VOLCANIC PRODUCTS. 



349 



window-panes. In other cases they unite in radial groups about cer- 

 tain centers, and thus build up globular masses to which the name 

 of " spherulites " has been given. 



In Figure 1 No. 1 represents a glass through which microliths or 

 crystallites of different dimensions and character are diffused. In 



Fig. 1. Sections of Igneous Rocks, illustrating the Passage from the Glassy to tiik 

 Crystalline Structure. 1. Vitreous rock. 2. Semi-vitreous rock. S. Vitreous rock with 

 spherulites. 4. Rock with cryptocrystalline base. 5. Rock with microcrystalline base. G. 

 Rock of granitic structure buiit up entirely of crystals. 



Nos. 2 and 3 the crystallites have united to form regular groups. 

 No. 4 represents a glassy ground-mass containing microliths (a crypto- 

 crystalline base), through which distinct crystals are distributed, and 

 is typical of the features presented by most lavas. Nos. 5 and G ill us- 



