ART. 28 



MINERALOGY OF TRIASSIC LIMESTONE SHANNON 



29 



datolite. The first and simplest type of these forms transparent pale 

 amber "dog-tooth" crystals resting thickly upon free surfaces of 

 small white datolite crystals. These average 4 mm. in length and 

 have the habit shown in figure 7. Those which were measured gave 

 the following forms and angles : 



3Ieasuremetits of calcite from Leesburg, Figure 7 



Another habit of calcite occurs as clear colorless and transparent 

 to translucent crystals up to 2 centimeters in length resting on the 

 larger datolite crystals. These calcites, which are associated with 

 barite plates, have in general the habit shown in figure 8. The 

 averages of the angles measured on several of these are given in 

 the following table : 



Angles of Calcite crystals of the Tiahit of Figure 8 



A later deposit of calcite occurs as a minutely drusy botryoidal 

 crust. This varies from a continuous crust which may envelope 

 large crystals of the last described form, to minute spherical glob- 

 ules, and ranges in color from white through yellowish to smoky 

 gray. The surface of this crust is made up of minute curved flat 

 rhombohedral crystals. This deposit of calcite was formed after the 

 removal of the anhydrite and occurs in places in the anhydrite 

 cavities. 



