556 DR. ^' H. HATCH ON THE SPHEE01D-EEAE1N& 



examination in convergent polarized light shows that the trace of 

 the optic axial plane is coincident with one of the cleavages (P). 

 Parallel to the cleavage-lines are two sets of straight, strongly 

 refractive markings, reticulated as in fig. 1. These lines are dark 



Pig. 1. — Section of Felspar showing the cleavages P and M and the 

 markings parallel thereto. 



in transmitted, and white in reflected light. They do not extin- 

 guish with the main mass of the felspar, the latter hecoming dark 

 between crossed nicols at an angle of 5°, the former at one of 14° 

 with the basal cleavage-lines. As Becke has already pointed out, 

 this lineation has nothing to do with microperthitic structure. It 

 may perhaps be due to a deposit of secondary crystalline material 

 along the cleavage-planes. 



The decomposition of the orthoclase shows itself in the formation of 

 cumulous aggregates of minute flakes of a brightly polarizing 

 substance — muscovite or kaolin. 



The oligoclase occurs only in isolated crystals of rather small 

 dimensions ; in colour it is white. Easily recognized by its sharply 

 defined twin-lineation, its place in the soda-lime-series is at once 

 given by its small-angled extinction. Its specific gravity is 2-6-18. 

 These figures fall within the limits assigned by Tschermak and 

 Max Schuster * to oligoclase, viz., 2*645, 2-671. 



The quartz, distinguishable by its pellucidity and its numerous 

 liquid-inclusions, belongs to two phases of crystallization, since it 

 occurs both in large allotriomorphic masses and as infillings of the 

 small irregularly shaped interspaces between the larger grains. 

 This younger quartz, representing the residual magma of the rock, 

 is also found sending tongue-like processes into the quartz of earlier 

 origin, thus afifording ocular demonstration of their separation in 

 point of time. 



The hornblende and mica are scarcely to be separated with the 

 unaided eye. They are generally aggregated in little patches of 

 black shining scales. Isolated and examined in convergent polarized 



* " Ueber die optische Orientirung der Plagioclase," Tscbermak's Min. und 

 Petr. Mitth., Bd. iii. 1881, p, 1.53. 



