ART. 2. 



PETROLOGY AT GOOSE CREEK SHANNON. 



69 



The second lot of datolite specimens was obtained from a north- 

 south fissure in April, 1923, and contained numerous crystals of 

 datolite as the earliest mineral of the veins, covered by later prehnite, 

 laumontite, stilbite, and calcite. These are greenish transparent crys- 

 tals of acute pyramidal habit as illustrated in figure 20. They 

 average 3 mm. in length and greatly resemble the crystals of datolite 

 from Bergen Hill. Entirely similar crystals line narrow veins later 

 solidly filled with white apophyllite. 



The crystals are fairly simple in combination with the forms a 

 (100), m (110), n (111), and x (102) prominent with smaller faces of 

 M (Tl4), e (Tl2), m^ (Oil), and g (012). The crystals of this type 

 which were measured gave the following angles : 



Measurements of datolite crystals, Figure 



20. 



When overlain by prehnite or other minerals these crystals often 

 have some of the faces etched to complete dullness, although those 

 which are completely covered by apophyllite are brilliant with all of 

 the faces lustrous. 



The second type of datolite, which invariably rests upon a columnar 

 or bladed green crust of earlier prehnite, was first seen in a little speci- 

 men picked up on the quarry floor by W. S. Burbank in April, 1923. 

 These crystals have the same yellow-green color and are transparent. 

 They differ in habit, however, and are more prismatic by elongation 

 on the a axis, making the clinodomes prominent. In this respect 

 they somewhat resemble the crystals from Westfield, Mass. One 

 such crystal was measured and had the development shown in the 

 drawing, figure 21. This gave the following angles: 



