REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 



61 



double angle hardly exceeds 10° as a mean. We have just said that several of the 

 plagioclase crystals showed a zonary arrangement : the interior zones have more faces 

 than those on the periphery. This fact seems to indicate that the plagioclastic mixture 

 was modified during the growth of the crystal. The largest crystals of augite are 

 greenish, as is generally the case in pyroxenic andesites ; they are sometimes twinned 

 according to the ordinary law, and the mineral here presents a very pronounced prismatic 

 form. The augite is generally altered and coloured brownish yellow by iron. The 

 little microliths of plagioclase in the ground-mass are, like their larger congeners, usually 

 twinned according to the aibite law, and related by their extinction — which takes place 

 at very small angles — to the microporphyritic plagioclases. 



The examination of another specimen of pyroxenic andesite has enabled us to make 

 observations which confirm what has just been said. As in the preceding specimen, 

 the microscope showed the ground-mass to be composed of an accumulation of plagio- 

 clastic and augitic microliths and small sections of magnetite. In this mass there were 

 large plagioclases, some of which gave good opportunities for studying their characters ; 

 others, on the other hand, formed irregular grains composed of colourless granules, as if 

 the crystals had been crushed ; and others were much corroded by the action of the 

 magma, presenting curves and sinuosities in outline in place of the right lines of 

 crystalline faces. This corrosion has been followed by a deposit of inclusions, surround- 

 ing the nucleus which has resisted solution. After the corrosion and deposit of 

 inclusions a fresh deposit of plagioclastic substance, of a more basic character than that 



Fig. 11.— Andesite of Ascension. Sections of plagioclase corroded by the magma, with a zone of small scales 

 of hematite ; the external felspathic zone is labradorite, the internal part of the plagioclase is more acid, the 

 extinctions of which are those of andesine. J B crossed nicols. 



forming the nucleus, took place. Indeed this very thin external zone, which closely 

 follows all the contours of the primitive crystal, extinguishes at an angle of about 16° 



