2 / 2 FINLAY 



(J?) Vegonia Type 



This rock is exposed along the brook course lOO yards south 

 of the Vegonia Mine. It is found as narrow dike-like stringers 

 and larger masses irregularly intruded into the San Narciso 

 type of andesite described above. 



Megascopic Description. — When seen in the hand specimen it 

 appears as an even-grained bluish-gray porphyritic rock with 

 inconspicuous phenocrysts of feldspar and smaller grains of the 

 dark silicates. The feldspars are at times a quarter of an inch 

 long, but this is unusual. They average not more than one 

 sixteenth of an inch in length. They often show the character- 

 istic twinning of the plagioclases to the unaided eye. The 

 darker silicates are by no means so abundant as the feldspars. 

 Except for an occasional yellow crystal of titanite they are 

 black. Taken altogether the phenocrysts make up half the rock. 



Microscopic Description. — In thin sections the principal con- 

 stituents of this andesite are found to be plagioclase, augite 

 and hornblende, in conspicuous phenocrysts, with titanite, mag- 

 netite and apatite as accessories, in a fine-grained holocrystal- 

 line ground-mass consisting principally of feldspar, with lesser 

 amounts of augite and magnetite in a second generation. 



The plagioclase phenocrysts are in tabular crystals which are 

 equidimensional. J/(oio), T(\\6) and /(no) are conspicu- 

 ously developed faces. Polysynthetic twinning occurs on the 

 albite law and carlsbad twins are not uncommon. More rarely 

 lamellae which result from twinning on the pericline law have 

 been noted. The lamellae are generally broad, with intercalated 

 fine bands. The extinction angles indicate labradorite. When 

 a crystal cut at right angles to Mipio) which shows twinning 

 on the albite law and carlsbad law at the same time can be 

 found the concurrent extinction angles correspond to a labra- 

 dorite with the composition Ab^An^. The double refraction is 

 high, often yellow of the first order. The labradorite crystals 

 almost always show zonal structure. In habit they are usually 

 microtinic and quite free from alteration products, but the core 

 of a crystal has at times been shattered and afterwards sur- 



