20 



GEOLOGY 



Some igneous rock looks like dark colored glass. Such rocks 

 are indeed glass, and, like manufactured glass, were solidified from 

 a liquid. Volcanic glass (obsidian) is one phase of solidified lava. 

 It is formed when the liquid lava solidifies quickly, before the cry 

 tals have time to grow. Some igneous rock is made up partly of 

 glass and partly of crystals, and between the rock which is all glass 

 and that which is all crystals there are all gradations. Whether 

 the lava becomes glassy or crystalline on hardening, or whether it 



Fig. 11. Scoriaceous texture. About natural size. (Photo, by Church.) 



is partly the one and partly 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. 11 is called scoriaccon*. 

 Rock of this sort is really lava froth, solidified, and the pores are 

 the spaces occupied by gases when the lava hardened. The bubbles 

 were sometimes large and sometimes small. Pumice is porous 

 volcanic glass, the pores being small. 



Besides these varieties of texture which originate as lava han lens, 

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



