0. Andersen — Aventurine Feldspar. 387 



The feldspar. — The cleavage pieces were generally fresh and 

 clear, of a strong, red color. 



Extinction angle on (001) = +1° 

 " " " (010) = +3-5° 



fi = 1-545 

 Composition : Ab 76 An 24 , oligoclase in agreement with 

 Scheerer's analysis.* 



The oligoclase was generally twinned both after the alhite 

 law and the pericline law, most of the twinning lamellae being 

 broad (often more than l mm ). The pericline striation on (010) 

 formed +10° with the a-axis. Sometimes broad cleavage 

 pieces, without any twinning, were seen. 



The hematite lamellae. — In some specimens the hematite 

 lamellae were very scarce; in others they were densely crowded 

 and of large size, producing the most brilliant aventurization. 

 Lamellae measuring 3 X 2 mm or even more were frequently 

 seen. The outlines were hexagonal, rhomb-shaped or irregular. 



The lamellae after (010) were often larger and more irregu- 

 larly outlined than the others. 



Some few lamellae were opaque, but most of them were 

 transparent with the usual absorption colors. 



Orientation of the lamellce. — The determinations of the 

 planes of orientation made by Scheerer (see p. 353) and Tertsch 

 (see p. 354) have already been mentioned. Of these only 

 Scheerer's have been partly confirmed by the present study 

 (viz., the faces (001), (010) 'and (110) ). The planes given by 

 Tertsch, (538) and (417), could be found neither on the Tvede- 

 strand sunstone nor on any of the other varieties examined. 

 That Scheerer's determination of (221) as a plane of orienta- 

 tion is wrong was intimated by E. Reuschf and further proved 

 by Tertsch4 



According to my measurements (of which Table 6 contains 

 those made with goniometer) the following forms were planes 

 of orientation : (001), (010), (112), (112), (150), (150), (110). Of 

 these (112) contained the largest number of lamellae which 

 cause the brilliant aventurization on (001) ; (112) contained but 

 few. After (150) and (150) there were numerous lamellae pro- 

 ducing a strong aventurization on (010). Along the other 

 faces (001), (010) and (110) there were comparatively few, none 

 causing aventurization on the cleavage faces. 



The face (110) was identified as plane of orientation by 

 microscopic measurements : (j> P = 56° — 57° ; 4> u about 62°. 



The projections on (001) of the elongated edges of lamellae 

 after (112) were often parallel to the a- axis ; some formed +72° 



*Loc. cit., p. 155. f Pogg. Ann., cxvi, 396, 1862. 



JLoc. cit., p. 248. 



