IGNEOUS ROCKS 



249 



I-'iu'. 237. Granitic rock, about half natural size. The white patches represent 

 crystals of one or two kinds of mineral; the dark parts represent crystals of others. 



crystals in which the larger ones are set may be so small as not to 

 be readily distinguished (aphanitic), or they may be visible sepa- 

 rately (phaneritic). 

 Some igneous 

 rock is, in reality, 

 volcanic glass. Vol- 

 canic glass (obsid- 

 ian) is one phase of 

 solidified lava. It 

 is formed when the 

 liquid lava solidifies 

 quickly, before the 

 crystals have time 

 to grow. Some ig- 



Kitf. 238. Porphyritic texture, pp = phenocrysts of 

 feldspar. The smaller crystals are of feldspar, mica, 

 and quartz. (Watts.) 



M Q 



neous rock is made 

 up partly of glass 

 and partly of crys- 

 tals, and between the rock which is all glass and that which is all 

 crystals there are all gradations. Whether lava becomes glassy or 

 crystalline on hardening, or whether it is partly the one and partly 



