ART. 28 MINERALOGY OF TRIASSIC LIMESTONE SHANNON 



27 



Under the microscope the transparent unaltered crystals are uni- 

 axial and positive with e =1.535, o =1.532. They are for the most 

 part poised upon needles of diopside and include the colorless vein 

 diopside as numerous fibers. 



The crystals, as shown in figure 6, are dominated by the unit 

 pyramid f (HI) with a short prism zone composed of small faces 

 of m (110) rounded into faces giving angles approximating the form 

 (780). The measurements follow. 



Measurements of apophylUte, Figure 6 



The apophyllite is definitely later than datolite and diopside and 

 is probably earlier than all of the calcite. 



Barite was found in a number of specimens, all apparently from 

 a single narrow but persistent vein. It rests upon crystallized da- 

 tolite and is doubtless later than the datolite but its age relation to 

 diopside, apophyllite and calcite could not be made out. The barite 

 forms flat tables which reach a diameter of 3 centimeters, some of 

 them being very tliin. Many of the plates are curved and they 

 tend to aggregate in sheaves (pi. 3). The surfaces of the plates are 

 etched with a silky sheen but in^e they are transparent and color- 

 less with good cleavage. Small free plates are square tables with 

 round corners and show no bounding faces. 



The forms assumed by the barite are precisely those shown by the 

 empty cavities assumed to have originally held anhydrite. The 

 barite is, however, a rather insoluble mineral. There is no indica- 

 tion that it is removed in solution and its age relation to the datolite 

 is different. 



CALCITE 



Calcite is an abundant mineral in the veins, probably as abundant 

 as datolite, which is not surprising as the limestone of the walls is 

 capable of furnishing any amount of calcium carbonate to be re- 

 crystallized in the open spaces. Some of the small filled veins con- 

 tain only calcite. One flat seam in diopside rock above the upper- 

 most of the two basalt dikes on the east side of the quarry averaged 



