608 MR. A. X. COOMAKA-SWAMY ON [Aug. IQOO, 



with a low power in a thin section, that this appearance is due to 

 the presence of parallel rods and spindles (of plagioclase) . If 

 ordinary convergent light is used, and all but the central rays are 

 cut off by a diaphragm, the plagioclase stands out very clearly 

 against the orthoclase by reason of its higher refractive index. 

 Between crossed nicols the structure becomes still more evident. 

 The plagioclase-strips are usually spindle-shaped and pointed at 

 both ends, more rarely thejr are lamellar, or irregular. In hand- 

 specimens of the rocks containing them, these felspars appear 

 greenish-grey with a rather greasy look, resembling the felspars of 

 the well-known rock from Laurvig, but without their blue sheen. 



The structure is best seen in sections parallel with M (0 1 0, oo P 06 ). 

 The orthoclase-cleavage on P (0 1, OP) is clearly marked, and 

 crosses the less evident cleavage on 1 (00 P 00 ) at about 110°. 

 The plagioclase-^pindles are seen to have their long axes parallel to 

 the latter cleavage, that is, they run parallel to the vertical axis of 

 the orthoclase. 1 A basal cleavage is observed also in the plagioclase, 

 and it is parallel to, or continuous with, that of the orthoclase (see 

 PI. XXXIII, fig. 5). The spindles may be of uniform size, or there 

 may be many smaller Avith larger individuals among them. They 

 may be few and scattered, or so numerous as to exceed the ortho- 

 clase in amount. 



The homogeneous-looking orthoclase background gives rather high 

 extinction-angles (up to 8°) from the cleavage parallel with P (0 1, 

 OP). The extinction-angle of the plagioclase-spindles is not so easy 

 to determine. Cleavage- flakes or sections for measurement must be 

 very thin and of uniform thickness, otherwise the higher refractive 

 index of the plagioclase renders it difficult to be sure of the exact 

 position of extinction. In a suitable flake from the Kalawewa 

 rock (sec p. 51)7), the following data were determined: — The 

 cleavage-flake is parallel with M (0 1 0, 00 P 06 ) and is bounded 

 above and below by the basal cleavage, which makes an angle of 108° 

 with the fine striation representing the orthopinacoidal cleavage of 

 the orthoclase, to which the plagioclase-strips are parallel. The 

 extinction-angle of the orthoclase from the trace of OP was 8°, 

 that of the plagioclase from the same trace (that is, on 00 P 06 ) 

 was 14°. 



In a specimen from the Buena-Vista rock (p. 597) the orthoclase 

 cleavage-angle was 112°, the orthoclase extinction- angle from the 

 trace of OP 7°, and the plagioclase extinction-angle from the same 

 trace 14°. These results agree with those of Dr. Diersche [30] 

 p. 250, who says : 



' extinction-angles of 13° to 17° in relation to the trace of OP were measured 

 iu the lainella?, showing their identity with an albite or lahradorite ; ' 



that is, the extinction on M(0 1 0, ooPoo ) is from 13 d to 17°, 

 leaving the ambiguity between albite and lahradorite. 



He says, however, that in basal sections where the orthoclase has 



1 In the Ragedara pyroxene-granulite the intergrowth seems to be a little 

 more complicated. 



