BY \V. G. WOOLNOUGH. 503 



The crushing of the rock has given rise, as above noted, to mole- 

 cular movements in the felspars, and the resulting abnormal 

 extinctions make optical determinations of the felspar very 

 difficult. A large number of very satisfactory measurements in 

 sections from the zone perpendicular to (01 0)"^" gave 20^^ as the 

 maximum. Sections parallel to (010) are beautifully zoned, 

 the zoning indicating the existence of many more crystallographic 

 forms than are shown by the present boundaries of the crystals, 

 even where the latter are well enough developed to be recog- 

 nisable. The (001) cleavage is sharply defined, and the (110) 

 parting is indicated by the arrangement of decomposition pro- 

 ducts, thus enabling the section to be oriented. The extinction 

 angle varies from 0*^ to- 2° for the outer zones to- 14*^ for the 

 central portions. These extinctions agree in indicating that the 

 felspar varies from basic andesine in the centre to oligoclase at 

 the periphery. 



In agreement with this determination are the facts that the 

 refractive index of the peripheral zones is in all cases lower than 

 that of quartz, while that of the central nucleus is in all cases 

 higher. 



Interpositions are not very numerous in the felspar. The 

 principal individualised forms are small acicular prisms of apatite 

 which range down to ultramicroscopic dimensions. The larger 

 individuals do not appear to possess any regular arrangement, 

 but the smaller ones seem to lie with their long axes parallel to 

 the crystal faces as indicated by the zones. There is a little 

 magnetite in grains and crystals, but this is not plentiful. 



In the more undecomposed parts the felspar contains fairl}'- 

 numerous unindividualised inclusions in the shape of liquid and 

 gas-cavities, the former with rapidly moving bubbles. 



* The measurements of the felspars throughout the whole of this investiga- 

 tion were carried out according to the methods elaborated by M. A. Michel 

 Levy in his "Etude sur la Determination des Felspath dans les Plaques 

 Minces '* (Paris, 1894-1896). 



