ART. 2. 



PETROLOGY .\r GOOSE CREEK SHANNON. 



77 



The analyzed material, examined microscopically, was found to be 

 biaxial negative ( — ), 2V medium, r<v strong, /3= 1.515-1.518. The 

 mineral powders to laths by splitting on the prismatic cleavage and 

 these give extinction varying from 32° to 44°. 



OPAL (hyalite). 



Hyalite opal was noted on a single specimen where it coated a 

 joint crack in normal diabase with a thin small botryoidal or globu- 

 lar layer of colorless transparent globules reaching 1 mm. in diameter. 

 This layer is in part overlain by globular calcite. 



The hyalite, which is brittle with a conchoidal fracture, is trans- 

 parent and colorless under the microscope, with a concentric struc- 

 ture. It exhibits a very faint birefringence with a sweeping extinc- 

 tion cross. The index of refraction is variable, between 1,452 and 

 1.458. 



APOrHTLLITE. 



Apophyllite, which is an abundant mineral in the veins, occurs in 

 a variety of forms, both as simple crystals and as platy cleavable 

 masses without distinct crystal outhnes. Veins solidly filled with 

 the latter may be 3 cm. wide. Sometimes the cleavage surfaces are 

 irregular with a structure resembling the "A" or "feather" structure 

 in mica. Ocassionally platy blades of the apophyllite are arranged 

 radially, giving rosettes up to 2 cm. across. 



Pure white apophyllite from a solid vein was analyzed and gave 

 the following results, which are compared with the theoretical com- 

 position given by Dana : 



Analysis of apophyllite. 



