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University of California . 



[Vol. 3. 



no description in the literature that would apply to the present case. 

 Yet this mode of alteration, most clearly exemplified in this 

 specimen, is the typical one in the feldspar of the Clarno volcanic 

 rocks. 



The feldspar substance that remains is perfectly clear and 

 fresh. Every crystal, however, contains areas, irregular and 

 jagged in outline, of a clear and colorless mineral, which by 

 reason of its very low refractive index appeal's deeply sunken 

 below the surface of the feldspar. The refractive index is found 

 by the Becke method to be much lower than that of balsam. A 

 distinct pinacoidal cleavage is often visible. Between crossed 

 nicols the extinction is found to be parallel to, or nearly parallel to, 

 the cleavage. The interference colors, while in most individuals 

 lower than those exhibited by the feldspars, sometimes reach 

 yellow of the first order, which by the comparative method is 

 found in the slides used to indicate a double refraction near .010. 



Similar material also occurs in fairly large irregular patches, 

 in small veins, and occasionally forms a fine lace-work of minute 

 veinlets in the groundmass. While the mineral evidently fills 

 original cavities and small fissures, it seems in some cases to 

 have replaced metasomatically a portion of the groundmass. 



Examination of cleavage flakes from the larger masses leads to 

 the discrimination of two different minerals. Heated before the 

 blow-pipe, all the fragments exfoliate, swell, and fuse to a white 

 enamel. Treated with acids, they all dissolve without gelatin- 

 ization. But when their cleavage flakes are examined in 

 convergent polarized light, they do not all exhibit the same 

 behavior. A certain portion are found to lie parallel to the 

 plane of the optic axes, as in stilbite. The remainder, in which 

 the lamellar cleavage was more nearly perfect, show the emergence 

 of an acute negative bisectrix, and the optic axial angle, even in 

 flakes from the same individual , varies from nearly zero to about 

 50°. This optical behavior is characteristic of heulandite. 



It will be recalled that part of the zeolitic material has a 

 double refraction near .010, and therefore higher than that 

 assigned in the books either to stilbite (.005) or heulandite 

 (.007). But the double refraction is a function of the axial 

 angle, so variable in the case of heulandite, and may be expected 



