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PHYSICAL GEOLOGY 



Intrusive or Plutonic Rocks 



Igneous rocks have either been extruded on the surface in the form 

 of volcanic products and lava flows, or they have failed to reach the 

 surface and have consolidated beneath it. The latter are called 

 plutonic (after Pluto, the Greek god of the lower world) or intrusive 

 rocks. The quantity of lava which failed to reach the surface is 

 probably many times greater than that which was poured upon it. 

 The deep-seated intrusive rocks are never vesicular (full of gas blebs), 

 since their contained gases were prevented from expanding by the over- 

 lying pressure. They are coarsely crystalline because they cooled so 

 slowly, owing to the fact that they were deeply buried, so that the 

 crystals had time to grow. Such rocks are exposed at the surface 

 only by the erosion of the rock strata which formerly covered them. 

 Doubtless many such masses, now exposed at the surface, were at 

 one time the deep-seated reservoirs from which the lava of volcanoes 



came. There are some rocks 

 which link the extrusive and 

 the plutonic rocks and may 

 be classed simply as inter- 

 mediate. 



The mechanics of igneous 

 intrusions is discussed on 

 page 334. It will be shown 

 that intrusions probably work 

 their way toward the surface 

 largely by sloping (p. 337). 

 When they have reached 

 within a few thousand feet 

 of the earth's surface, they 

 take advantage of any planes 

 of weakness, such as joints 

 and faults, and continue their 

 journey through fissures. 



/. Injected Masses 



Dikes. — Dikes are masses 

 of igneous rock which have 

 hardened in more or less verti- 

 cal cracks or fissures(Fig. 319). 



Fig. 319. -A vertical, branching dike. 

 (Photo. F. B. Sayre.) 



