IGNEOUS ROCKS 



247 



the nick of the individual sheets shows little indication of arrange- 

 ment in layers. Some extruded rock has a structure developed by 

 tin' (low of the lava after it had become stiff from cooling. This 



Fig. 234. Flow structure in volcanic glass. About half natural size. (Photo, 

 by Church.) 



Fig. 235. Columnar structure in igneous rock. Giant's Causeway. 



