OCCLUSION OF IGNEOUS ROCK 411 



the lower First Order, but sometimes reaching reddish orange 

 or bright brown. Extinction of individual blades parallel or 

 normal to chief plane of polarization of the nicols. On selec- 

 tion of small isotropic spots an obscure biaxial interference 

 figure may be shown in convergent light. This substance was 

 identified as serpentine (antigorite) and predominates in some 

 thin sections, but its higher interference colors suggest slight 

 but general intermixture with a more birefringent mineral, 

 brucite. 



In many thin sections, irregular colorless patches with little re- 

 lief occur, sometimes 2 mm. across, connected by narrow veinlets, 

 0.2 mm. in breadth, generally in parallel arrangement. A dis- 

 tinct pleochroism, colorless to pale yellowish, indicates the pre- 

 dominance of a chlorite. Smaller bunches of the same mineral 

 may occur in apparent isolation here and there. In large part 

 they present aggregates of minute plates and blades of exceed- 

 ingly fine fibrous texture, microchrysotile or perhaps micro- 

 nemalite, often with abrupt terminations against the veinlet wall, 

 and revealing in polarized light an abundance of short cross- 

 partings, delicate but sharply defined. 



In some veinlets a symmetrical cross-fibration appears, normal 

 to the walls ; but the fibers are commonly bent, broken and 

 mixed up, as if by rock movements. A somewhat high bire- 

 fringence appears in interference colors of upper First and lower 

 Second Orders, from white up to sky blue or turquoise blue. 



Prevalent extinction parallel to the fibers and wavy, but vary- 

 ing, to 7° in curved fibers. C ^^is parallel to the fibration. In 

 convergent light, the straight bars of a uniaxial figure are com- 

 monly seen, most distinct in the more unbroken plates, and 

 very rarely a dim cross opening out on rotation into a biaxial 

 figure. In most cases the direction of the optic axis coincides 

 nearly or exactly with the fibration, /. e., parallel to c, show- 

 ing the double refraction to be positive, the mineral being ap- 

 parently uniaxial. 



These chloritic veinlets appear to be the " colorless, low polar- 

 izing bands," for which another explanation has been offered.^ 



' Newland, toe. ciL, 317. 



