290 FINLAY 



The small lath-shaped plagioclases in the ground-mass, which 

 are fresh and glassy, have low extinction angles, near io°. 

 They are therefore much more acid than the feldspars of the 

 first generation. They are often twinned in three or four fine 

 lamellae. They are automorphic and by their distribution they 

 cause the rock, at times, to approximate the diabasic texture. 

 Altogether plagioclase makes up about 50 per cent, of the 

 whole. 



The augite, which was once the most abundant dark silicate 

 throughout the ground-mass, is now nearly all changed over 

 to chlorite. This mineral is faintly pleochroic from yellowish- 

 green to green, and shows but slight effects of double refrac- 

 tion. 



The hornblende of the ground-mass has a rich brown color. 

 It occurs as xenomorphic grains and patches which have a low 

 extinction angle. The interference colors are high. The pleo- 

 chroism is inconsiderable, various shades of brown prevailing. 

 Square pieces and irregular grains of magnetite make up about 

 5 per cent, of the rock. Calcite is very generally present as an 

 alteration product. Minute needles which give high interfer- 

 ence colors are epidote. 



4. Vogesite 



The rock to be described under this name occurs as a four 

 and a half foot dike in the andesite along Bretafia Creek. It is 

 aphanitic and almost black. It carries no phenocrysts discern- 

 ible to the unaided eye. 



Microscopic Characters. — Under the microscope it is found to 

 be holocrystalline and to contain hornblende, augite and ortho- 

 clase, with magnetite, some plagioclase and titanite and minute 

 foils of biotite. There are no well-marked porphyritic develop- 

 ments. The orthoclase, which was the last mineral to take crys- 

 talline form, encloses all the other constituents. It is without 

 good crystallographic boundaries. Carlsbad twins may occa- 

 sionally be noted. 



The augite crystals are light buff-colored, xenomorphic, 

 .25 mm. in length by .1 mm. in width, and not pleochroic. 



