VARIATIONS OF TEXTURE IN IGNEOUS ROCKS 599 



rock crystallized slowly under conditions of slight viscosity. 

 After the exhaustion of the feldspathic material residual quartz 

 crystallized in the remaining spaces. 



Tabulatio7i. — The following table shows the gradation from 

 fine-grained rhyolite to granite : 



Specimen No. Character of groundmass. 



164 N(a) 



165 N(a) 

 162 N(a) 

 169 N 



171 N - 



172 N 

 175 N 



Glassy. 



Cryptocrystalline. 



Finely microgranular. 



Microgranular ; micropegmatic. 



Granular; grains average .5""" diameter. 



Granular ; grains average .5""" diameter. 



Large quartz grains, average 2.25""°. 



Afialyses. — The following are analyses of the two fairly typi- 

 cal specimens as above described (analyst, Dr. H. N. Stokes): 



It will be seen that No 2 is slightly more siliceous than No. i ; 

 nevertheless the two rocks are intimately related. In No. i the 

 relation of K^O + Na^O : CaO= i : .26. In No. 2 the same 

 ratio equals i : .15. Similarly, in No. i the relation of Kg O: CaO 

 = I : .75. In No. 2 the same ratio equals i : .32. 



Conclusions. — Many of the bands in this granite -rhyolite 

 series show by their structure that they have undergone no break 



