258 A NATURALIST IN THE PACIFIC chap. 



The pyroxene phenocrysts, which are at times infrequent, may be 

 5 mm. in size. They are of pale yellowish-brown augite, giving 

 extinctions of 40 . The felspar-lathes are rarely lamellar ; but in 

 one such case the angle of extinction was 17 (medium andesine). 

 The grains of augite in the groundmass average '02 mm. in 

 diameter. 



The only species represented is that with the felspar-lathes 1 

 to '2 mm. long. 



*&■ 



15. Genus of the Olivine-Basalts 



FORMULA. — Oliv, matr, cop, flu, gran, non-phen, tnagn. 



CHARACTERS. — Abundant olivine. In the groundmass the 

 felspars are in fiow-arrangement and the pyroxene is granular. No 

 plagioclase phenocrysts, but large phenocrysts, over 2 mm., of 

 olivine and pyroxene. 



Description. — This genus includes the most basic rocks 

 represented in my collection. They are compact heavy blackish 

 rocks with sp. gr. 3 to 3*1, and display large porphyritic crystals of 

 olivine and pyroxene often 3 or 4 mm. in size. The olivine 

 phenocrysts may be fairly fresh with clean outlines, or they may be 

 deeply eroded and stained by iron oxide, or they may be passing into 

 serpentine. The pyroxene phenocrysts may be either eroded or 

 possess regular outlines. They are of pale brown augite and give 

 extinctions over 30 . The pyroxene granules, which average *02 

 mm. and are very abundant, are of the same augite. The felspar- 

 lathes are relatively scanty. In the two rocks here included they 

 average in length *o6 mm. and *o8 mm. When lamellae can be 

 recognised they give extinctions of 30 — 40 (labradorite). The 

 untwinned lathes give extinctions of 20 — 28° (labradorite). 



The only species represented is that with felspar-lathes less 

 than 'i mm. in length. 



'to" 



16. Genus of the Olivine-Basalts 



Formula. — Oliv, matr, cop, flu, gran, nonphen, parv. 



CHARACTERS. — Abundant olivine. In the groundmass the 

 felspar-lathes are in flow arrangement and the pyroxene is granular. 

 There are no plagioclase phenocrysts ; but there are numerous 

 small phenocrysts, under 2 mm. in size, of olivine and occasionally 

 a few of pyroxene. 



DESCRIPTION. — As a highly basic genus this ranks next to 

 the preceding one, the specific gravity of the rocks ranging from 



