78 



consisting of rods and grains of iron-oxide (black), micro- 

 lites of felspar, etc., embedded in a colourless substance 

 probably rich in soda. 



Fig. 5. The large felspar represented in Fig. 3 more highly 

 magnified. Polarised light. Mag. 30. 



Regarding the figure as a map the nicols were 

 crossed so that their shorter axes lie N-S. and E-W. In 

 this position the centre of the crystal is dark and the 

 margin light. The crystal is seen to be twinned and 

 zoned with certain parts slightly displaced. 



Fi & . 6. The same after rotating the stage of the microscope so 

 that tne centre is light and the margin dark. The optical 

 characters revealed by rotating the section between the 

 crossed nicols and measuring the angles of extinction 

 show that, during the growth of the crystal, the composi- 

 tion of, the successive layers changed, but not continuously, 

 from pure or nearly pure anorthite to a felspar (andesine) 

 containing from 60 to 70 per cent, of albite. 



