112 



light the higher birefringence of the plagioclase is most 

 noticeable. 



The composition of the oligoclase varies, and in most cases 

 it is very acidic. The extinction angles in the zone perpendicu- 

 lar to <i(010) rise to 10°, with refractive index distinctly be- 

 low Canada balsam ; this indicates the variety oligoclase-albite 

 with a composition of Abg^ An^,,. In other rocks maximum 

 extinction angles perpendicular to (010) were 6° or 4°, giv- 

 ing an approximate composition of Ab,, -^^^isj ^iicl in a few 

 slides even greater basicity was noted. The angles in the same 

 zone were 10°, while the refractive index was distinctly 

 higher than that of Canada balsam, facts which indicated 

 that the composition was Ab^^ Ang^, a species intermediate 

 between oligoclase and andesine. Rarely two sets of felspar 

 are indicated. In one slide about half the readings of ex- 

 tinction angles in the zone perpendicular to (010) approached 

 4° (Abg An^), while the remainder lay between 4° and 9° 

 (Abg An^). The texture of the rock, however, though but 

 little affected by crushing, gave no sign of two epochs of 

 crystallization, nor did individual crystals appear to be zoned. 

 Absence of zoning, it may be remarked, is a common, if not 

 universal, feature of the plagioclases of these rocks. 



Straining and crushing have often induced rather shadowy 

 extinctions, and bent or faulted lamellae are sometimes seen. 

 Marginal granulation is very frequent. 



Alteration. — The kaolinization of the oligoclase and posi- 

 tion of the liquid inclusions are in every way similar to 

 that described for albite. 



Microcline is another very abundant felspar. It forms 

 white or greenish crystals and anhedrons, rarely recognizable 

 macroscopically. It must be regarded as soda-bearing, for 

 the composition of the rock does not show the percentage of 

 potash that would be present were the microcline a pure pot- 

 ash felspar. The soda content is present, either included 

 in the microcline as an albite molecule or as albite micro- 

 perthitically intergrown with the microcline. Sections suitably 

 oriented for optical investigation show that the microcline 

 does not depart from the normal type. The extinction angle 

 of 15° -16° is plainly observable on (001) sections, and sec- 

 tions perpendicular to the acute bisectrix show a negative 

 biaxial figure, whose axial points lie near the edge of the 

 field of view, with an objective whose angular aperture is 

 134°. Such sections have an extinction angle of 5°. 



Besides its occurrences in the plagioclase as described 

 above, the microcline is noteworthy for its inclusion micro- 

 perthitically of thin, colourless strips, whose birefringence 

 and refractive index are both greater than those of its host, 



