276 FINLAY 



The plagioclase of the older generation occurs in equidimen- 

 sional crystals which often are 2 mm. in length. The albite, 

 Carlsbad and pericline laws are all represented in the twinning. 

 The lamellae on the albite law are excessively fine. The sym- 

 metrical extinctions given by them in the two halves of a carls- 

 bad twin make possible the determination of the plagioclase as 

 labradorite Ab^An^. Zonal structure is very common. The 

 substance of the plagioclase is often quite clear and glassy at 

 the center, while the outer border of the crystal is commonly 

 crowded with inclusions. Again the outer zone may be ob- 

 served to extinguish at the same instant as the center of the 

 crystal, while one or two intermediate shells have angles of ex- 

 tinction at variance with them. The outlines of the plagioclase 

 individuals, as of all the other phenocrysts, are ragged and 

 irregular. 



Porphyritic crystals of orthoclase show twinning after the 

 Carlsbad law. Quartz is frequently intergrown with the ortho- 

 clase after the fashion of microgranite, where all the enclosed 

 pieces of quartz extinguish at the same time. Perthitic inter- 

 growths with plagioclase on a microscopic scale also occur. 

 The orthoclase is always partly kaolinized. 



The pyroxene does not usually show even an approach to 

 good crystal outlines. Irregular grains of this mineral are 

 common. It is light green, non-pleochroic, extinction 40°. 

 Embayments produced in the crystals by the caustic action of 

 the magma are frequently observed and the secondary minerals 

 thus produced are magnetite and some little hornblende. By 

 alteration the augite yields a dull green pleochroic fibrous 

 chlorite. 



Patches of a dull green slightly pleochroic hornblende, with 

 an extinction of 15°, are rare, but may be as large as 2 mm. 

 They are always ragged on all sides. Occasional poikilitic in- 

 tergrowths between the hornblende and the pyroxene should 

 be noted. 



Patches of biotite as large as the hornblendes may be found 

 at times. Secondary augite and magnetite grains occur around 

 them as products of magmatic resorption. 



