MATERIALS OF THE EARTH 15 



the rock has undergone since it was formed. The structure shown 

 in Fig. 4 is known as schistosity a structure characteristic of 

 metamorphic rock. The planes of cleavage, often irregular, are 

 independent of the original planes of stratification in those cases 

 where the rock was originally stratified. Metamorphic rock may 

 be derived from igneous rock, as well as from sedimentary. 



More commonly than otherwise, metamorphic rocks lie beneath 

 sedimentary beds, or come to the surface from beneath them, 

 and they are often broken through by igneous rocks. 



Igneous Rocks 



How they come to be at the surface. Some of the igneous rocks 

 which appear at the surface and beneath the mantle rock, have 

 been extruded from volcanoes. From some volcanoes, such as 

 those of Hawaii, lava flows in streams, and as it cools, hardens 

 into solid rock. From others, such as Vesuvius, fragments of hot 

 rock are hurled high into the air and fall about the vents. Frag- 

 mental material ejected from volcanoes forms pyroclastic (fire- 

 fragmented) rock. Pyroclastic rock is, in many cases, lava blown 

 into small pieces which harden separately. 



Lava sometimes flows out on the surface through long fissures, 

 which extend down to great depths. Such extrusions of lava are 

 called fissure eruptions. There have been few great fissure eruptions 

 in historic time, but in recent geologic time, there were great erup- 

 tions of this sort in the northwestern part of the United States, 

 as well as in some other parts of the earth. All rock matter ex- 

 truded from volcanoes and fissures is extrusive rock. 



Besides being thrown out or poured out from volcanoes and 

 fissures, igneous rocks appear at the surface in other ways. Much 

 lava which rises from the deep interior does not reach the surface, 

 but is intruded into rock which lies above the source from which 

 it started. If intruded lava fills cracks in the earth, it forms dikes. 

 If it forces itself in between layers of rock, it may take the form of 

 intruded sheets, or sills. If it accumulates below the surface in 

 masses, arching up the overlying beds of rock, it forms laccoliths 

 (Fig. 5). If it breaks and lifts its cover, instead of arching it up, 

 the lava is called a bysmalith. Laccoliths and bysmaliths may 



