Handbook of Paleontology 63 



either in a volcanic eruption or through large fissures in 

 the earth's crust are known as extrusive igneous rocks or 

 lava rocks; those derived from molten material that does 

 not reach the surface but cools within the crust of the 

 earth are known as intrusive igneous rocks. In certain 

 intrusive forms of rocks the magma has cooled essen- 

 tially where formed. They enlarge downward and no 

 base has ever been found. These are the deep-seated or 

 abyssal rocks, also termed subjacent. The older rock 

 into which molten rock is intruded is termed the country 

 rock and sometimes blocks of this country rock are found 

 included in the igneous rocks. These are known as inclu- 

 sions. The surfaces bounding the igneous rock are con- 

 tacts and the zone is known as the contact zone. Because 

 changes are brought about both in the country rock and 

 the igneous rock in the contact zone, that is, meta- 

 morphism has taken place, the contact zone is also known 

 as the contact metamorphism zone. 



Intrusive igneous rocks occur as intrusive sheets or 

 sills, interformational sheets, laccoliths, dikes, necks, 

 batholiths, stocks and bosses (figure 11) ; among extru- 

 sive igneous occurrences are necks, flows and cones. An 

 intrusive sheet or sill is formed by igneous material that 

 has been forced upward into sedimentary rocks and has 

 spread out in a layer between and along the beds of a 

 series of sedimentary rocks. They may vary from a foot 

 or less to several hundred feet in thickness. The rock 

 forming the Palisades of the Hudson river is an intrusive 

 sheet. When the layer of igneous material is intruded 

 along a surface of unconformity it is termed an inter- 

 formational sheet. Laccoliths are like sills, but they are 

 thicker in comparison to their width, giving lenticular 

 shaped masses of igneous rock between stratified beds, 



