Page:Popular Science Monthly Volume 20.djvu/363

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

 1. Vitreous rock. 2. Semi-vitreous rock. S. Vitreous rock with spherulites. 4. Rock with cryptocrystalline base. 5. Rock with microcrystalline base. 6. Rock of granitic structure built 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