376 PERLITIC CRACKS IN QUARTZ DISCUSSION. [Aug. 1 894, 



Meissen pitchstone. There, according to Sauer, the generally peri- 

 pheral arrangement of the cracks within the quartz-crystals indi- 

 cated a greater contraction in the mineral than in the surrounding 

 glass. In the rock now described, the frequent continuity of the 

 cracks through glass and crystal alike pointed to a smaller difference 

 between their respective coefficients of contraction. 



Prof. J. F. Blake also spoke. 



The Author, in reply to Mr. Rutley and Prof. Blake, pointed out 

 that the perlites in quartz were of the same nature as those in the 

 surrounding glass, that the two substances were often involved in 

 the same perlite, but that the quartz cracks were often quite inde- 

 pendent and frequently concentric. As the cracks traversed every 

 structure in the rhyolites, including trichites and microlites, he felt 

 no doubt that the felsitic structure was in this case original. 



