THE IGNEOUS OR ERUPTIVE ROCKS 53 
away (Hig. 13). Water percolating through the layers, 
deposits a cement of lime or iron, which binds the frag- 
ments firmly together; and so the geologist can draw 
no line between unconsolidated and solidified rock. 
Fig. 12: 
To illustrate process of solidification at present. <A group of railway spikes ce- 
mented by a deposit of iron, after a burial of a few years in a railway 
embankment. 
The same is true of lava. There is every grada- 
tion between the liquid lava and the cold, solid rock; 
and it would be difficult indeed to say just when a 
lava ceased to be a liquid and became a rock. 
