56 ELEMENTARY GEOLOGY 
vealed, the kind of rocks that are formed under these 
conditions. Our granite quarries furnish illustrations 
of such intrusions. 
A molten rock that reaches the surface is said to be 
erupted or extruded; and the rock that is formed is 
Fig. 15. 
A lava rock with many gas-made cavities caused by expansion of steam. 
called a volcanic, an eruptive, or an extrusive igneous 
rock (sometimes effusive). If it flows away, somewhat 
like water, as in the Hawaiian Islands, it is a lava flow, 
and the eruptive is a lava (Figs. 14, 201, and 202). 
In volcanoes one of the important elements is steam ; 
for, as with a bursting boiler, the sudden eruption of 
