PETROLOGY AT GOOSE CREEK — SHANNON. 



65 



Fig. 17.— Prehnite. 

 a side view of 

 crystal a g g r e- 

 gate similar to 

 that shown in 



FIGURE 16. 



barely perceptibly. The main portion of the crystal shows inter- 

 ference colors mostly in subnormal blue and liver brown with a 

 very little first-order yellow. All portions of the pattern yield 

 a similar interference figure in convergent hght, 

 biaxial positive, 2V approximately 10°, acute bisec- 

 trix perpendicular to the table, axial plane parallel 

 with the long direction, r>v strong. These inter- 

 ference figures are somewhat hazy and confused. 

 This is most probably like the preceding, but the 

 portions of scales making up the "hourglass" are so 

 exceedingly thin that they do not greatly obscure 

 the optical properties of the main crystal, even 

 though the latter is itself very thin. 



The explanation implying an overgrowth of scales 

 isnotpurely hypothetical, especially since any of the 



crystals when care- 

 fully examined un- 

 der a lens do show 

 such scales, indeed usually a group 

 of them, curving upward. This is 

 similar to the tendency of the crys- 

 tals of prehnite to form 

 sheaves and the flat 

 crystals, with a thin 

 overlying scale, grade into bundles of curved, scalelike 

 crystals. Two such aggregates are illustrated in figures 

 16 and 17; the form shown in figure 17 is very much 

 more common than any other and has been referred to 

 as "dumb-bell" prehnite. Drusy surfaces are often 

 made up of this type grading into still more globular 

 forms and the crystals of the preceding descriptions 

 rest, in most cases, upon such shapes. 



In an east-west shear zone exposed just south of the 

 center of the quarry in August, 1923, there were found 

 some specimens of prehnite made up of pale yellowish- 

 green columnar bladed crusts up to 1 cm. thick lining 

 an open space. The surfaces of these crusts are smooth 

 botryoidal but are made up of the terminations of innum- 

 erable closepacked crystals. Attached to this crust as 

 though later are single. well-definecT crystals of prehnite 

 up to 3 mm. broad, which are more abundant and more perfect where 

 the space between the crusts is narrow. These have the crystal habit 

 shown m figure 11, showing the prism m (110), the front pinacoid 

 94110—24 5 



Fig. 18.— Prehnite showing optical struc- 

 ture OF CRYSTALS OF TTPE 2. 



c 



mc: 



Fig. 19.— prj-jj. 

 nite showing 

 optical struc- 

 ture of crys- 

 tals of type 3 



