26o A NATURALIST IN THE PACIFIC chap. 



Description. — About two-thirds of these rocks have a common 

 facias, being closely similar in appearance, brownish-black in colour, 

 and with spec. grav. usually between 2-84 and 2-92. They belong 

 to the species with the felspar-lathes "i to '2 mm. in length. They 

 are essentially the rocks of the old submarine basaltic flows ; and 

 they are often columnar, the columns being 2 to 4 feet across. 

 My remarks will mainly apply to this predominant group. 



To the eye they are somewhat compact and show scattered 

 porphyritic crystals of plagioclase. In the slide they display numerous 

 phenocrysts of plagioclase, with a few of olivine and pyroxene, in a 

 groundmass formed of stout lamellar felspar-lathes and small prisms 

 forming a plexus with granular augite in the meshes. There is a 

 good deal of magnetite and generally scanty residual glass. The 

 plagioclase phenocrysts are usually 2 to 3 mm. in size, but they 

 may be smaller (i to 2 mm.) or larger (3 to 5 mm. or more) when 

 the rock has a porphyritic appearance.^ They are often cross- 

 macled and at times show zoning. In many slides two kinds are 

 distinguished by the extinctions which indicate in one case medium 

 andesine(i5° to 22°) and in the other acid labradorite (27° to 32°). 

 They contain inclusions of the magma and are often eroded. The 

 pyroxene phenocrysts are of pale brown augite, scanty and small, 

 and give extinctions of 30° to 40°. They are sometimes twinned 

 and may be eroded and contain inclusions of the magma. The 

 olivine phenocrysts, which do not usually exceed 2 or 3 mm., are 

 mostly rounded, but sometimes have the regular outlines, and are 

 in various stages of serpentinisation. The felspars of the ground- 

 mass, which average 'ij mm. in length, are mostly stout and 

 lamellar ; but they exhibit all transitions from the lathe-shape with 

 one or two lamellae to broad multi-lamellar prisms where the 

 breadth is half the length. They give lamellar extinctions averag- 

 ing 15° to 18° (andesine). The augite granules of the groundmass 

 are pale brown and average "02 to "03 mm. in size. In a few cases 

 they are larger ("05 mm.) which is an indication of an approach to 

 the ophitic type. In most slides occur a few small augites of 

 prism-form, two or three times the size of the granules, which give 

 extinctions of over 30°. Where the phenocrysts of augite are very 

 scanty or absent, there exist large grains ('i mm.) of an inter- 

 mediate size. The magnetite is often abundant, occurring in 

 crystals, rods, and irregular masses, the last associated often with 



' According to the size of the plagioclase phenocrysts, whether averaging 

 less than or more than 3 mm. in size, these rocks may be divided into a non- 

 porphyritic and a porphyritic sub-genus. 



