PETROLOGY AT GOOSE CREEK SHANNON. 



63 



chlorite and contains embedded " hour-glass " epidote crystals. 

 Apophyllite crystals rest upon the surface of the prehnite. 



In the material collected from the east-west shear zone in the north- 

 central portion of the quarry face in October, 1922, prehnite is abun- 

 dant, especially as sheeted vein fillings made 

 of pale green material having a trans verse- 

 bladed structure and showing evidence of 

 repeated reopening of the fissure during its 

 deposition by the inclusion of many layers of 

 chlorite and partings parallel to the walls, giv- 

 ing a sheeted structure. Small open spaces in 

 this material are lined with minute tabular 

 and very thin crystals. The thinner of these 

 have the form shown in figure 10 and give 

 very poor but definite reflections from the 

 prism faces indicating a prism angle of about 

 80° ±2°. The faces of pinacoids, though vis- 

 ible, are etched dull and give no reflection. 

 When these small euhedral crystals are ex- 

 amined in polarized light they give very re- 

 markable effects. Crystals like those indi- 

 cated in the drawing, figure 10, are shown in 

 figures 13 and 14 below, and in figure 15 is 

 shown a variant which is bounded by only 

 the two pinacoids, a (100) and h (010) giving 

 rectangular plates. These peculiar crystals 

 are referred to as " hour- 

 glas" prehnites by 



analogy with the associated epidotes described 

 above. The cause of the appearance is very 

 different in the two cases as will be apparent 

 from the following description. 



The crystal, figure 13, consists of an hour- 

 glass pattern at the ends and continuing as a 

 narrow line through the center of the crystal to 

 connect with an identical area at the corre- 

 sponding opposite end. The main portion of 

 this crystal, marked c — c^ in the drawing, 

 shows, at the position of maximum illumina- 

 tion, a uniform pale-yellow birefringence color of the first order. The 

 extinction is parallel with the sides. The figure, in convergent fight, 

 shows an optically biaxial positive character, with 2V medium, acute 

 bisectrix perpendicular to the plate, r>v distinct. Axial plane paral- 

 lel with the long direction which makes the optical orientation X = a, 

 Y = &, Z = c. 



Fig. 12.— Prehnite. Type 3 

 elongated on the a axis 

 showing striation and crys- 

 tal habit in orthographic 

 and clinographic projec- 

 TION. 



■\N£r4ir2>^'' 



FiG. 13.— Prehnite. Type i 



SHOWING COMMON 'HOrK- 

 GLASS" FORM. 



