274 FINLAY 



plainly porphyritic by reason of the larger crystals of feldspar, 

 with flashing cleavage faces, which are just large enough to be 

 recognized. A hand lens shows that their centers are some- 

 what weathered, while the outer rims are fresh. The dark 

 minerals are not determinable in the hand specimen. They 

 appear as minute black patches sparsely distributed through the 

 ground-mass. 



Microscopic Characters. — The study of thin sections reveals 

 the presence of plagioclase, augite and quartz phenocrysts in a 

 ground-mass, which is an intimate mixture of quartz and feld- 

 spar. Apatite, epidote, titanite and magnetite are the accessory 

 minerals. 



The plagioclase crystals are 1.2 mm. long, often in highly 

 complex forms with only (010) and (i 10) distinguishable. The 

 center is often deeply corroded, and secondary muscovite pro- 

 duced inside a clear outer rim. Many of the medium-sized 

 individuals of the mineral are, however, quite glassy. Twinning 

 according to the albite law with extremely fine lamellae is very 

 common, but many other lamellae are often intercalated on the 

 pericline law. The two halves of the crystals twinned at once 

 on the albite and carlsbad laws show different interference colors 

 in the 45° position. One is usually yellowish, the other blue 

 gray. Symmetrical extinctions on the lamellae of the two halves 

 gives readings for an acid labradorite Ab^An^. 



The quartz phenocrysts are seldom above .1 mm. in diam- 

 eter, but they are very common. Their substance is clear and 

 glassy. It encloses needles of apatite, and only rarely shows, 

 gas or liquid inclusions. The mineral is never automorphic. 

 It has generally grown side by side with deeply kaolinized feld- 

 spar which comes in crystals much smaller than the labradorite 

 phenocrysts. No twinning was observed on this second feld- 

 spar, which is identical with the feldspar of the ground-mass. 



The pyroxene makes up a very small part of the rock. It 

 is in irregular deep green twinned grains that are pleochroic 

 yielding light and dark shades of green. Apatite in minute 

 needles is a rare accessory, as is also epidote in short stubby 

 grains, Titanite is present in irregular pieces not above .1 mm. 



