of pale yellowish augite, which give the large extinc- 

 tions of that mineral, exhibit but little alteration, although lying in 

 the same slide with those of the plagioclase, . . . The pyroxene 

 prisms of the groundmass are of the same yellowish augite. They 

 are broad with rounded extremities and are arranged in a loose 

 plexus. . . . The felspars of the groundmass, which average "i mm. 

 in length and give extinctions of medium andesine, are either lathe- 

 shaped or short and broad when they display lamellae. The last- 

 named approach the orthophyric type, and such rocks come near 

 the porphyrites ; but I do not feel justified in placing them in a 

 separate orthophyric order. 



Ill Ophitic Sub-order of the Augite- Andesites with the 

 Felspars of the Groundmass not in Flow-arrange- 

 ment. 



Formula. — Ati^, matr, non-flu, oph. 



These rocks form generally ancient flows. They are for the 

 most part semi-ophitic, large ophitic " plates " being uncommon. 



9. Genus of the Augite-Andesites 

 Formula. — Aug, matr, non-flu, oph, phen, vitr. 



Characters. — The felspar-lathes and prisms of the ground- 

 mass are not in flow-arrangement. The augite of the groundmass 

 is ophitic or semi-ophitic. Glassy plagioclase phenocrysts. 



This genus may be divided into two sub-genera, 



{a) Porphyritic, where the average size of the plagioclase pheno- 

 crysts is 3 mm. and over. 



{b) Non-porphyritic, where the size is less than 3 mm. 



A. Porphyritic Sub-genus 



Description. — Coarse-looking brownish-black porphyritic 

 rocks displaying large plagioclase crystals that often show a play 

 of colours. Their sp. gr. is about 2 '8. None of the rocks in my 

 collection are vesicular. On account of the considerable porphy- 

 ritic development of the plagioclase, the groundmass is relatively 

 diminished, the large phenocrysts occupying about a third of the 

 mass. They form ancient basaltic flows more especially in the 



