16 



GEOLOGY 



be large enough to form good-sized mountains. The Henry Moun- 

 tains of Utah are laccoliths. Very great intrusions of rock of 



massive form are sometimes called 

 batholiths. Rock solidified from lava 

 well below the surface is called pln- 

 tonic rock. 



Sills, laccoliths, bysmaliths, bath- 

 oliths, etc., are formed beneath the 

 surface; but if the covering rock is 

 worn away, as it may be by erosion, 

 the igneous rock appears at the sur- 

 face. Much of the igneous rock 

 which is now at the surface or concealed by the mantle rock only, 

 is intrusive rock, laid bare by the removal of its original cover. 



Fig. 5. Ideal cross- section of a 

 laccolith with accompanying 

 sheet and dikes. (Gilbert, 

 U. S. Geol. Surv.) 



Fig. G. Flow structure in volcanic glass. About half natural si/c. 



by Church. 1 ) 



The larger structural features of igneous rock. Igneous rocks 

 have certain structural features which distinguish thorn from other 

 rocks. Thus laccoliths, bysmaliths, and batholiths are massive. 



