250 



MATERIALS AND THEIR ARRANGEMENT 



the other, depends on the conditions under which it solidifies. All 

 liquid lava contains the materials out of which crystals may be 

 formed, under proper conditions. 



Glassy and partly glassy rock may be compact or porous. 

 Porous rock of the type shown in Fig. 239 is called scoriaceous. 



Fig. 239. Scoriaceous texture. About 4/5 natural size. (Photo, by Church.) 



^ Rock of this sort is really lava froth, solidified. The pores are the 

 spaces occupied by gases when the lava hardened. Some of the 

 bubbles were large and some small. Pumice is porous volcanic 

 glass, the pores being small. 



Besides these varieties of texture which originate as lava hardens, 

 there are the textures peculiar to pyrodastic rocks. When quanti- 

 ties of volcanic dust, etc. (sometimes called volcanic ash}, become 

 coherent, as by cementation, the resulting rock is called tuff (or 

 volcanic tufa}. If the constituents are largely coarse, the resulting 

 rock is volcanic agglomerate. 



Liquid lava ( = liquid glass). Liquid lava is essentially fluid 

 glass. It is analogous to common glass, which is a silicate of potash, 

 soda, or other base, except that manufactured glass is relatively 

 free from iron and other coloring substances which abound in lavas, 

 rendering them dark and more or less opaque. Lavas, too, are 



