284 A NATURALIST IN THE PACIFIC ch. xix 



flow-arrangement and the augite is ophitic or semi-ophitic. Glassy 

 plagioclase phenocrysts. 



Description. — Dark rocks, sp. gr. 276 — 2'8, forming ancient 

 flows and displaying at times a columnar structure as at Yanutha 

 Point (page 123). The ophitic character is only in part developed, 

 which may be connected with the flow-arrangement of the felspars. 

 These rocks come near to the blackish ophitic basalts with scanty 

 olivine (genus 33 of the olivine sub-class). 



They all belong to the non-porphyritic sub-genus where the 

 plagioclase phenocrysts are less than 3 mm. in size. These 

 phenocrysts, which often contain abundant magma-inclusions, give 

 extinctions of andesine labradorite (20° — 30°). The augite pheno- 

 crysts are small and composite in character as often happens with 

 these ophitic rocks. They sometimes invest the smallest felspar 

 phenocrysts, and occasionally display intergrowths of rhombic 

 pyroxene. The felspar lathes are *i to "14 mm. in length, and give 

 extinctions of medium and basic andesine. The augite granules 

 are large (*02 — *o6 mm.), and tend to wrap around the lathes. 

 Interstitial glass exists in fair amount. 





