18 PEOCEEDINGS OF THE NATIONAL MUSEUM VOL.68 



action of gases upon viscous melts or glasses is of primary importance 

 in bringing about the crystallization of the lavas, and may, in fact, 

 be largely responsible for the crystalline character of their flows. 



DESCRIPTION OF PLATES 



Plate 1 



Large lithophysae and spherulites in obsidian glass. These lithophysae are 

 made up of feldspar rods and globular aggregates of tridymite crystals. 



Plate 2 



Lithoidite showing different forms of lithophysae. These lithophysae are made 

 up of sanidine and tridymite. The black crystals scattered through tlie 

 lithophysae are fayalite. 



Plate 3 



Lithophysae in lithoidite with numerous rings. The rings are made up of 

 tridymite and feldspar, while the black crystals are fayalite. 



Plate 4 



Figs. 1. Feldspar crystals in parallel growth, out of lithophysal cavity. ( X 40) 



2. Cristobalite pellets in lithophysal cavity. ( X 6) 



3. Cristobalite pellets and feldspar rods in smaU lithophysae. ( X 6) 



4. Tridymite, feldspar, and fayalite in small lithophysae. ( X 6) 



o • . 



