ART. 2. 



PETROLOGY AT GOOSE CREEK SHANNON. 



75 



These, in part, rested on practically unaltered basalt and in part 

 upon an earlier druse of small colorless chabazite crystals. The stil- 

 bite of these druses is in crystals averaging about 1 mm. in length, 

 which differ from those found in the wider siieared veins in the pos- 

 session of 4 pyramidal plains as shown in Figure 25. They are thus 

 typical crystals of stilbite. Oriented as drawn with the broadest, 

 pearly-lustered face, as the h (010) pinacoid, the measured crystal 

 gave the following angles : 



Measurements of stilbite crystal from druse. 



The lack of agreement in the angles is 

 due to the poor quality of the faces, even 

 the smallest of these stilbites, like the others, 

 exhibiting irregularities showing that they 

 are made up of numerous smaller units not 

 quite in parallel position. 



Lying on the h (010) face, these crystals, 

 like the simple rectangular ones, give varia- 

 ble very small extinction and some show 

 visible twinning while others do not. The 

 cleavage is almost equally good parallel to 

 (010) and (100). The optical properties 

 are biaxial, negative ( — ), 2V medium; r>v 

 weak, a = 1 .500, jS = 1 .504, 7 = 1 -508, 7 - « = 

 .008; orientation (as drawn) Xac = 0°-3°, 

 Y = h, Z = a. 



Occasionally the stilbite of the druses is 

 overlain by later crystals of calcite. 



LAUMONTITE. 



Laumontite is one of the commoner min- 

 erals of the locality and was found in a 

 variety of situations. It is all in the form of white prismatic crys- 

 tals varying from 1 mm. to 1 cm. in length, sometimes in radiating 



Fig. 27. — Apophyllite of cubic 

 form showing only prism and 

 basal pinacoid. 



