51S 



W. S. Bayley — Fibrous Intergrowth of 



eral surrounded by it. This is very narrow and consists of a 

 highly refractive substance with strong double refraction (tigs. 

 1 and 3). This zone is too narrow to admit of the nature of 

 its material being discovered by its optical characteristics ; but 

 every gradation can be traced between wide enveloping man- 

 tles of diallage and thin seams with all of the characteristics 

 of the intermediate zone. Moreover, it is frequently noticed 



Fig. 3. Fibrous intergrowtli around 

 magnetite? Between the latter mine- 

 ral and the fibrous rim can be seen a 

 narrow zone of diallage. Section 

 10,439. x 20. 



Fig. 4. Olivine partly sur- 

 rounded by narrow rim of 

 augite, which is continuous 

 with large plate of same mine- 

 ral. 8803. x ca. 18. 



that the narrow rim widens out and expands into large wedge- 

 shaped plates of whose diallagic nature there can be no doubt 

 (fig. 4). Since the narrow zones between the fibrous envelope 

 and the surrounded minerals are similar in every respect to 

 these narrow rims of pyroxene, it is safe to assume that the 

 former, like the latter, are composed of diallage. Conse- 

 quently, the fibrous growth, if it be a reaction rim, is one be- 

 tween pyroxene and plagioclase, rather than between olivine 

 and feldspar, as in the cases referred to in the opening para- 

 graph of this article. But the intergrowth is rarely found 

 around well developed diallage plates, while on the other hand 

 it occasionally occurs around olivine, biotite and magnetite 

 without the intervention of the seam of augite between these 

 minerals and their envelope, and, again, it is sometimes found 

 in the midst of feldspar as little granophyre-like bunches, not 

 associated with any iron-bearing mineral whatsoever. It is 

 thus quite evident that the fibrous growth is not a reaction 

 rim between diallage and plagioclase, and, since it occurs indis- 

 criminately around so many different minerals, that it is not a 

 reaction rim at all. 



