PLATE XXII. 



Fig. a. 



(Sp. No. 32732. Without analyzer, x 18.) 



Aporhyolite showing beautifully developed perlitic parting. The perlitic cracks are brought 

 out clearly by the chlorite which has accumulated in them. (Described, p. 87.) 



Fig. B. 



(Sp. No. 32732. With analyzer, x 18.) 



Aporhyolite showing perlitic parting, wheu viewed between crossed nicols. The groundmass 

 lesolves itself into a finegrained mosaic of quartz and feldspar, showing luicrogranitic characters. 

 Tlie perlitic parting is thereby almost completely obscured. (Described, p. 87.J 



274 



