THE COMPOSITION OF THE EARTH 



49 



buried by the lava floods were in some cases mountains of 

 considerable elevation. Lava flows built an even larger pla- 

 teau in the peninsula of India. 



1 NEVADA UTAH 



FIG. 25. Lava flows of the Northwest. 

 UNDERGROUND STRUCTURES OF IGNEOUS ROCKS 



Most of the lava forced upwards from great depths fails 

 to reach the surface and solidifies underground. Igneous 

 rocks formed from lavas deep below the surface are called 

 plutonic rocks. Such rocks may 

 be exposed at the surface 

 through the wearing away of 

 the rocks which overlay hem. 

 Indeed, much of the igneous 

 rock at the surface is intrusive 

 rock. Intrusions of lava vary 

 in shape and in their relations 



FIG. 26. Diagram of dikes and 

 sills, d, dike ; s, sill. 



to the inclosing rock. These differences have given rise to 

 special names. Lava hardens in cracks and fissures in the 

 rock to form dikes (Fig. 26). Dikes vary in thickness from 



B. & B. GEOL. 4 



